Figure 7.1Pentagon of Power
Having discussed machinery, amplifiers, and sensors, we can now focus on the top of the Pentagon of Power: Operator Interface and Control. In the next few chapters we will explore simple control methods as well as sophisticated systems before tackling operator interfaces. Throughout the discussion of controls we’ll necessarily stumble into some aspects of operator interfaces. Controls are the electronic bridge between the operator interface and the amplifier. Commanded by the operator interface, controls translate the desire of the operator into electrical signals which, in turn, describe how much power should be supplied by the amplifier to the machine to move at the requested speed. Controls are also responsible for tracking the position of the machinery by reading the electrical signals of the sensors. Since controls are connected to the operator interface, amplifier, and feedback sensors, we’ll consider the related points of the Pentagon of Power because keeping controls artificially isolated from the rest of the automation components would be a bit silly.
The first control to consider is the simplest. Not only is it valuable conceptually to grapple with a rudimentary system, but simple controls have practical applications in single-axis effects like roll drops, trap doors, turntables and scissor lifts. In some shows, or facilities, programmable positioning isn’t required, so a simple pushbutton control fits the bill nicely. Knowing when and how to utilize these fundamental controls gives you the option to achieve the most bang for the buck, and wisely select the proper system for the task at hand. A small venue with limited budget, or one-time gag at an event where quick setup and simple operation trump sophisticated programmability, are a couple of examples where using a simple control is preferred.
In a simple control system, the operator interface and the control circuitry are often melded together in the same devices. For example, a pendant with a speed knob, a forward/reverse selector switch, and a GO button could be hardwired directly to the amplifier. As you manipulate the operator interface, pressing buttons and turning knobs, you are directly triggering control signals that the amplifier can use to figure out how fast to run the motor and in what direction. Commonly, switches are used for selecting direction and a potentiometer is used for speed control.
Figure 7.2Simple control
Opening up an electronics catalog and browsing “switches” can be a little overwhelming; there are a lot of switches in the world. While there are many options, and a large subset of those options would be appropriate, if you’re not sure where to start, a good choice would be the generically named industrial switches. Industrial switches are modular products that have separate actuators, bases, and contacts which allow you to configure exactly the device you want. This modularity makes industrial switches flexible and reusable. Let’s walk through the options to get an understanding of what’s possible with these handy, fundamental components.
Figure 7.316 mm and 22 mm switches
The switch actuator is the bit that the operator presses, twists, or flicks to make the switch work. Switch actuators mount to the control panel with a round hole of either 16 mm, 22 mm, or 30 mm diameter. Currently the most popular are the 22 mm switches, though 16 mm can be really useful in pendants or control panels that have limited space. Because they take up an awful lot of room, 30 mm switches aren’t typically used. Once you’ve decided on a size (pick 22 mm if unsure), your next step is to select the type of actuator. Here’s a list of the most common switch actuators:
•Pushbutton – Available in round or square, pushbuttons actuate electrical contacts when pressed. The button head can be flush with the housing or projecting or recessed. A flush or recessed head is most useful to prevent accidentally pressing the switch, like when you lean a flat against a wall, because the head starts flush with the housing and presses inwards into the switch body. A projecting head stands proud of the housing and, when depressed, comes flush to the housing. Projecting buttons can be easier to locate and mash, which makes them useful for stop buttons (not Emergency Stop; those come later) but a terrible choice for buttons that start movement. Pushbuttons can be momentary or maintained (aka latched). A momentary button springs back when released, while a maintained button stays put until pressed again, like a clicky pen. Momentary buttons are more common in automation, but latched can have uses in some circumstances. Pushbuttons come in a variety of stock colors, including red, green, blue, yellow, white, and black. Most of the color options are available as opaque plastic, or translucent plastic that can be illuminated with either an incandescent lamp or LED, both of which may be available as snap-in modules.
Figure 7.4Projecting, flush, and recessed pushbuttons
•Twist – Round with a projecting tab, twist actuators need to be turned, rather than pressed, to actuate the switch contacts. They are commonly available in either two or three positions. More exotic flavors exist with more positions, but the principle is the same. A two-position switch can be used as an ON–OFF switch, while a three-position switch might be used as an UP–OFF–DOWN, or FORWARD–OFF–REVERSE switch. There are usually a few options for the degree of twist, so you can choose how far the actuator has to be turned to activate the switch. In a three-position actuator, the center condition is relaxed with no electrical contacts activated. Twisting to the right activates one set of contacts, twisting to the left activates another set of contacts. The switch movement may be maintained or momentary, or in the case of a three-position switch, it may be both maintained and momentary, meaning that the one actuated position is maintained, while the other springs back to the center condition. Twist switches have the same color choices as pushbuttons and can be either opaque or illuminated. A twist switch can also have its handle replaced with a key to securely lock out access to the switch, except by operators that are authorized.
Figure 7.5Twist selector switch
•Mushroom – Emergency Stop circuits must use a mushroom head switch that is large, red, projecting, and actuated by pressing, or more likely mashing. The switch is maintained in its mashed state and releases only when twisted. This special behavior is reserved, and mandated, for Emergency Stop circuits to both make it easy to depress and hard to accidentally release. Additionally, it’s worth noting here that an Emergency Stop switch must use direct-acting electrical contacts. That means that the contacts to do not rely on a spring to actuate, rather the force of the actuator is directly transmitted to the electrical contacts. Why? Well, if the electrical contacts were welded shut, hitting the mushroom head harder will physically break the welds instead of relying on the spring force built into the contacts. Though I’ve never seen it done, you could take a hammer to an Emergency Stop switch to break badly welded contacts. Rather than a modular switch, you might consider a unibody switch for the Emergency Stop to avoid the risk of the contacts on a modular switch unknowingly coming loose.
Figure 7.6Mushroom switch
Figure 7.7Unibody E-stop
•Biometric – If you need to secure access to a switch beyond a traditional key, there are actuators with biometric sensors that can be programmed to actuate on a fingerprint or pulse detection.
Figure 7.8Biometric switch
Regardless of the actuator, an industrial switch has a base that snaps onto the end of the actuator and provides a mounting location for the electrical contacts. The base may be held on by a spring clip or mounting screw, though I prefer bases that mount with a spring clip since they are faster to install. The base has multiple slots, usually three, that hold both electrical contacts and illumination modules, if used. It’s a piece of plastic that doesn’t deserve much fanfare, but important to order since it is the glue that holds the actuator to the electrical contacts.
Figure 7.9Switch base
As noted above, the Emergency Stop button may be purchased as a unibody switch to mitigate the risk of the base coming loose from the switch. Alternatively, some manufacturers provide additional hardware to secure the contacts on an Emergency Stop switch.
The electrical contacts that snap into the base come in either Normally Open (NO) or Normally Closed (NC). Each contact block has two terminals to connect into your electrical circuit. The terminals may be screw-type for wires, or solder-type for mounting to a printed circuit board (PCB). As we previously discussed when considering limit switches, the two terminals in an NO contact are disconnected, or open, in the relaxed state. When the actuator is pressed or twisted, the NO contact closes, allowing current to flow across the two terminals. In an NC contact the opposite is true; the terminals are connected (or closed) in the relaxed state and activating the switch will open the connection between terminals.
Figure 7.10NO and NC switch contacts
The contact blocks fasten to the base either by snapping into place, or secure with a screw. Again, I prefer contacts that snap into place since they are quicker to install. Though the base may only accept three contact blocks, usually the blocks can stack onto each other, making it easy to add more contacts to a switch assembly.
Figure 7.11Stacked contacts
Because you can mix and match both NO and NC contacts on the same switch in varying numbers, you can accommodate some complex circuit logic with just switch contacts. For example, if you have Forward and Reverse buttons on a winch pendant you probably don’t want both directions enabled at the same time. You can use a Normally Open contact on each button to signal the direction, but feed the Reverse direction with a Normally Closed circuit from the Forward button. In this way, the Reverse button will be disabled whenever the Forward button is pressed.
Figure 7.12Schematic of interlocking FWD/REV circuit with NC and NO contacts
Common Brands
The 16 mm, 22 mm, and 30 mm industrial switches are available from many manufacturers and distributed widely through both local electrical supply houses and online stalwarts like Digi-Key, Mouser, Newark element14, McMaster-Carr, and Grainger. Schneider Electric is my preferred brand because of the quality of the components, speed of installation, and decent pricing. However, Omron and Allen-Bradley are very popular brands as well. AutomationDirect carries a line of switches which, like most of their offerings, are very inexpensive and of decent, if not great, quality. IDEC makes a good-quality line of industrial switches, but notably they make unibody Emergency Stop switches that are available in the common required switch configurations (1 NC+1 NO; 2 NC; 2 NC+1 NO) in a monolithic design to avoid the risk of modular parts coming apart.
Figure 7.13Potentiometer
To adjust speed, a potentiometer is the common choice. A pot is a variable resistor made from a strip of resistive material and a wiper that can be moved along the strip to achieve a range of resistance values. Pots are rated by their resistance and power capacity. The resistance rating is given in Ohms (Ω) or Kilo Ohms (kΩ) and describes the resistance across the entire resistor. The power capacity is given in watts, which can be computed by multiplying the voltage of the source by the current running through the device (W = VA, easily remembered with the mnemonic West VirginiA).
Motor amplifiers will typically have speed control terminals that accept a potentiometer. The user manual will specify what resistance and wattage should be used for a speed knob. Selecting one is just a matter of matching the given specs and the mounting method that works best for your control panel or pendant. Typically, a pot will have a threaded collar surrounding the stem that you turn to operate it. That threaded collar fits in a hole that you drill in the front panel of your case and then a nut snugs it up. Additionally, there is often a small anti-rotation tab on the pot that fits into a second, smaller hole to prevent the body of the pot from rotating when you twist the stem.
Common Sources
Some motor amplifiers come with a speed pot as part of the installation kit, which makes it easy to select the right pot (just use the one that came in the box). If you need to buy a potentiometer, there are thousands to choose from so I’d recommend using the parametric search on a distributor like Digi-Key, Mouser, Allied, Farnell, or Newark to sift through the options until you find the one that matches the dimensions you want with the resistance and power rating you need. Note: The site octopart.com is a handy tool for finding electronic components. It compares inventory and pricing across popular online electronic suppliers.
Relay switches are used in all types of control systems from the simplest to the most sophisticated. Relays are electrically controlled switches, meaning that the switch can be actuated by a control voltage. There are two major types of relay switches: electromechanical relays, and Solid State Relays. Both types of switches serve similar purposes. A relay switch is operated by a control voltage; when the control voltage is presented, switch poles are activated. Conceptually, the relay switch is a remote-controlled switch. The low-voltage control circuit is isolated from a high-voltage power circuit. The control signal can be run to a button and then fed into a relay coil. When the button is pressed, the relay is activated and it can turn on a high-power circuit to start a motor. As well as isolating the different voltage and current of a control circuit from a power circuit, a single relay coil may operate multiple switch contacts, or poles, to switch several circuits simultaneously. Each pole in the relay is isolated from the others, so you can have several circuits of different voltages all controlled from a single control signal.
Figure 7.14Electromechanical relay
Electromechanical relays have an electromagnet or “coil” that, when energized, operates the switch by physically pulling the switch contacts together. When the coil is not energized, a spring returns the switch contacts to the original state. The coil of the relay has an operating voltage. Common coil voltages are 5 VDC, 12 VDC, 24 VDC, 24 VAC, 120 VAC, or 240 VAC, but other voltages can be found for more exotic installations. This voltage is used to switch the relay “on,” or flip the contacts of the relay switch.
Figure 7.15Relay coil with double-throw contact
The action of the contact is called the throw. A single-throw contact has two terminals. The connection between those terminals is either Normally Open or Normally Closed. When the switch is activated, a normally open (NO) single-throw (ST) switch will close the connection between the terminals permitting current to flow. As you would expect, a normally closed (NC) single-throw (ST) switch opens the connection between the two terminals, thereby interrupting current flow when the switch is activated.
Figure 7.16Schematic of single-throw NC and NO contacts with current flow
A double-throw contact has three terminals: a common terminal, a normally open terminal, and a normally closed terminal. A double-throw, or transfer, contact can direct current to two circuits that share a common source. Let’s say you wanted to make a red/green stoplight. The red light could be connected to the NC terminal, the green light could be connected to the NO terminal, and power is supplied to the common terminal. Normally, the traffic light is red, but when the relay is activated the red light turns off and the green light turns on.
Figure 7.17Schematic of double-throw contact
When specifying a relay, you choose the control voltage for the coil, the voltage and current rating of the switch contacts, the type of contacts (single-throw or double-throw), and the number of switches or “poles” that are operated by the coil. A 24 VDC SPDT relay has a control voltage of 24 VDC, a single switch pole with both NC and NO terminals. A 120 VAC 4PST has a control voltage of 120 VAC, four switch poles each with a set of NO terminals. Unless otherwise specified, an ST relay contact is normally open. With either relay switch, the voltage and current rating of the switch contacts will be specified, such as 250 V 3 A, or 120 V 10 A.
Relays that are rated for larger currents (>10 A) are referred to as “contactors.” Most commonly, contactors have only single-throw contacts and come with three or four poles. Contactors operate on the same principles as a standard relay, but are used for heavy lighting loads, motors, and other inductive loads.
Figure 7.18Contactor
When checking the rating of a contactor or relay, the current rating is dependent on the way in which the device will be used. The International Electrotechnical Commission (IEC) has described distinct “utilization categories” in the standard IEC 60947. Manufacturers use that standard when rating contacts in relays and contactors. Inductive loads, like motors, have different relay ratings than resistive loads, like lights. Below is a table of common IEC utilization categories that you may come across when searching for a relay rated to handle the load you need.
| Current type | Category | Use | ||
| AC | AC-1 | Non-inductive loads. Switches can easily break the current. | ||
| AC-3 | Starting and stopping a squirrel cage motor. Still easy for the relay to break the current. | |||
| AC-4 | Inching a squirrel cage motor by quickly switching the relay. Or stopping the motor quickly by reversing incoming power connections to the motor. This is a demanding application. | |||
| AC-15 | Switching AC electromagnetic loads. | |||
| DC | DC-1 | Non-inductive loads. Switches can easily break the current. | ||
| DC-2 | Starting and stopping a DC shunt motor. Still easy for the relay to break the current. | |||
| DC-3 | Inching a DC shunt motor by quickly switching the relay. This is a demanding application. | |||
| DC-13 | Switching DC electromagnetic loads. |
In a standard electromechanical relay, each switch pole is physically independent of the others. The contacts all activate at the same time when the relay coil is energized because they experience the same magnetic pull. When the coil de-energizes, the springs force the contacts back to their normal state. As the contacts separate, the current that had been flowing through any closed switch arcs from the switched terminal to the common terminal. This arc creates a couple of problems. First, it acts just like an arc welding machine and can fuse the two contact points on the switch together. This could leave a single pole on a multi-pole relay stuck. The control signal powering the relay coil would cease to have any effect on the state of the switch, and there is no way to detect that failure other than seeing a motor that won’t shut off or a safety brake that won’t engage. If the relay is used in a portion of the control circuit where it is critical to detect failure, then you can use a “force-guided” relay, or a relay with “mirrored contacts.” Both of those terms describe the same feature, which mechanically connects all switch poles together. If a switch pole is welded shut, all other poles will remain shut as well and you can use another switch pole as a testing circuit to detect that the relay did not open when it should, then take appropriate action. We’ll look more at force-guided relays in our chapter on safety circuits (Chapter 10).
Figure 7.19Mirrored contacts vs. standard contacts
The second problem created by the electrical arc jumping across a switch contact as it opens is electrical interference. The arc created by the opening contact briefly radiates a lot of energy that can interfere with sensitive electronics. At one point, Creative Conners discovered a problem with our Stagehand motion controllers whereby they would very occasionally reboot after the motion of the cue was completed and the failsafe brake was engaged. After many days of troubleshooting and analyzing every component in the system, as well as running days of cycle testing to catch a single glitch, we found the source of our grief. Through a seemingly small change in the layout of the control cabinet, the relay responsible for firing the safety brake moved about 2 inches closer to the motion control computer. When the relay was de-energized, to engage the brake, sometimes a large burst of interference was transmitted. Because the motion control computer was closer to the source of the interference, it would reset. The fix was to replace the relays in the affected control panels with one that was manufactured for low arcing.
The coil in an electromechanical relay is another source of electrical interference. When a relay coil is turned off and the electromagnetic field collapses, a large amount of interference radiates from the coil. This can have disastrous effects on nearby circuitry inside a control panel. Creating sufficient space between relays and sensitive components is important, but further abatement may be necessary to insure reliable operation. Snubbing diodes can be used across the coil to lessen the interference, in fact many manufacturers make relay models with such diodes pre-installed.
Figure 7.20Snubbing diodes across coil reduces EMI
Along with the interference problems associated with the moving parts of an electromechanical relay, the more predictable problem of component failure exists as a result of the internal construction of a relay. As with all moving parts, eventually things wear out. Relays are rated with a mean time to failure (MTTF) given in the number of cycles the switch can be activated before you should expect to replace the component. In the theatrical business, we often ignore the need to replace components from wear since many shows have limited runs. However, when building automation equipment that will be used for years, or decades, the anticipated lifespan of key components should be considered and designed for easy maintenance. Electromechanical relays are available in many different packages, but the most commonly used in automation control panels have a base that mounts onto DIN rail with wiring terminals and a replaceable relay switch (aka ice cube relays). This packaging makes it easy to replace a blown relay without disturbing the rest of the wiring since the wiring terminations are made in the mounting base. Contactors are usually housed in a monolithic piece of plastic and can’t be replaced without unwiring the old device and wiring the new device into the control cabinet.
Figure 7.21Ice cube relay can be easily replaced
For longer operating life, a relay with no moving parts can be used. The Solid State Relay (SSR) is a pre-packaged transistor circuit that can be mounted and wired like a mechanical relay, but without any moving parts. Since there are no bits of copper making and breaking contact in the switch, no electrical arc develops when the switch operates. Without an electrical arc, the electrical interference from switching is practically eliminated in most circuits. To achieve this clean switching, the SSR uses transistors or TRIACs or SCRs. These electronic devices have to be specifically designed for the load being switched. SSRs are rated for switching either DC or AC, but not both so you must pick the right relay for the job. If you need to switch a heterogeneous mix of signals you will need several SSRs. The solid-state circuitry inside an SSR operates substantially faster than mechanical switches and without any audible noise. The contact arrangement is limited to SPST, though multiple circuits can be package in the same physical device. The silicon switches are not as forgiving of short overloading and care should be taken to keep the load current and voltage within the switch ratings.
Figure 7.22Solid State Relay (PCB-style)
Because the Solid State Relay eschews springs and copper switch contacts it has a much longer mean time to failure (MTTF) and often lasts an order of magnitude longer than its mechanical brethren. However, nothing is forever and Solid State Relays will fail eventually. Unpleasantly, the Solid State Relay typically fails with the output circuit closed. This means that when it eventually dies, the relay will leave the output circuit connected, so the motor will perpetually run or the brake will remain energized. The failure mode of the SSR makes it impossible to use in failsafe circuits.
In practice, I’ve found that mechanical relays still dominate in automation control designs. You will find mechanical relays in most control cabinets, and SSRs are relegated to very specific uses where the long-life, lack of interference, or fast switching speed outweigh the disadvantage of their “fail on” failure mode.
We have amassed enough general knowledge to design a simple automation system. If you’ve been following along sequentially through the preceding chapters, relish in the satisfaction that we can now build something real and useful. This is our first of several systems we will assemble as a mile marker on our way to automation enlightenment. Enough chat, let’s build our simplest system.
Requirements
Our first system is a variable speed controller that uses limit switches for position feedback and a pushbutton pendant for control to open and close a traveler track with a soft curtain. Typically, the operator will select a speed with the knob and then hold down either the OPEN or CLOSE button. When the curtain fully opens or closes it will stop. The operator can change speed at any time without affecting the ultimate stopping position for either open or close directions. Here’s the bullet list of features we need to implement:
•Variable speed for 1 HP AC induction motor with a holding brake, top speed 3 ft/sec
•Knob for selecting travel speed
•Pushbutton for open
•Pushbutton for close
•Limit switches for positioning
•Limit switches for slow-speed
•Emergency Stop
Let’s walk through some of the design considerations.
Since the motor for this traveler machine is an AC induction motor, the amplifier will be a VFD (variable frequency drive). Though it may not be completely necessary to include a holding brake in this application, building the control circuit for a holding brake certainly makes this control more versatile. Holding brakes are failsafe, and require a voltage to release. For this project, a brake with a coil voltage of 200–240 VAC at 60 Hz will be used. That voltage will have to be switched on and off independently of the voltage powering the motor since the motor will be powered by a variable frequency drive, and that power would cause the brake to work somewhere on the spectrum between not-at-all and erratically. Thus it will need a separate power circuit controlled with a relay switch.
Since the operator may choose to select a speed before pressing a direction button, we should program an acceleration time in the VFD. This will make the curtain gracefully ramp up to speed and eliminate startup jerkiness. When stopping, the VFD should gracefully ramp down with a programmed deceleration time. Using the acceleration and deceleration ramps in the VFD is convenient, and smart use of the included functionality of the amplifier. However, using a deceleration affects the positioning accuracy. If we program a 2-second deceleration rate, the curtain will travel different distances depending on the traveling speed. For example, let’s consider a traveling speed of 36 in/second. When decelerating to a stop, our average speed is 18 in/second. Over 2 seconds the curtain will travel 36 in.
Figure 7.23Overtravel at 36 in/second
If we reduce the speed to 9 in/second, then our average speed drops to 4.5 in/second. In two seconds, the curtain will only travel 9 in.
Figure 7.24Overtravel at 9 in/second
Using limit switches for positioning, in this example the open and close positions would change radically by 25 in and the difference in position worsens as the traveling speed decreases. Setting limit switches for that wide range of speeds would be impossible. To insure proper positioning, we will add two more limit switches that reduce the speed to a preset creeping speed of 3 in/sec. When the curtain opens, it will travel at the operator-selected speed until it strikes the slow-down switch. Then it will track open at a preset low-speed of 3 in/sec. When it hits the stop limit switch it will decelerate to a stop 3 in past the switch. Because the speed will be consistent between the slow-down and stop limit switches, the switches can be placed such that positioning is predictable and repeatable.
Figure 7.25Slow-down limits
The VFD has control inputs and configuration parameters that make it possible to select between a speed derived from a pot input and a programmed fixed speed. Once configured, when the slow-speed signal is activated the VFD will lock onto the programmed speed until the input signal is deactivated, at which point it will accelerate to the speed described by the pot. To make our system work, we will install a 40 in long striker bar on the curtain to activate the switches. This will insure that the slow-down switch remains engaged until the stop switch is struck at high speed. You may wonder if the curtain will travel at the slow-speed when reversing direction away from the stop switch. The answer is yes, unless we’re clever and design the circuit to ignore the slow-speed signal when traveling away from a stop switch. Food for thought as we develop the schematic.
For the operator pendant, we will pick a pot for speed selection and buttons for open and close. The knob is easy and doesn’t warrant any more discussion until we select components. The buttons are a little more nuanced. What happens if the user selects both OPEN and CLOSE at the same time? There is no sensible way to determine what direction is desired and the only logical action is to disallow any motion until the operator regains his senses. One way to make sure that only a single direction is selected is to use a selector switch for the direction and a separate “GO” button. In such a case, we could use a twist button for OPEN/CLOSE and a pushbutton for GO. This certainly makes the wiring easier, and is a valid choice, but I personally find it less appealing. As an operator, I prefer two pushbuttons: one for OPEN and one for CLOSE. This may make the schematic trickier, but it results in a better interface (where better = my preference).
Lastly, we need to provide an Emergency Stop button on the pendant and an Emergency Stop subsystem. This is the first time we are bumping into the topic of Emergency Stop subsystems, and we’ll discuss it more later. For this project, we are going to implement a Category 0 Emergency Stop. The idea behind Emergency Stop is simple: when something goes wrong, slapping a big red button will stop the motor. There are three different categories of stopping methods:
1.Category 0: Remove power from the motor and engage the brake. This is an uncontrolled stop, no deceleration, slam the brakes on and remove power from the prime mover. This is suitable for emergency, provided the sudden halt doesn’t present a danger. In systems with high momentum, the force created by a Category 0 can break mechanics or cause harm to people. However, it is the simplest Emergency Stop to implement, and should be considered in the design since a loss of power will initiate an unintended Category 0 stop.
2.Category 1: Use power from the amplifier to quickly decelerate to a standstill, then remove power from the prime mover. This is a controlled stop, and suitable for emergency, but much less violent and therefore gentler on machinery. Some circuitry must be implemented to remove power after the intended deceleration time has elapsed. If there is a problem with the amplifier, which necessitated the stop, then it will effectively be a Category 0 stop.
3.Category 2: Use power from the amplifier to decelerate to a standstill, and leave power available to the prime mover. This is a controlled stop, but not suitable for emergency since it leaves the system energized.
Pentagon Dissection
This simple automation system has all five points of the Pentagon of Power. This is a good example of single physical components encompassing more than one conceptual responsibility in the Pentagon. Splitting or combining logical functions across physical boundaries is commonplace in automation systems, but doesn’t lessen the need to analyze the system and mark the logical boundaries. In fact, it makes it all the more necessary to keep a clear head while designing and troubleshooting. Knowing how a component is functioning in a system, and the role it plays, is fundamental.
The Operator Interface in our first system is the pushbutton pendant. The operator expresses the desire to open or close the traveler track by pressing the corresponding button. The speed is described by turning a knob. To inform the operator that the system has power, an amber indicator light glows. A second indicator light, green in color, is used to show that the motor is moving. The third and fourth indicators are red and signal that either the OPEN or CLOSE limit switch is engaged. These indicator lights are simplistic, but the information they provide is a lot of bang for the buck.
The operator will likely be in direct line of sight and will see if the motor is moving, but the interface should give some visible cue that the machine is operating, or attempting to operate. If the operator’s view is obstructed, the indicator is obviously helpful. Less obvious, the indicator is helpful when troubleshooting. If the operator presses the button and the green light glows, but the motor doesn’t move, you know that the control is working, and that the problem lies with either the amplifier or motor. Conversely, if you press the OPEN button and the green light doesn’t come on, but the OPEN limit indicator is glowing red, you know that the motor is at its extreme position and can’t move any further. If the limit indicator isn’t glowing, then there is a problem with the control and troubleshooting should start there. If the amber light is off, then go and find out who kicked the power cord out of the wall.
With a little bit of planning, and a few extra bucks, our simple operator interface has decent functionality with enough sophistication to make operation pleasant and offer guidance for troubleshooting.
Figure 7.27Developed sketch of pendant interface
The VFD has enough control logic built in that we can leverage the on-board circuitry, along with a few relays to create a controller. The buttons are connected to digital inputs on the VFD to signal that motion should start. The button signals are interrupted directly by the limit switches wired in series. Another digital input is used for creeping at slow speed when either of the slow-down limits is struck. A little extra wiring is needed to make the motor only creep when it is heading in the same direction as the limit switch. We want the motor to run at slow-speed when opening and the OPEN slow-down switch is engaged. When you press the CLOSE button, the OPEN slow-down switch is still engaged, but the motor is free to move at any speed.
Digital outputs from the VFD are employed to power the green indicator light and fire the brake relay to release the failsafe brake.
Figure 7.28VFD control terminals
Filling its second role in the Pentagon of Power, the VFD acts as an amplifier powering the AC induction motor. It interprets the speed command signal described by the pot on the pendant as an analog voltage and converts that into variable-frequency, three-phase power.
Figure 7.29VFD power terminals
The machine in our example is intentionally vague. It is some nebulous curtain track winch that runs forward and backward. From an automation control perspective, the only pertinent detail is that the prime mover in the machine is a 1 HP AC induction motor. The motor type is critical to know so that we can pair it with the correct amplifier. Beyond that information, a motor is a motor, regardless of the mechanical design.
To close the loop in our control circuit, limit switches signal back to the control when the motor has either reached its destination or triggered a reduction in speed. While I was glibly unconcerned with the details of the winch, I am keenly interested in the mechanical design of the striker that interacts with the limit switches. There are a couple of criteria for the limit switch and striker design that have to be considered. First, the striker must be placed so that it doesn’t physically damage the limit switch if the motor travels too far. An oft-made mistake when first designing a limit switch mount and striker is to place the limit switch at the end of travel and directly in line with the striker. It’s tempting to believe that the motor will stop precisely when the limit switch is activated, but physics still applies and momentum can provide enough energy to destroy the switch as the machine lumbers to a stop in the final milliseconds between switch activation and complete halt. So, the switch body should be placed out of harm’s way and the limit actuator placed in the path of the limit striker.
Figure 7.30Good limit placement avoids damage to switch
The next consideration to be made when designing the limit striker is its length. It must be long enough to keep the slow-speed limit engaged until the stop limit is activated. Our control circuit is as simple as possible, and doesn’t have the ability to latch onto the slow-down event, rather that signal must be maintained mechanically. The motor will only move slowly when the switch is activated; if it springs back to its normal state, the motor will return to cruising speed. If the striker is too short, it will contact the slow-down limit, the motor will slow down, and then the limit will release and the motor speeds back up. When the motor hits the stop limit, the motor will decelerate to a stop, but the positional repeatability we diligently designed into our system will be lost.
To determine the minimum striker length, you could experiment and determine the length empirically, or use a little math. The top speed of our machine is 36 in/sec. We will set our slow-speed to 3 in/sec. We will set our deceleration time to 2 seconds in the VFD. The VFD will apply any change in speed over 2 seconds, giving a brisk yet graceful deceleration. When the limit striker contacts the slow-down limit at 36 in/sec, how much distance will it travel in the next 2 seconds before it hits the stop limit? We calculate the average speed in the deceleration trajectory:
Figure 7.31Velocity over time
Figure 7.32Distance over time
Once we hit the stop limit, we will decelerate from 3 in/sec to 0 in/sec in another 2 second ramp.
Figure 7.33Velocity over time
Figure 7.34Distance over time
Figure 7.35Striker design
This calculation shows that the minimum striker length is 42 in, and that the slow-down limit and the stop limit have to be 42 in apart for consistent positioning if you want to be able to operate the curtain machine over the entire speed range. In practice, the striker should be a little be longer to insure engagement, so I’d make a striker bar 48 in long.
When designing a new control panel, I pick the amplifier and develop a schematic to work out the logic of the circuitry. Below is a schematic for this simple controller. If you aren’t familiar with reading schematics or wiring diagrams, skip ahead to Chapter 14 for a quick explanation of the symbols used and then come back to dissect this schematic.
Figure 7.36Simple control schematic
Component Selection
Below is a chart of the components selected for the project and a brief description of the deciding factors. All of these components have equivalents from other manufacturers, but they are a decent selection and serve as a good example or starting point for deviation in your own design.
| Component | Model |
| VFD | Mitsubishi D700 |
| Buttons | Schneider 22 mm Harmony Plastic Series |
| Pot | Bourns 1K Single Turn |
| Indicator lights | APEM LED 24 VDC & 220 VAC |
| Brake contactor | Schneider LP4K0601BW3 |
| Limit switches | Schneider Osiswitch – Roller Arm |
| Emergency Stop relay | Preventa |
| Terminal blocks | Phoenix Contact |
| Fuses | Bussman |
The Mitsubishi D700 meets our needs in an affordable package. The key points for this project are price, decent speed control (but we don’t need precise performance), and enough inputs and outputs to handle our modest control needs.
Figure 7.37Mitsubishi D700 VFD
The Schneider Harmony buttons are high quality and tough. Though not the cheapest option, spending a little extra on good buttons is wise. The operator interface will be handled frequently and risks damage from the abuse taken backstage. As well as being tough, the contact blocks are pleasant to work when wiring up the panel, they mount easily and securely but are quick to unmount if you need to re-wire at any point.
Figure 7.3822mm switches
The specifications for the speed pot should match the recommendations in the Mitsubishi installation manual. The manual specifies a ½-watt 1 -kΩ pot, so that’s our primary criterion. Next, I choose a panel-mount style with solder tabs for ease of installation. These options make it easy to drill a hole in the electrical cabinet, mount the pot, and then solder on wires for connecting to the VFD.
Figure 7.39Potentiometer
Selecting the right indicator lights is a matter of matching up the voltages from our schematic, choosing a lamp type, and picking the mounting style. The power indicator on the control panel is 220 VAC; the rest of the indicators will run on 24 VDC. LED lamps are the only sane option these days for low-voltage indicators because they look good, last a long time, and are cooler than incandescent lamps. For the high-voltage indicators, there are some nice LED options now, or neon indicators are a reasonable second choice. For control panels like this one that won’t have a custom circuit board printed for the operator interface, panel-mount indicators that fit into a drilled hole and secure with a nut are easiest to install.
Figure 7.40LED indicator
The brake contactor needs to switch the current drawn by the failsafe brake. Based on the rating of the contactor in this utilization category, and the current draw of the brake, the Schneider part is well suited. Though not necessary in this design, the Schneider contactors are constructed with mirrored contacts, or force-guided contacts, which is handy if you need to add in some monitoring logic to verify that the contactors are operating correctly. The contactor has three power poles and auxiliary NC and NO contacts for signals. Mirrored contacts all operate in unison, so if a power pole is welded shut, the signal contact will stay shut also. By comparing the coil voltage with the state of the auxiliary contacts, you can discern if the contactor is operating properly. If there is no coil voltage, there should be current flowing through the NC contact otherwise there is a problem.
Figure 7.41Brake contactor
The compact limit switches from Schneider are available in a variety of styles and are of decent quality without excessive expense (and are available from your favorite electrical supply house). The roller lever arm style is good for this application where the striker bar will travel several feet past initial contact with the switch. The roller wheel rides easily in either direction without getting stuck on the striker bar. The contacts in the switch will carry just a few milliamps (mA) of current at 24 VDC, and the switch can easily handle that load.
Figure 7.42Roller-arm limit switch
To save space in our control box, and meet modern safety standards, we will use a pre-packaged Safety Relay in our Emergency Stop circuit. The Safety Relay is a composite module that internally contains redundant, force-guided relays with integral monitoring circuitry inside a single module that neatly clips onto a 35 mm DIN rail. If either of those internal relays fails to operate, the module can still interrupt power during an Emergency Stop, but the internal logic circuitry will not allow the relays to be reset and re-energize the output. This failsafe design is critical for the Emergency Stop circuit.
However, the contact rating is not high enough to interrupt power directly to the VFD. Instead, safety relays are intended to interface either with compatible power devices, or monitor the operation of larger contactors that have high current capacity. The D700 VFD has a feature known as Safe Torque Off (STO). By wiring the safety relay into the STO terminals, the VFD will reliably remove power from the machine when the Emergency Stop button is engaged.
Figure 7.43Safe-torque off hookup
The safety relay has two sets of inputs that are wired through two sets of identical NC contacts on the Emergency Stop button. Following the philosophy of redundancy, two sets of contacts are used and their operation is compared by the safety relay. The contacts should always operate in unison. If the timing between opening of the circuits varies beyond tolerance, the safety relay will engage the E-stop and not allow it to reset until normal operation returns.
The Preventa series from Schneider is a decent safety relay with all the required features. Like all the other components, alternatives exist and should be considered, especially when pricing these modules. Safety relays are tightly regulated and as long as the relays have the same safety certification (e.g., SIL3, PLd, etc.) you can get the one that fits the budget.
Figure 7.44Safety relay
To provide power for our signal circuits, we need a 24 VDC power supply. In this simple design, we can utilize the small, yet sufficient 24 VDC power supply built into the D700 VFD and available on terminals PC (+24 VDC) and SD (common). Using the onboard power supply is convenient and cost effective, but it only provides 2.5 W of power. For more complex designs, a dedicated power supply would be required. When you need a dedicated power supply, the Omron S8VK-G06024 power supply is an impressively compact DIN rail mountable component with a nice complement of features such as overload protection and short-circuit protection. And it comes in black, so it looks cool in the cabinet.
Figure 7.45Power supply
Power supplies can be fussy animals. In control cabinets, we rely heavily on power supplies to function consistently; without control power, nothing will move on stage. Junky power supplies that die too soon are a bane of the automation technician. Spend a little extra and get a power supply from a reputable manufacturer. Everything else in your show relies on the power supply. The Omron supplies are one of several good-quality power supplies that I have used for decades (you’ll hear endorsements for other brands from automation veterans; use any of those recommendations and reap the benefit of hard-won experience). Every time I cheap-out and use a lesser brand, I regret the decision while cursing at a pile of wires in the trap room du jour with a flashlight in my mouth!
To distribute power and signal throughout the control panel, terminal blocks are needed. Like the contactor, safety relay, and power supply, terminal blocks are available as DIN rail mounted parts, and you should use them. Phoenix Contact makes my favorite terminal blocks. Terminal blocks are more a system of parts than a single component. There are end clamps that should be placed on your DIN rail like book ends to stop all the other components from sliding around. Feed-through blocks conduct from one side to the other.
Figure 7.46DIN rail terminal blocks
Singly, these are useful for terminating wires in the cabinet that will require wires to be added during final installation. For instance, if the limit switch wires are permanently installed, a pair of terminal blocks could be used. The circuit is wired in the cabinet and ends at the terminal blocks. The other side of the terminal blocks are left empty until final installation at which point the limit cables are brought into the panel and the individual wires are terminated into the terminal blocks.
Figure 7.47Terminal blocks used for field termination
Terminal blocks can also be used to create a bus for any signal or power source by installing jumpers between the terminal blocks. These jumpers are installed from the top and fit down into adjacent terminal blocks leaving the wiring terminals unobstructed. This will be handy when we need to distribute the 24 VDC power supply to several wires. We can use the terminal blocks to build a bus for the +24 VDC and Common power wires.
Figure 7.48Jumpers used to build a bus
Terminal block dividers are available to separate groups of blocks that either share the same signal or related signals. It’s a housekeeping tool that cleans up a panel nicely and suggests some instinctive order when looking inside the cabinet. Be sure to use terminal block covers to insulate the last terminal block in a row.
Figure 7.49Fuse
Fuses are needed to protect the conductors feeding the panel and the components inside from excessive current that could cause damage or fire. Fuses must be sized according to the voltage, current rating, type, and protective class. The voltage and current rating are straightforward, but the type and protective class deserve some explanation. Fast-acting fuses essentially blow instantly when current rises above the fuse rating. Slow-blow fuses will allow excessive current for a defined amount of time before blowing. Some devices, like motors, will draw large currents at startup but quickly lower the demand. Choosing the proper type requires reading the specifications for the devices in your cabinet and following the recommendations.
Fuses are classified as either branch circuit protection or supplemental protection. Branch circuit protection means that every conductor downstream from this fuse is protected. A supplemental fuse can be used to protect devices, but must itself be protected by another fuse upstream. The National Electric Code and manufacturer documentation are good resources when determining which fuse to select. If you don’t include branch circuit protection in your cabinet, you must label the cabinet appropriately so that anyone using the cabinet knows to power it from a circuit that has the correctly sized fuse or circuit breaker.
From the Mitsubishi manual, a 10 A Class T (fast-acting) fuse or UL489 molded case circuit breaker (MCCB) is recommended for the 1 HP VFD. We’re using a 10 A Class T fuse instead of an MCCB to reduce cost at the expense of inconvenience when fuses blow and need replacing. Note that fuses are an excellent protective device and exceedingly reliable at preventing damage or fire. Circuit breakers are more convenient, but are much more complex devices and thus more prone to failure.
Setting Up the VFD Parameters
To achieve the functionality that we require, the VFD needs some parameters configured. Below is a chart of the parameter configuration and brief description of each.
Pre-packaged Options
This is a book focused on providing you with the knowledge and tools to create, implement, and troubleshoot automation systems. While it is not meant to be a catalog of commercial products, I think it would be silly to omit the discussion of popular automation products that are built for our industry in a quest for academic purity. Much like our important discussion of components which I hope serve as a starting point for your own investigation during system design, a quick look at a pre-packaged simple controller may spark your curiosity or expose you to a thing previously unknown.
The Deck ChiefTM from Creative Conners, Inc. is functionally equivalent to the hypothetical device we built in this chapter. Available in a wide range of horsepower, it offers a simple pushbutton interface and variable speed. Accepting both slow-down, and stopping limits, it provides consistent positioning across the full range of speed.
Figure 7.50Deck ChiefTM simple control
Source: Courtesy of Creative Conners
From this point forward our pace quickens and the topics get richer. With the foundation established, we can expand into more sophisticated systems. Though more sophisticated systems are possible, the knowledge of how simple systems work and when they are appropriate will be useful in shows that benefit from easy operation and quick setup. If you’re confused at this point, go back and read again; it only gets harder from here on (but in a good way).