With the push of a button, we will make an entire stage filled with scenery move autonomously into a choreographed position. That is the goal. Night after night, performance after performance, we want scenic choreography with predictable precision and timing. This exact repetition of movement is a goal shared with factory automation. But, unlike our industrial counterparts, theatrical automation technicians need to be able to rapidly adjust the position and timing of those movements during rehearsal to match the artistic intent of the production.
Automation, however, is complex. It seamlessly blends machinery, electronics, and software into motion on stage. Tackling an automation project as a single, monolithic problem is daunting. The number of physical components and the extent of engineering knowledge required to piece them all together is dizzying at first blush. But if we break an automated system into simpler chunks, each can be easily understood. There are five chunks that can be found in any automated effect, and it’s important to give these names in order to understand how each contributes to the whole. Because there are five (and because I have a penchant for both geometry and alliteration), I like to refer to this composition as the Pentagon of Power. And when I say it in my head, I rather like to imagine a little reverb to make it sound like a superhero.
Figure 2.1Pentagon of Power
The Pentagon of Power consists of an operator interface, control circuit, power amplifier, machine, and feedback sensor. Every automation system has components that fill these roles to perform specific functions. From a simple knob-and-button curtain winch to a Las Vegas spectacle with dozens, or even hundreds, of automated machines, these systems share the same conceptual structure. Once you learn the pillars of a generic automation system, you can dissect any specific system and figure out how it works in detail.
When we discuss an automation system, we often speak of an axis, or many axes of automation. An axis is a single automated effect. Why don’t we just call them motors, or winches, or something more concrete? Not all automated effects are driven by motors, some are hydraulic or pneumatic. The source of mechanical power the machine uses to move the effect is a detail that isn’t relevant to other parts of the Pentagon. So, an axis – or the plural, axes – is a more generic term we can use without getting into the mechanical specifics. Another reason why axis is an appropriate term is that the movement of each automated axis is restricted to a single direction. If we consider a Cartesian grid, an automated axis can translate (push or pull) a piece of scenery along the X, Y, or Z axis, or it can spin the scenery around the X, Y, or Z axis. We might make a curved track in the stage that doesn’t stay straight like a Cartesian axis, but if a machine moves along that curved track it will always follow the same path.
Figure 2.2Cartesian axes of motion, 6 degrees of freedom
With some vocabulary established, let’s dig into each of the five roles in the Pentagon of Power before looking at specific implementations. As you read this, you may encounter unfamiliar technical terms, but the overall picture should be clear. Later chapters dive into much greater detail of these components and I encourage you to return to this overview of the Pentagon of Power frequently as you read the book.
The Operator Interface is the bridge between us fleshy humans and the machines. During load-in, the operator will need to run the winches, turntable, or elevator axes to test that the mechanics are working well and confirm that set pieces can move without a collision. Next, once the show is in technical rehearsal, the operator will need to run each axis of automation into position on stage to recreate the set designer’s desired look. Those looks need to be recorded as cues and the specific timing of the motion will need to be sculpted to suit the artistry of the show. When the show opens, the operator then needs to run the automation cues and replay all the recorded motions night after night – after all, that is the goal of an automation system. In between shows, or on dark days, the crew will undoubtedly need to do maintenance either on the automation equipment, scenery, or other less important stuff on stage, and the operator will want an easy way to shuffle the set pieces around. The machines are installed – might as well use the machines to push the set around when it helps the crew do their work.
The operator interface gives the operator the tools to accomplish all these tasks. Depending on the size of the show, the number of axes, the budget, and the existing equipment that may be pressed into service, the operator interface could be as simple as a pushbutton and a knob, or as sophisticated as a large touchscreen interface with commercial automation software, replete with live 3D visualization of the stage and all the scenery on it moving in real-time. In either extreme, the operator interface is what gives you the power to move the machinery on command. But, as you can tell by the different tasks required during load-in, rehearsal, performance, and maintenance, the operator interface probably has a few different modes to cater to the task at hand.
Figure 2.3Deck ChiefTM
Source: Courtesy of Creative Conners, Inc.
Figure 2.4The Rhody: a sophisticated automation system
Source: Courtesy of Creative Conners, Inc.
Though it is most common to have a person pressing a button to initiate cues during a performance, it’s also possible for automation cues to be triggered by a larger show control system that is synchronizing other aspects of the performance, such as lights, projection, and sound. In a production where automation is driven by a show control system, the operator interface during performance may be just a single “ENABLE” button that the operator holds down to allow motion to continue automatically. The precise timing of when that motion starts would be controlled by the show control system, but the operator is there to confirm that it is safe for the automation to execute and grant permission for the movement.
Figure 2.5Show control with enable button
The Operator Interface takes its input either directly from the operator or from a higher-level control system and outputs a signal to the Control Circuit that indicates what you want to happen on the stage.
If the operator interface gives you the ability to command the machinery on stage to move, where does that command go? Take the simplest example: a pushbutton could be used to start and stop a motor-driven winch. This would work just like a table saw in a carpentry shop. Pressing a start button would make the motor run and pressing the stop button would halt movement. That sort of control is appropriate for a table saw, but clearly it would be a disaster if you were moving a two-story set across stage at high speed. You need precise control of the position and speed of the scenery as it moves from one spot to another. For example, one cue may need to move a wagon from the wings to center stage at 3 ft/second; the next cue may require it to scoot it 10 ft stage left of center at a crawling pace of 1 in/second. This is the role of the control circuit, to take input from the Operator Interface and regulate the speed and position of the Machine so that it moves where you want at the speed you desire.
To regulate the speed of a machine, the Control Circuit sends either an analog or digital electrical signal that dictates the speed. This low-power signal is sent to the amplifier. As the machine moves, a feedback sensor will provide the Control Circuit with data that reports the movement resulting from the speed signal. The feedback data could just inform the Control Circuit when the machine has arrived at various positions along its travel, or it could provide richer information that includes speed, direction, and position. As we will see later, the last of these is a requirement for anything but the simplest systems.
To recap, the Operator Interface takes input from the operator and then sends an output signal to the Control Circuit. The Control Circuit takes input from the Operator Interface and now sends output to the third component in the Pentagon of Power, the Amplifier.
The Amplifier has a singular duty. It takes a low-power input signal from the Control Circuit and generates the electrical energy necessary to move the machine. The input signal can dictate either the speed or the force of the machine. The most common case, and the one we will discuss here, is an amplifier that receives a speed signal from the control circuit.
The Amplifier is connected to the electrical service in your theatre. It transforms that energy into the proper power to move its machine at the speed commanded by the Control Circuit. Whether the electrical power output is alternating current (AC) or direct current (DC) will vary depending on the type of machine. If the amplifier is powering an induction motor-driven machine, then it will produce three-phase AC with a varying frequency to spin the motor faster or slower. If it’s a hydraulically operated machine, then the amplifier will produce low-voltage DC to shuttle a proportional valve spool between closed and open positions in small increments to regulate oil flow. The amplifier in automation is analogous to the amplifier in a sound system. In a sound system, a signal from a microphone can’t directly drive a loudspeaker. Instead, an amplifier boosts the mic signal and powers the speaker. Similarly, a motor can’t be powered from the control circuit. Instead, a motor amplifier boosts that speed signal and powers the motor.
Amplifiers are commonly referred to as “drives” or “drivers.” You’ll encounter that term when searching through catalogs and talking to industrial suppliers. There are DC motor drives, variable frequency drives (VFD) for AC motors, brushless servo drives, proportional valve drivers, and more. Unfortunately, the term “drive” is often overloaded and overused backstage to mean the mechanical system, such as a “turntable drive” or perhaps the entire control circuit and amplifier when built into an electrical box. Be aware of who your talking to and make sure you mean the same thing when using the word “drive.”
The Control Circuit can be connected to a variety of amplifiers, but the amplifier must be precisely matched with the machine. Hooking up a hydraulic valve amplifier to an AC motor would not work, but the same speed signal from the Control Circuit could be used on either a motor amplifier or a valve amplifier.
Figure 2.6Mitsubishi A800 VFD
Figure 2.7Minarik RG5500UA DC regen drive
Figure 2.8Vickers hydraulic valve drive
Figure 2.9Lexium 32 Servo Amplifier
Figure 2.10Control, amplifier, machine
The amplifier, or drive, takes input from the control circuit (typically a speed signal) and sends output power to move a machine.
The machine is probably the most familiar piece of the puzzle when you are first getting started in automation. Understandably so, it is usually the thing that is most obviously doing the work of pushing, pulling, lifting, or spinning scenery around on stage. Winches and turntables are two of the most common machines used in scenic automation, but every machine must have an actuator that takes power from the amplifier and converts it into movement. Electric motors convert current into rotary motion. Electro-hydraulic valves take current and convert it into linear motion that slides a valve spool to vary oil flow to hydraulic cylinders or motors.
In addition to the actuator, machines usually have a series of linkages that convert the primary motion into the movement you need on stage. A motor on a winch might have a gear reduction and then possibly a couple of sprockets connected with a chain that spin a drum that itself winds a cable. A turntable machine might have a motor with a gear reduction and then a rubber wheel that presses into the edge of the revolve platform to spin it around. A scissor lift will have a hydraulic cylinder that pushes on a series of hinged steel link bars that raise and lower a platform. Each of these examples has an actuator that is powered by the amplifier, but that actuator is physically connected to mechanical parts that convert the raw motion of the actuator into the desired movement. It is this combination of actuator and mechanical components that creates the machine.
Figure 2.11Basic winch anatomy
Figure 2.12Turntable machine anatomy
Figure 2.13Scissor lift anatomy
The machine takes input power from the amplifier and outputs physical motion. That motion is tracked by the final element in the Pentagon of Power, the Feedback Sensor.
The Feedback Sensor takes input from the physical movement of the machine, or from the scenery that it’s moving, and generates an electrical signal that is returned to the Control Circuit. This closes the loop between the original output of the control circuit and the eventual motion that it caused. If you’ve heard the term “closed-loop control,” this is what it means. There is an alternative to closed-loop control, which is predictably called “open-loop control.” In open-loop control, the control circuit never receives any indication of the result but rather just assumes that everything worked as expected. There are situations where open-loop control is fine (pneumatically operated snow bags spring to mind), but it is less common in scenic automation than closed-loop control. The Feedback Sensor is only required when implementing a closed-loop control system, but since that’s most of the time, it’s good to learn it as one of the pillars of the Pentagon of Power.
Feedback sensors come in all shapes and sizes and generate a similarly wide variety of output signals: Limit switches close an electrical contact when struck by indicating “I am here!”; encoders generate speed and direction signals for precise positioning and speed control; inclinometers output their angle relative to level, describing the incline of a platform that is being tipped from vertical to horizontal. There are sensors that can indicate just about any type of data you want to measure, but the control circuit must be able to make sense of the electrical signal format used to describe that data. When choosing a feedback sensor, you must match both the movement you are trying to measure and the output signal that will be fed back into the control circuit.
Using the Pentagon of Power as a Map
We have neatly summarized the five roles of the Pentagon of Power. These roles are filled in any and every automation system. If we look at a single axis of a system, we may not see precisely five pieces of gear. While you could have a system that maps each role to a physical thing, it’s equally likely that some roles are combined in a device. For instance, the control circuit and amplifier could be built into a single electrical box. Or a machine could be packaged with the amplifier on board. The operator interface may be spread across a pendant, a computer, and rack of knobs and switches. Regardless of how the physical components are packaged and distributed, each component can be classified into one of the five roles of the Pentagon of Power. Understanding where a specific component fits into the puzzle is critical when either designing a new system or troubleshooting an existing one.
Let’s take a look at how we can apply the Pentagon of Power to deconstruct a few different automation systems and thereby make this idea concrete.
Pushbutton Turntable
Figure 2.14Simple turntable with tire drive and manual control box
A classic tire-drive turntable machine powered by a DC motor and a pre-packaged speed control is a prototypical, simple, mechanized effect. Though this isn’t quite sophisticated enough to be considered automation, it’s such a common scenario that I think it is worth dissecting.
The operator interface is the on/off switch, forward/reverse direction switch, and speed knob. The controls are immediately recognizable and within a few moments you would know how to get the turntable spinning.
In this instance, the control circuit is combined with the operator interface. The switches that the operator flips with her finger are feeding signals directly into the amplifier.
Built into the speed controller, the amplifier is a DC motor drive that takes AC power from the wall, rectifies it to DC power, and then varies the voltage based on the resistance of the speed potentiometer knob.
The machine is a DC motor with worm-gear speed reducer and an inflatable rubber tire mounted directly onto the output shaft of the speed reducer.
The Feedback Sensor in this system is simply the operator’s eyes. The operator watches the turntable and perhaps lines up some tape marks on the revolve platform with the surrounding show floor.
PLC-Controlled Trap Door and Scenery Lift
Figure 2.15Multi-axis lectern lift
Source: Courtesy of Creative Conners, Inc.
In this example, two machines need to be coordinated, so I introduce some real automation, but still at a relatively simple level.
Here we have a pendant with three buttons: Up, Down, Emergency Stop. Pressing the Up button will open the trap door and raise the lift so that it is flush to the stage. Pressing the Down button will lower the lift and then close the trap door. Pressing the Emergency Stop button at any point will stop all motion. When the Emergency Stop button is reset, no movement restarts. Rather, you must deliberately press either the Up or Down button again to restart motion.
Figure 2.16PLC control for lectern lift
The Control Circuit is composed of a Programmable Logic Controller (PLC) that commands when to run the lift motor, when to run the trap door motor, and in which direction. Inside the electrical enclosure, there are two potentiometers that set the speed signal. These allow for occasional adjustment but aren’t meant to be altered often. The PLC connects to the Operator Interface through the hard-wired buttons on the control pendant.
Figure 2.17Mitsubishi D700 VFD
The amplifier role for each motor is performed by a VFD (variable frequency drive), and it follows the speed signal set by the potentiometers in the electrical enclosure.
There are two machines used in this effect. A hoist raises and lowers the lift platform. A winch pulls the trap door into and out of the trap opening.
Figure 2.18Limit switch
Since this effect only has two desired positions, up and down, the feedback can be simple. Limit switches are used to detect when the lift is up or down and when the trap door is open or closed.
Computer-Controlled Deck Tracks
Figure 2.19Musical production with multiple deck winches
Source: Courtesy of Creative Conners, Inc.
In this case, a musical production needs several cross-stage deck tracks that haul various pieces of scenery from the wings into view. Because of the number of cues and the need to rapidly adjust positioning, speed, and acceleration, a computer-based system is required to store, recall, and edit the automation cues.
Figure 2.20SpikemarkTM automation software
Source: Courtesy of Creative Conners, Inc.
The operator interface is a PC running commercial automation software, a console with dedicated buttons for editing and running cues, and a touchscreen display.
Figure 2.21Stagehand Pro AC motor controller
Source: Courtesy of Creative Conners, Inc.
The control circuit is a motion controller that is built into the same electrical enclosure as the amplifier. It connects to the operator interface over Ethernet.
The amplifier is a VFD, or variable frequency drive, built into the same electrical enclosure. The VFD receives an analog speed and direction signal from the control circuit and amplifies that signal to spin the motor.
Figure 2.22PushstickTM v2 zero-fleet deck winch
Source: Courtesy of Creative Conners, Inc.
The machine is a typical theatrical deck winch with an AC induction motor, gear speed reducer, and helical grooved drum to wind up wire rope.
Figure 2.23Motor-mounted encoder
An incremental encoder coupled to the rear of the motor shaft provides a speed and direction signal that is fed to the control circuit. Additionally, a limit switch provides simple switch closures for end-of-travel limits to detect if the winch has moved further in either direction than intended because of possible encoder failure.
The Pentagon of Power is a map for plotting a course from automation concept to physical manifestation. When devising a new system for moving a piece of scenery, this map can guide your planning so that you design, purchase, and build all that is required. When troubleshooting an existing system, this map helps to isolate a problem to the correct chunk, so you aren’t distracted by irrelevant components. Because you have a basic idea of how a car works, you wouldn’t replace the windshield wipers when a car stalls. Similarly, you shouldn’t look for a software bug if a motor failed to move because it was simply unplugged. You need a mental model to help direct your efforts when a motor stops spinning, and the Pentagon of Power is a convenient, simple model to learn.
Something that is conspicuously absent from the discussion thus far is safety systems. Automated scenery carries huge potential for injury and damage, and, as technicians, we absolutely must implement appropriate safety systems to limit such risk. However, we are going to delay that discussion, not because safety should be an afterthought, but for two other reasons. First, it’s such a large topic that it deserves in-depth coverage that’s hard to squeeze into this overview. Second, I think it’s hard to understand safeguards until you understand how automation works. Once we have a solid understanding of how an automation system works, we can then discuss the inherent dangers and how best to mitigate them.
We’ve covered a lot of ground in these few pages. Hopefully, the principles are starting to sink in. If some of the specific terms or examples seem fuzzy, press on. The five pillars of the Pentagon of Power will be used as road markers as we drive further into the details of automation.