Support-focused enterprise controls: command control
Each group of logic circuits enables a command signal. Designers use the enabled command signals to turn on output signals. Enabled output signals activate external circuits that cause objects and mechanisms to move. A control circuit comprises one or more logic circuits that work together to enable, disable, and/or re-enable an output signal.
Regardless of the output signal, most groups of control circuits provide similar control characteristics. Recognizing and understanding the inherent and differing characteristics of these circuits is important. One key characteristic revolves around how applied control circuits start movement. The next obvious characteristic is how control circuits stop something from moving when an object or mechanism reaches its expected end or stop position.
A less evident characteristic centers on how control circuits prematurely stop something after it is moving. This characteristic is important when other stationary objects or mechanisms inadvertently change position while an object or mechanism is moving. Lastly, an obvious restart characteristic centers on the need to get something that prematurely stopped moving again.
In most cases, the fundamental design characteristics behind various control circuits remain hidden from support personnel, unregulated by manufacturers, and poorly documented by control system designers. Some manufacturers use example templates to govern control circuit fundamentals. This is the only way most manufacturers can react to avoid having designs go off in many different directions.
Without knowing the rules behind designs, control circuits will always appear chaotic to support personnel. This is especially true when manufacturers task them with supporting various applications from different machine suppliers. So what are the obvious differences in control circuits? In most cases, it is the number of circuits designers use and the various ways they configure, sequentially order, and combine them.
The term "combine," means how designers unknowingly use rung substitution to create visually different circuits. An obvious visual difference revolves around the many ways a designer can document circuits. Some less evident but extremely important differences center on the way output signals enable and turn over control to electric, hydraulic, and/or pneumatic circuits. As a result, the real-world integration of an output signal forces designers to understand how the premature disabling of an output signal affects an external circuit. The following terms explain various types of external circuits:
- A stoppable circuit needs a controller’s output signal to stay enabled in order to keep an object or mechanism moving.
- A nonstoppable circuit does not need an output signal to stay enabled in order to keep the object or mechanism moving.
- A reversible circuit reverses the intended direction an object moves based on the disabled state of the output signal that enabled it.
Designers usually evaluate the physical mass of an object or mechanism and the distance it must travel before applying a set of control circuits. Designers typically apply stoppable circuits to control long movements of high-mass objects or mechanisms. Designers often apply nonstoppable circuits to enable short movements of low-mass objects or mechanisms.
The same is true for reversible circuits. In most cases, the external circuit selected revolves around the momentum force of the moving object or mechanism. Objects or mechanisms that have a high momentum force have an increased potential to cause damage to equipment. To avoid potentially damaging equipment, designers apply stoppable circuits to disable movement when a control circuit detects another object or mechanism out of its expected position.
There are many forms of external circuits. A stoppable electric circuit usually has a relay integrated with a motor starter and brake release solenoid. Mechanical movement starts when an output signal energizes the relay. The relay simultaneously directs the flow of electric current to the brake release solenoid and to the motor. Mechanical movement stops when the output signal de-energizes the relay to cut off electrical current to both the motor and the brake solenoid. A hydraulic or pneumatic nonstoppable circuit uses a detent solenoid valve integrated with cylinder or motor. Movement starts when an output signal electrically energizes the valve’s solenoid. The energized solenoid magnetically shifts a detent valve spool, thus enabling the continuous flow of air or liquid to the cylinder or motor. After the spool shifts, fluid continues to flow, even if a control circuit disables the output signal.
Aside from the external circuit differences, control circuits have six primary control roles. These roles involve starting something moving, changing the speed, keeping it moving, prematurely stopping it while it is moving, restarting it moving if it inadvertently stops, and disabling the circuit when it reaches its expected final position. To fulfill these roles reliably, control circuits must contend with the known transition effects of steady and variable-state signals.
The roles of various circuits are important in order to control the movement of an object or mechanism. To start something moving means having a control circuit that initially examines the largest number of appropriate steady and variable-state signals. To keep it moving means having a cutoff control circuit that continually examines a large number of steady-state signals. In some cases, keeping a mechanism or object moving means having a control circuit that dynamically examines variable-state signals. Prematurely stopping movement means having a circuit that interrupts an external stoppable circuit.
Restarting movement after it stops part way means allowing it to resume automatically or in a manual mode. Stopping movement normally means having a control circuit that examines a small number of high-priority, variable-state signals. To understand control roles, designers must use a generic and systematic approach to developing unique role-assigned control circuits. The following terms describe the roles of various control circuits:
- A position-ready circuit examines the position of an object or mechanism before it moves.
- A sequence-control circuit permits automatic motion based on all sequenced mechanisms moving in sequence.
- A start-position circuit examines many steady and variable-state signals needed to start an object or mechanism moving.
- A trigger-ready circuit examines downstream trigger circuit signals to ensure they will work properly before releasing an object.
- A start-state circuit sums up the start-position signal with other steady and variable-state signals.
- An end-position circuit examines a small number of high-priority, variable-state signals needed to stop a moving object or mechanism.
- A clear circuit examines the steady-state signals that must not change state to avoid prematurely stopping a moving object or mechanism.
- An auto-enable circuit allows a command circuit to re-enable an output after a clear or operating mode signal prematurely stops the movement of an object or mechanism.
- A command circuit integrates operational mode signals with start-position, end-position, safety, and auto-enable circuits before activating or deactivating an output circuit.
- An output circuit integrates a command circuit signal and an opposing motion signal with an output signal that enables an external circuit.
After reviewing the possible number of control circuits, many control systems designers conclude that their design styles are more efficient. Most with this view believe that a style that has fewer circuits is superior, especially if application tasks are repeatable. Fewer circuits are always more efficient, but how do these efficiencies improve the abilities of support personnel to understand and interact with applications? Does the repeatability of one application trump the ability of support personnel to understand all designs? The answer to both questions is no.
Anyone who writes an application in any programming environment knows that code efficiency generates less structured code. Less structure automatically means the application is harder to understand. A harder to understand application creates confusion and makes it more difficult to support.
Although the systematic design strategy in this article proposes 10 types of circuits to control movement, designers often use direct, inverse, and seal circuit substitution techniques to reduce their numbers. This is one way to achieve design efficiency. This sometimes means they can reduce the number of control circuits to one or two large circuits. These circuits typically blur the functional control roles because there are no clear demarcation lines between circuit contacts and branches. On the other hand, if designers use many circuits, each circuit has a single purpose.
Single-purposed circuits enable support personnel to demarcate their individual roles while allowing them to gain confidence in their ability to recognize, understand, and modify them. Manufacturers can expect to gain many manufacturing advantages when they promote this type of design strategy because it greatly enhances the ability of support personnel to understand and interact with control applications.
The need to improve the supportability of designs is paramount to the safety of personnel. Will manufacturers continue to train support personnel on many differently styled applications? The fundamentals behind application styles have more to do with look and feel issues or how individual designers use substitution to combine or compress control circuits.
Some control system suppliers have been using standard circuits for so long they are not able to reverse engineer their compressed designs. Instead, they tout the benefits of faster executing code, smaller controller memory sizes, and single-purpose training requirements. They never state how their efficient design strategy will improve the ability of support personnel to interact safely with all control applications. The term "safe," means personnel who interact with the design will recognize predictable machine behaviors.
Developing position-ready circuits
Developing a position-ready circuit is the first step toward systematically designing a set of control circuits. This circuit identifies the position of an object or mechanism before it moves. For example, if a mechanism extends and retracts, there are two separate position-ready circuits. One circuit identifies the retracted position before it moves, while the other identifies the extended position after it moves. Each circuit verifies the pre-move position sensors are on, and the post move sensors are off. Many designs use generated circuit signals to enable annunciation logic to turn on lights or indicators on intelligent panels. The lights or indicators enable someone to recognize the position of objects or mechanisms.
When designers use these circuits for annunciation, they force control applications to reproduce all positional state conditions. There is a significant benefit to control applications when they substitute each set of examinable input conditions with a generated, position-ready circuit signal. In most cases, direct substitution enables designers to replace the two input conditions with one ready signal. The substitution strategy reduces the number of examinable elements per circuit, improves further substitution abilities, minimizes controller memory utilization, and physically shortens the length of circuits. Shortening the length of circuits improves the ability of support personnel to see displayed control logic without having to zoom out or scroll screens. As a result, support personnel are quickly able to see the entirety of a circuit and focus on problem elements.
Figure 1 shows the position-ready circuits needed to control an object moving between two stations and a mechanism between two positions. The number of circuits per command varies between conveyor and machine designs. Machine designs typically have one position-ready circuit that represents the position of a mechanism before it moves. Conveyor designs use two circuits to represent the status of sensors at two different stations. The rules for determining the examinable conditions needed for position-ready circuits differ slightly between each type of application. For conveyor applications, designers can systematically extract all design information from application models using the property field data assigned to sensor activation bars. For machine applications, designers can extract design information from application models using motion bar assigned properties.
Interlocks can be both steady- and variable-state signals depending on the perspective of the application. For a conveyor application, a station clear interlock is always a steady-state signal set by a machine application. For a machine application, a stopped, in-position interlock is an examinable, steady-state signal set by a conveyor application. On the other hand, a machine application sets a cycle-complete interlock signal. A conveyor application treats this interlock as a variable-state signal, whereas a machine application treats it as a steady-state signal.
Figure 2 shows the conveyor and machine clear circuits designers produce after they extract bar property information from the application models. Sequence-control circuits ensure various sets of control circuits force physical movements to occur in a fixed sequence. For some manufacturers, it is not enough to be in a valid state to allow a mechanism to move. When manufactures impose rigid sequencing rules, they force control system designers to develop sequence-control circuits for machine applications.
These circuits enable mechanism motions and template activities to occur only if the machine reaches the start-position state following a normal sequence. These designs have a sequence-control, pointer circuit for each movement. Each circuit has both pointer latch and unlatch instructions. Once the machine is in the correct state, the circuit simultaneously disables the current state’s pointer signal and enables the next state’s pointer signal. For these designs, the machine will not operate when it skips a state. For simplification purposes, the integration of sequence-control signals into command circuits is not part of this discussion.
The next step in the design process involves creating an object or mechanism clear circuit. This circuit sums together all the steady-state signals that must not change state while an object or mechanism is moving. Most designs start by programming a clear circuit to execute before the start-position and command circuits. All the signal preconditions on this circuit are a subset of those conditions examined by the start-position circuits. The conventional process is to put this circuit first and use the resulting enabled clear signal as examinable condition in the start-position circuit.
For conveyor designs, clear circuits must examine all interlocks to ensure machine controlled mechanisms are in a position to allow the unobstructed passage of an object. The circuit must remain enabled while the object is moving into or out of a station. When entering a station, the circuit checks the deactivated state of the exit sensor to ensure the previous object has successfully cleared the station. If a clear circuit uses a normally closed exiting sensor contact, designs must include a sensor power on input signal. This power on signal guarantees the sensor is active, and the deactivated state of the sensor input is valid. For a machine design, the clear circuit must examine an object in-position interlock and the selected sensor states of other mechanisms. These checks ensure the object and station mechanisms remain stationary while the mechanism is changing position.
Start-position, triggers-ready, and start-state circuits have unique purposes. A start-position circuit examines all object and mechanism sensor signals needed to start an object or mechanism moving. A triggers-ready circuit verifies downstream trigger circuits are set to fire. A start-state circuit combines the start-position, triggers-ready, with other steady or variable-state signals. For conveyor applications, the start-position circuit examines object position sensors for two stations. Object position signals include the deactivated state of the next downstream station sensors and the actuator encoded in-station state of all current station sensors. The triggers-ready circuit verifies the state of all trigger circuits. The associated start-state circuit combines the start-position and triggers-ready signals with other signals that must be true before an object starts to move. For machine applications, the start-position circuit examines mechanism position signals and critical template-based process interlocks. The associated start-state circuit combines the start-position circuit signal with the valid state of cycle complete signals.
Figure 3 shows how modeled application information translates to examinable start-position and start-state circuit conditions. The machine design start-position circuit represents the state of mechanism sensors and process interlocks at the beginning of the "extend locator" motion bar. For the conveyor design, the triggers-ready circuit examines the armed ability of downstream station triggers to fire. This feature reliably enables a trigger-first design strategy.
Many manufacturers are unaware that their machine suppliers are providing control applications that do not check the state of arming signals before they move objects. As a result, many are suffering adverse operational consequences for triggers failing to fire. Some programmers argue for using a single signal that indicates that all arming signals were set is sufficient for allowing the next object to enter a station. This is analogous to suggesting that since they were armed, they still are.
So how do control system designers resolve the inadvertent firing of triggers? Why do many designs allow stations to accept new objects if station triggers are not going to fire when they arrive? These are examples of the kind of events that setup chaotic conditions that lead to huge production losses. For many manufacturers, the answers to these questions are "who knows!" To avoid this type of situation, manufacturers must specify a standard methodology for re-arming triggers and subsequently enabling the movement of the next object into a station.
An end-position circuit enables a signal when an object or mechanism stops moving after it reaches its destination position. For conveyor and machine designs, this means examining a minimal number of stop position sensors for the downstream station.
Figure 4 shows the end-position circuits each type of design uses, but not all end-position circuit conditions are as simple as the circuits depicted in this example. If designs need other stop conditions, listed property information for individual bars can provide additional criteria. Designers integrate clear, start-state, and end-position circuit signals to produce auto-enable circuits. This circuit enables the automatic movement of an object or mechanism from its start state. However, when an interruption occurs to stop movement prematurely, the circuit either re-enables or disables further movement. In most cases, the stoppable or nonstoppable nature of the external circuit determines how designers configure auto-enable circuits. If an output signal enables a nonstoppable circuit, the design uses a latch circuit to ensure the automatic re-enabling of the output. If an output signal enables a stoppable circuit, the design uses a seal circuit signal that shuts off the output to prevent automatically re-enabling movement.
Figure 5 shows an auto-enable seal circuit enabling a signal when the programmed conditions are true. The seal is broken when any of the stop, clear, or mode conditions change state to disable the auto-enable signal. For the machine design, the latched instruction guarantees the auto-enable signal is enabled and stays enabled until the locator reaches its extended position. When the locator is extended, the unlatch circuit disables the auto-enable signal.
Most designers clearly understand the role of auto-enable signals when they see them integrated with command circuits. Command circuits enable output circuits using start-position, end-position, clear, mode, and auto-enable circuits. Designers then use the resulting command signal to enable an output circuit.
Figure 6 shows two example command circuits. Designers configure these circuits using simple rules. The first rule requires output branches to include normally closed contacts for opposing movements. The next rule requires the circuits to have a unique set of mode-specific input branches, or one for each mutually exclusive operating mode. Each branch includes a normally opened mode contact. Bar-specific properties define the applicable operating modes.
Automatic and semi-automatic mode branches must have adjacent auto-enable and clear signal contacts. Manual and unrestricted mode branches must have neighboring pushbutton contacts. Aside from having a pushbutton contact, a manual mode branch must also have a clear circuit signal contact. Per the definition, an unrestricted mode branch does not have a clear or auto-enable signal contact.
Lastly, designers use command circuit signals to produce output circuits. These output circuits produce signals that enable external circuits. The major configuration rules behind these circuits come mostly through the stoppable or nonstoppable nature of device-connected circuits. Minor but equally important circuit configuration rules come through the application of designer-specified bar properties.
Daniel B. Cardinal works as an engineering consultant for Insyte Inc., implementing integrated scheduling and part identification applications in the automotive industry. Edited by Chris Vavra, production editor, Control Engineering, firstname.lastname@example.org.
Designers use command circuit signals to produce output circuits, which produce signals that enable external circuits.
Assigning control circuits requires a generic and systematic approach.
The major configuration rules behind these circuits come mostly through the stoppable or nonstoppable nature of device-connected circuits.
What applications would benefit from command control circuits the most?
See prior stories in this series by Daniel Cardinal linked below.