Support-focused enterprise controls: command control
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. Understanding the number of circuits that designers use and the way they are configured can provide clarity to users.
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.
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