Support-focused enterprise controls: Control system triggers
Upper-level system applications rely on movement detection circuits to produce dependable triggers when objects enter, stop in position, and exit process stations. This is part 3 in a series on standardizing development of programmable logic controller (PLC) programming for controlling discrete manufacturing processes. See 5 ways to arm a sensing trigger. Link to part 1 and 2, below.
Manufacturers cannot expect to be competitive and force control and information system designers to develop applications around the inferior mechanics of machines. Strategists must recognize that mechanical actuator and sensor placements can have a negative effect on all applications. Control system designers must recognize inferior mechanics and work to improve machine designs.
Over time, mechanical designers become aware of the sensor actuation needs of control system applications. Since most companies produce common machines, machine designers routinely place sensors in needed locations. Similarly, some control system designers habitually develop control circuits that use these sensors, whereas others simply cut and paste circuits from the last application to make the next. In most cases, control system designers thoroughly understand the sensor needs of their circuits, but they are unaware of the global impact these sensors have on controller-based trigger circuits and system applications.
Movement detection is merely a label assigned to the electromechanical triggering characteristics of all automated process machines. It is also the hidden fundamental behind reliable and robust trigger circuit designs. Upper-level system applications rely on movement detection circuits to produce dependable triggers when objects enter, stop in position, and exit process stations. Most control applications always produce two of the three possible station triggers.
For state-based control systems, mechanical machine designers place sensors in positions needed to verify the physical positions of mechanisms. To detect objects moving from position to position, designers strategically place sensors to enable them to detect objects entering, stopping in position, and exiting machine stations. It will become evident that most control applications detect objects in only two of these three common positions.
Movement detection triggers
Most control applications include circuits that produce two movement detection triggers. Each trigger activation relies on two mutually exclusive object positional events. Most of these events come directly from sensor inputs, or from circuits that combine multiple sensor inputs into a unique programmed event. Either way, the two mutually exclusive events identify moving objects at two positions. Mechanical movements guarantee sensor inputs sequentially activate and deactivate for each object passing through a station. This sequential two-event detection characteristic is fundamental to the activation of all movement detection triggers.
Early relay controlled machines had control system design traits that emulated the two-step movement detection characteristic. These machines needed an operator to load parts before they initiated an automatic cycle. After each cycle, these machines needed the operator to remove parts and manually transport them to the next machine.
Control system designers quickly realized how these two critical events needed to activate sequentially to guarantee the continuous operation of these machines. Designers developed a special circuit that focused on the activation and deactivation of the cycle start pushbutton and part in-position sensor(s). The developed circuit improved reliability, and it prevented the operator from using rope or tape to tie down and permanently press a cycle start button. This circuit feature guaranteed the first step of the two-step input activation and deactivation sequence.
To make sure machines did not repeat or double cycle on the same part, designers created a similar circuit that guaranteed the operator removed the finished part before allowing the next automated cycle to start. This circuit also made sure the operator did not physically tie down the arm of a part in-position limit switch. This anti-tie-down feature guaranteed the second step in the two-step activation and deactivation sequence. Although designers did not call the sensor and button toggling process movement detection, this inherent sequential dual-step characteristic was present.
Movement detection triggers can directly activate system applications, or be the basis for arming an unlimited number of ancillary triggers. It will become clear that these triggers are critical to reliably triggering both control and upper-level system applications. However, as designers can effectively influence mechanical characteristics of trigger designs, they must understand how control system applications produce movement detection triggers. This means understanding the effects of sensor placements, actuator lengths, and circuit programming methods.
The physical placement of sensors decides how control system circuits arm and fire movement detection triggers. Arming is the logical act of having a circuit change the state of a signal and keeping it in that state until a triggering event occurs to cause the circuit to change the signal's state. It is analogous to pulling back the hammer on a gun and then keeping it retracted until someone pulls the gun's trigger. Both events are mutually exclusive and they occur sequentially. Simply stated, arming and firing is a two-step circuit characteristic that generates movement detection triggers at each step.
Most control applications are rigid in their ability to prevent automated machines from repeat cycling on parts. This built-in rigidity prevents machines from cycling multiple times to change the part. To most machine processes, this can be disastrous. To ensure this does not happen, control applications only allow repeat cycles to occur through deliberate manual initiation. To guarantee a machine does not automatically repeat a cycle, control system designers use movement detection circuits to sense station objects and generate triggers at two unique station positions. "Unique" refers to an object's sensed position that occurs only once when a part enters, stops, or exits a station.
Therefore, the only way a repeat cycle can occur automatically is through the rare and unlikely chance someone forces an object to move forward or backward to another position, and then back to the original position. As a result, sensing objects entering, stopping, or exiting a station is critical to arming and firing movement detection triggers.
The following names are applicable to each sensed trigger position.
Entering trigger: A signal that activates when one or more sensors detect an object approaching a process station.
In-position trigger: A signal that activates when one or more sensors detect an object arriving at, or stopping in position, at a process station.
Exiting trigger: A signal that activates when one or more sensors detect an object leaving a process station.
All movement detection circuits are capable of producing two triggers. Typically, one occurs when a circuit sets an arm signal on, whereas the other occurs when the circuit resets the arm signal off. For explanation purposes, an arm signal goes on when a trigger fires and then goes off after another trigger fires. This allows applications to use an enabled arm signal to identify objects between trigger positions.
Arming a sensing trigger 5 ways
Movement detection circuits are able to arm a trigger in one of the following five ways:
1. Arm when entering — A trigger circuit design that uses a momentary entering event signal to arm an in-position or next-to-exit trigger.
2. Arm when in position — A trigger circuit design that uses a momentary in-position event signal to arm a next-to-enter or a next-to-exit trigger.
3. Arm when exiting — A trigger circuit design that uses a momentary exiting event signal to arm the next-to-enter or next in-position trigger.
4. Arm post-exiting — A trigger circuit design that uses a momentary downstream event signal to arm a station's next-to-exit trigger.
5. Arm when firing — A secondary trigger circuit designs that uses the firing of a primary trigger to arm another ancillary trigger.
Some control system designs do not adhere to the generic movement detection arming methods described above. Instead, they use hybrid trigger designs. These special solutions sometimes use multiple sensors for each station position, while others use a common set of sensors for object transfer mechanisms that move parts for many stations.
Hybrid solutions often use sequencing pointer applications, while others use encoders to generate changing values that represent movement. Excluding cost savings, the trigger circuit design that a control application uses depends mostly on the functional and failure characteristics of a machine. Rarely do designers consider the negative effects failed triggers have on upper-level system applications. This is why designers must understand and recognize the various forms of control system triggers.
Trigger root forms defined
The following definitions provide a basis for understanding several root forms of triggers:
Simple two-step trigger: A two-trigger circuit design that relies on the physical placement of two sensors and short mechanical actuators.
Synchronous transfer trigger: A multi-trigger circuit design that supports a multi-station synchronous transfer machine.
Release-based trigger: A two-trigger circuit design that combines an in-position sensor off transition with release command output.
Gap dependent trigger: A two-trigger circuit design that needs a control application to hold back and gap objects to overcome the effects of using sensors at two unique stations.
Conveyor-timed trigger: A two-trigger circuit design that uses at least one object-actuated sensor and the timed movement of a conveyor to indicate an object moved to another position.
Robot-interlocked trigger: A two-trigger circuit design that uses at least one object-actuated sensor and one or more robot-activated interlocks to indicate the robot has picked up, dropped off, or transferred an object.
Read-event trigger: A two-trigger circuit design that uses one object-actuated sensor and a reader that is capable of reliably reporting good reads and no-read faults.
Encoder-based trigger: A multi-trigger circuit design that uses a sensor array, position processor, and object-attached encoder-based actuators.
The conveyor-timed trigger design is the most unreliable form of movement detection. Manufacturers accept these designs for production processes that can tolerate object collisions. These designs make the assumption that the object is moving if a conveyor's drive motor is running and a sensor's input signal changes state.
A typical design has a circuit that counts the number of seconds an object is supposed to be moving after it activates or deactivates the sensor. The change in state of the input signal enables the first of two triggers. A typical design also examines a controller's output signal that enables the conveyor's drive motor to run, or an input signal that indicates the motor is running. The assumed distance an object travels depends on the speed of the conveyor and the number of seconds counted while the motor is running. When the counter reaches a preset value, the circuit enables the second of two triggers before the next object repeats the cycle.
The robot-interlocked trigger design is one of the most reliable forms of movement detection. This method is prevalent in process areas where robots control the movement of parts. This method ensures that expensive robots do not collide, and associated controller applications accurately track the movement of parts. A machine controller is usually part of the design for connecting sensors, interlocking robotic processes, and tracking parts by type or by their specific identifiers. In most cases, a robot drops off a part at a hold table and another robot picks it up. To detect movement, hold tables and robot part-grasping tools are equipped with part sensors wired to the machine controller. When the robot drops off a part, the machine controller detects the absence of the part in the robot's grasping tool and the newly sensed part on the table. Robust designs use interlock signals that signify the robot has just released the part. The machine controller responds by activating an acknowledge interlock signal to indicate the part and associated data has successfully moved.
Most experienced designers rarely use the read-event trigger method because such designs force processes to rely on a specific type of reader device that identifies object-attached labels or tags. From a control perspective, the identifying labels or tags act as sophisticated actuators that have unique, device-compatible, data-encoded identifiers.
These designs tie the control system's ability to reliably sense the movements of objects to a device's ability to consistently detect the presence of labels or tags. In other words, each read-device acts like a momentary sensor after it detects and reports all data-encoded actuators passing through a station. Read-devices do not necessarily have to identify all actuators; they just have to reliably report all no-read and good read events. A reported read-event becomes one of two triggers, while designs incorporate a sensor or some other design scheme to generate the second trigger. These designs make it difficult for a manufacturer to declare this approach a standard, unless designers are willing to deploy a reader at all stations or release control points.
The encoder-based trigger method is an extremely reliable and precise form of movement detection. These designs rely on a sensor array that detects an encoder actuator attached to an object. The sensors feed discrete signal information to a special position processor. The processor decodes the discrete signals and provides a machine controller with the object's position information as it moves into or out of a station. The information can be numeric or designers can discretely interlock position processors with the machine controller. The machine controller's control application uses the position information to self-generate application trigger signals.
Many processors are integrated with drive controllers that directly control the movement and speed of objects. These drive-integrated designs can smoothly accelerate or decelerate an object as it moves from station to station. Some advanced designs have encoder-based actuators that have actuator-specific identifiers. When attached to an object, the position processor can supply a machine controller with both the position information and the unique actuator-specific identifier. As a result, these advanced encoder designs can be used as readers.
Many trigger circuit designs are prone to not fire when unexpected object movements occur. In most cases, unforeseen movements occur because someone manually repositions an object to provide clearance for accessing station equipment for maintenance. Other times, unexpected movements occur when power interruptions cause objects to coast off or pass by a powered-off sensor.
The following terms describe the types of object movements designers can expect:
Controlled movement: The expected and commanded motion of an object or mechanism.
Uncontrolled movement: The unexpected and non-commanded motion of an object or mechanism.
Each movement detection trigger is uniquely prone to inadvertently fire or not fire when uncontrolled movements occur. Those trigger designs that fire when sensors deactivate often misfire when a power failure occurs. This causes designers to condition trigger circuits with output signals that enable movement. This design feature ensures power failures do not deactivate sensors to fire triggers.
The enabled output signal ensures only the controlled movement causes the expected deactivation of the sensor. Those trigger circuits that use positive sensor activations to fire triggers are less likely to produce extra triggers. These unwanted triggers usually fire when someone moves a part forward then backward, causing the sequential activation and reactivation of both sensors. Designers guarantee that circuits do not produce extra triggers when they ensure triggers will not arm or fire when someone places the station in a maintenance or manual mode.
The previous definitions describe many forms of movement detection triggers. There are also many hybrid and special forms of movement detection that are derivatives of the ones described. The conveyor-timed, robot interlocked, encoder-based and read-event are special forms that are not critical to understanding the common forms.
Commonly used sensor-based methods typically employ simple two-step, synchronous transfer, release-based, and gap-dependent trigger designs.
Simple two-step triggers
Simple two-step trigger designs guarantee circuits arm and fire triggers using two sensors and one short mechanical actuator attached to each moving object. The spacing between sensors ensures actuators sequentially toggle sensors when objects pass through process stations. Short mechanical actuators and sensor spacing arrangements prevent trailing objects from activating sensors until lead objects exit and complete the sequential toggling of both sensors.
Short actuators make it easy for a control application to detect the exact position of an object stopped in position at a process station. However, this feature usually accompanies machines that need mechanical assistance to stop and position objects accurately. Once an object has stopped, a short actuator, combined with an activated sensor, provides constant and accurate object position information. Regardless, the continuous sensor activation is just a momentary detection step when an object moves through the process station. When an object exits, the sensor deactivates before a downstream sensor activates to detect the part exiting the station. The deactivation of this second sensor completes the two-step signal toggling sequence.
Figure 1 shows a trigger timing diagram for a simple two-step trigger circuit. The diagram shows the sensor activation and deactivation sequence for arming and firing two triggers. The momentary activation of a sensor fires the first trigger that arms the second trigger. Similarly, the brief activation of the next sensor fires the second trigger and rearms the first. This dual arm-and-fire method guarantees an application trigger cannot repeat fire unless both sensors sequentially toggle. The repeated activation and deactivation of any one sensor without toggling the other has no effect on the design.
Figure 2 shows a simple two-step trigger circuit that generates an object in-position and an exiting trigger. The optional maintenance mode condition shown on each rung allows operations personnel to generate controlled or uncontrolled movements without causing extra triggers.
If mechanical designers size sensor actuators to compensate for objects coasting to a stop when a power failure occurs, trigger circuits will not stop working after power is restored. As a result, simple two-step circuits can only fail to arm or fire when sensors electrically fail or when actuators mechanically fail to activate sensors. Extra triggers are possible when an object unexpectedly moves forward, backward, and again to repeat the normal activation sequence. Machine designs that use simple two-step trigger circuits create opportunities for circuits to use trigger actuators globally in other process areas. This is because these triggers are highly compatible with control and information system applications. This includes those applications supporting process areas where objects travel pushed or butted together. These easy-to-adopt designs become the impetus for globally using standard two-step trigger circuits while enabling designers to avoid hybrid movement detection and presence detection circuits.