Support-focused enterprise controls: sensor actuation charts
Many machine designs come with a sequence of operation drawing for each station. Mechanical designers use the drawings as a way to convey design information to control system designers. Other design-related drawings show the dimensional offsets of station sensors with respect to station centerlines. Most designs are missing an understanding of how control system designers use object actuators and sensor position information to create application triggers. Mechanical designers use the sequence diagrams to show the time in seconds allotted for objects to move between or into process stations. Sequence of operations drawings typically do not show an object’s variable travel speed as it activates acceleration, deceleration, or any other in transit sensors.
To document the speed transition points, designers remove the aspect of time and adopt sensor activation charts. These charts purposely convey an object’s station-specific movement information to control system designers. The bars found on sensor activation charts represent the length of actuators or the distance objects travel while not activating a sensor. The length of each bar does not represent the timed speed of a moving object. The combined bars also describe the sequenced activation and deactivation of applied sensors. The following definitions describe two types of bars found on a sensor activation chart:
- Sensor-activation bar is a drawing object that appears on a sensor actuation chart, representing the physical activation of a sensor.
- Sensor-deactivation bar is a drawing object that appears on a sensor actuation chart, signifying that no station sensors are active when an object’s actuator is between sensors.
Figure 2 shows how mechanical designers can construct sensor actuation charts using sensor-activation bars. Notice how each of the three independently controlled stop-stations use sensors to detect a carrier’s long sensor actuator. These actuators force control applications to create large gaps between moving objects. The same drawing shows three sensors assigned to each independently controlled station. The diagram shows a slow, stop, and exit position sensor for each station. The slow and stop position sensors detect both ends of a carrier’s actuator to signify an object is in position at a station.
The station’s robotic operation starts after the carrier stops and both sensors detect the end of the actuator. The slow position sensor signals the control application to decelerate the carrier moving towards the stop position. The second stop position sensor at each station detects the leading edge of a carrier’s actuator, and it signals the control application to stop the moving object. Each Exit Position sensor detects an object depart a station and enter the next. An object can only start to move into an empty station when the exiting carrier ahead is no longer activating that station’s exit position sensor.
The sequence of operations for the three stations shown in Figure 1 are the same. Each station’s sequence starts with a fast-moving object entering the station and activating a slow position sensor. The object immediately decelerates and continues moving until it activates a stop position sensor. Once stopped, a mechanical locator (not shown) extends, and two clamps (not shown) sequentially close to secure the stopped object. After the stopped object is securely located and clamped, station robots begin their operations. When the robots complete their work, the clamps open and the locator retracts. The new retracted state enables an object to move towards the downstream station. The conveyor moves the object forward when the next station is clear.
This means no object is present in the station, and the last object successfully exited. Similarly, the next object cannot enter this station until the current completed object exits. When the object exits, a control application enables the motors for each independently operated conveyor. If the control application is not able to confirm the exit, each motor starts within a predetermined amount of time and disables forward movement. After confirming movement, the object continues to activate and deactivate the station’s exit position sensor. Only after an exiting object deactivates can an exit position sensor become an upstream object that is allowed to enter the now emptied station.
Trigger-firing position information is not readily available to most system integrators. So, how do integrators know where control system applications will produce triggers for controller-based applications? Many integrators do not know the trigger positions forcing add redundant sensors or trigger circuits. To improve control system designs while simultaneously formalizing the trigger firing positions, manufacturers must insist that machine and conveyor suppliers provide a method for documenting trigger positions. The generation of a sensor activation chart is the first step in this process.
Figure 2 shows an example of how designers produce a sensor activation chart. This specific example is for the second station pictured in Figure 1. Each bar in the sensor activation chart represents the activated state of a station sensor. Since mechanical designers know the dimensional relationships between sensors, they are able to assign descriptive property fields to each bar object. The sensor’s name, actuator length, and leading edge offset dimension to an object’s stop position are examples of bar-specific property fields.
Figure 3 shows the sensor activation charts for two different conveyor examples and the timing for carriers that have small sensor actuators. Each chart uses arrows to show the location of possible triggers. The gap-dependent chart at the top shows the location of triggers for any of the stations pictured in Figure 1. Notice that when mechanical designs use these actuators, the sensor transitioning off has no positional value. The delta between activation and deactivation trigger positions represents a small travel distance.
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.
- Most designs lack understanding of how control system designers use object actuators and sensor position information to create application triggers.
- To document the speed transition points, designers remove the aspect of time and adopt sensor activation charts, which convey an object’s station-specific movement information to control system designers.
- To improve control system designs, manufacturers must insist that machine and conveyor suppliers provide a method for documenting trigger positions.
What other applications can sensor actuation charts be used for?
See prior stories in this series by Daniel Cardinal linked below.