Gate and Trigger Signaling in Automation

                                       Gate and Trigger Signaling Explained

                                                             Herbert Tonello

                                                      

Foreword:

One of the challenges in any machine design is the gate and triggering signal. Gating is simply a “gate” that the machine’s central control opens to allow a trigger signal to be recognized. The trigger is an electrical signal that signals to a machine’s control system that an event is to take place. That event may be any number of things from a robotic arm move, a vision device to do an inspection, or an electrical/pneumatic solenoid to open. This article will address design issues and practical applications with regards to implementation along with troubleshooting gate and trigger signaling design problems.

 

The Gate Signal:

The gate signal is so aptly named because it “opens a gate” for the machine to proceed through in a timely manner to conduct an operation. The signal can be a DC or AC voltage of varying magnitude, but generally a DC square wave is used due to well defined rising and falling edges. Machine programmable logic controllers (PLC) will often receive/send these signals to signal a “gate” to open so a trigger signal can be received. Once the time has elapsed in which the machine expects a trigger signal, the “gate” will close and no instances of the trigger signals will be recognized. The gate closing process is done to prevent unwanted operations due to electrical/mechanical noise, etc.

 

 

 

The gate signal is turned on by the rising edge of a pulse and tells the machine to accept trigger signals for one second in the above example and then close for another second and then repeat in an endless loop.

 

 

The Trigger Signal:

The trigger signal is so aptly named because it “triggers” an event to happen during the machine’s operations. In fact, the gate signal and trigger signal are very similar in their characteristics with the gate signal being an extended time signal in most cases.

Typical 24V signal used for Trigger Signaling

 

Trigger Signal Design Considerations:

The most critical design consideration is timing of the trigger signal. Timing is often the most misunderstood and trouble-prone area of machine design. The trigger signal must be able to be recognized during a PLC scan time while it must also be recognized by a host of other peripheral gear in an industrial machine or vision system. The trigger signal quality, shape, and pulse width are of paramount importance since it is possible that industrial machinery may need triggering for an event at speeds of tens to hundreds of times a second. Electrical noise in a system must also be considered since a noise spike can be recognized as a trigger signal pulse. False pulses are also a consideration and the designer must consider what happens when a false pulse is received by the industrial machine’s control systems.

Other considerations also include what sensor is being used to produce the trigger signal, and the footprint/location of the sensor.

Poor performance and false triggering of sensors are a common design problem in many start-up projects. Reflectivity of the target by laser sensors, capacitance variations, and positional errors all contribute to missing signals or create extra gate/ trigger signals.

Electronic circuit limitations that effect waveforms, capacitive and inductive time lags, etc. cause timing issues when high triggering speeds are needed.

Electronic timing charts are useful in choosing the right PLC input/output equipment needed for accurate trigger signal detection and gate/trigger signal generation. Using the technical specifications provided with the trigger signal activation sensors or the trigger signal generation devices ensures that the trigger signal is well recognized in a machine’s logic system. Determining how long a gate timing window is on so that a trigger signal may be recognized is primarily important.

 

Rising edge/leading edge definition and pulse width characteristics need to be defined. Pulse width and average voltage should be repeatable and easily identifiable. The gate/trigger pulse must be recognized within the scan time/response time of a PLC or peripheral devices or the signal may become lost.

 

Gate Signal Design Considerations:

Many of the factors that affect trigger signal conditions also affect gate signaling. Since timing of the gating pulses may be longer, signal quality during the time period the gate is open is important.

Also conditions that may shift the gate timing pulses must be considered like mechanical drift and hysteresis due to encoders being mechanically linked to gearing. Gate signaling is usually performed by PLC timing operations or by cam positioner/encoder circuitry and is generally much more reliable than gating trigger signals from a sensor.

 

Waveform time lag and edge definition on rising and leading edges of the gate signal can affect electronic signaling circuitry.

 

 

Trigger Signal Producing Principles:

Trigger signals are formed by hundreds of sensor types and electronic apparatuses, so this article will cover a few of the most common items found in use around the world.

 

• Sensors (Photoelectric, Capacitive, Hall Effect)

 

These sensors use DC or AC power to operate and send a trigger signal voltage to a control device. Typical DC sensors come in PNP/NPN configurations and are used to detect a moving part going past a point within the machine’s footprint.

 

https://www.bannerengineering.com/en-US/

 

 

• Cam Positioners

Typically used on larger industrial equipment, these devices use a turning gear/belt attached to a cam position sensor/encoder to output gate/trigger signals hundreds of times within a 360 degree move. These devices are very reliable and also used for gate timing windows.

 

 

Photo Courtesy of Omron Corp

 

• PLC

The PLC will send gate and triggering signals as well as accept them. A typical example of a PLC sending a trigger signal is when the PLC sends a signal to a vision system to acquire an image.

 

 

 

Photo Courtesy Schneider Electric

 

 

 

Gate and Trigger Signal Troubleshooting:

Once the gate and triggering circuitry has been designed it has to be tested in actual operation within a machine. Modern day HMI (Human Machine Interface) programming allows for easy identification and visualization of machine signaling. PLC manufacturers allow for data logging and graphical displays of signaling too, greatly assisting the engineer/technician to solve signal timing issues. Most problems lay in timing, when a trigger is sent and the gate is closed or when a trigger is sent and the PLC does not react quick enough to process the signal. The use of triggered oscilloscopes along with the aforementioned data collection techniques will further help in solving why a machine does not particularly work as designed during certain situations like ejection control and good-bad product decision making.

 

Actual Machine HMI screen showing gate signal timing (Courtesy Laetus Corporation)

 

The timing of the signals have to be in a sequence that the machine’s logic can understand. Troubleshooting and debugging includes observing when the trigger signals and the gate signals are turned on and off; and whether these signals are too long or short in duration. Electronic test equipment like signal tracers and oscilloscopes can aide in the troubleshooting task along with the aforementioned HMI. Once the signals are observed, they can easily be adjusted by manipulating software and hardware controls. Often these timing issues result in good/bad product verification issues and eject control systems failing to pass or eject good and bad products respectively.

Another issue common in initial and post design machine systems is false triggering due to electrical noise or misadjusted sensors and settings. Software settings can help reduce false triggers by using software de-bouncing of the trigger signal to where the trigger input signal is run through a “filter” so to speak. PLC and sensor manufacturers have also recognized this issue over the past several decades and have incorporated many features into their product offerings that help with trigger signal recognition and suppression of false triggering.

Actual hardware signal conditioning is also popular, where the trigger signals are electrically isolated via opto-coupling and then sent to signal forming equipment where the trigger signal is recognized, re-shaped, and sent to the machine’s control equipment. These signal formers also reject false triggers while sending perfectly shaped signals to the machine’s logic.

 

 

Conclusion:

Gate and trigger signaling happen at speeds measured in microseconds/milliseconds in most machine designs involved in the industrial sector. The correct consideration, design, and use of this signaling is just as important as any of the other major design stages during a machine build.

Important considerations have to be made by software and hardware designers to help the engineer, technician, and production operators understand and troubleshoot problems within the gate and trigger signaling system on any machine project.

 

 

   

Blog:

 

https://herberttonello.blogspot.com/2015/09/gate-and-trigger-signaling.html

 

 

End Article