Why use wireless pushbuttons in a factory setting?
The first thing that comes to mind when exploring remote pushbuttons is the garage door opener. There are many non-safety critical applications that could benefit from this type of control. Here are a few benefits:
- The operator is free to move about the equipment while retaining a level of control.
- The equipment wiring is simplified as there are no wires or conduit.
- The wireless control may be semi-permanently affixed to the equipment with the ability to move the switch assembly without rewiring the machine.
- A wireless switch block may be added to an existing Harmony pushbutton thereby passing the control signals from one machine to another. Stated another way, this solution eliminates the need for wiring a dry relay contact between machines.
In this engineering brief we will explore the Schneider Electric Harmony pushbutton system as shown in Figure 1. This picture features the ZBRRA receiver and the ZBRM22B0 wireless handy box. The receiver is mounted on a small relay / PLC trainer and powered by a B&K Precision 1550 power supply.
Tech Tip: The term “safety critical” is an important consideration for remote control applications. As per the Schneider documentation, the Harmony remote pushbutton system shall not be used for hoisting or emergency stop. These critical lifesaving controls must be hard wired to the machine following appropriate local, state, and federal guidelines.
Figure 1: Test setup for the Schneider Electric Harmony ZBRRA receiver and the ZBRM22B0 dual remote wireless pushbutton.
How does the Harmony wireless pushbutton work?
When the Schneider material arrived in the DigiKey lab, it caught the attention of technicians and engineers. The “wireless” and “no batteries required” specifications were intriguing. And so, we did the right thing and opened the remote pushbutton enclosure as shown in Figure 2.
Figure 2: Picture of the inside of the ZBRM22B0 pushbutton assembly. One of the ZBRT2 modules has been unclipped and placed into the foreground.
The code of the pushbutton assembly is the Harmony ZBRT2 module as shown in Figure 2. Here, one module has been removed and placed into the foreground of the image. Note that these modules have the same footprint and are compatible with the other Harmony switches. Figure 3 shows the wireless module installed on a Harmony XB5.
Figure 3: Picture of the ZBRT2 wireless module installed on a Harmony XB5 switch assembly.
The ZBRT2 features a Zigbee transmitter
The Schneider wireless system is based on a IEEE 802.15.4 Zigbee protocol. The ZBRT2 modules feature a low power transmitter and the DIN rail mounted receiver features a sensitive receiver. The system operates at 2.405 GHz. It would be interesting to capture an over-the-air packet – perhaps another day.
The ZBRT2 appears to use energy harvesting
The ZBRT2 module is batteryless. Also, there are no external connection of any kind for external power. We can therefor conclude that the energy for the Zigbee transmitter is derived from the user. Please know that data is sent when the button is pressed and when it is released as shown in Video 2. We can therefore conclude that energy harvesting is used for both button push and release. This is a tiny amount of energy requiring only enough to transmit a single packet of information. Specifically, the ZBRT2 must transmit its unique identification. Perhaps another day we can disassemble the transmitter to find out how it works.
Tech Tip: The Harmony wireless transmitters are rated for 100-meter operation in an ideal setting. This is not realistic, especially if the ZBRRA receiver is installed inside a metal (shielded) control panel. As per the Schneider data, 25 meters is a more realistic range. While not applicable to the ZBRRA receiver, Schneider does have devices featuring an optional external antenna connection such as the related ZBRN1 gateway.
How is the Schneider ZBRRA receiver physically connected inside the control panel?
The XBRRA output consists of two Single Pole Double Throw (SPDT) relays. Like all relays, the contact’s current rating depends on the applied voltage and the approval agency. As a general specification, the contacts are rated for 3 A allowing direct connection for small loads. The receiver could also be used to directly control a motor starter.
What are the operating modes for the ZBRRA?
The ZBRAA has 4 modes including:
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maintained: This is similar to single pushbutton control of a process. The first button press activates the while the second button press deactivates the associated system.
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momentary: The output will be active for the duration of the button press. This is the mode shown in Video 1.
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pulse: The output will send a single pulse upon detection of a button press.
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start / stop: This is like the traditional 3-wire motor start stop circuit with one button to activate and one button to stop the system
Video 1: The testbed’s red and green panel lamps are activated as long as the associate red or green pushbutton is held.
Interface with a PLC
The relay output of the ZBRRA provides a natural interface with a Programmable Logic Controller (PLC). One leg of the receivers output relay is connected to 24 VDC while a normally open contact is connected to the PLC input (11 to 14 pair for channel 1 and 21 to 24 pair for channel 2). From the PLC’s perspective the remote receiver is just like any other input field device. Close inspection of Figure 1 shows the 24 VDC from the red power block and the two wires connecting to the red and green panel lamps.
Tech Tip: The PLC may be used to provide creative feedback for the user. A simple form of feedback is for the PLC to momentarily activate a buzzer upon detection of a key press. Multiple key presses or patterns of key presses could then be used to perform multiple function. For example, holding a button for 10 seconds could transition the PLC into a maintenance mode. While there are many things that could be done, there is something to be said for simplicity.
How is the Schneider Harmony system configured?
Each Zigbee transmitter has a unique ID. For proper operation the receiver must be paired with transmitter. For example, the red pushbutton in Video 1 was paired with channel 1 and the green with channel 2. Please see the Schneider Electric produced Video 2 to guide you through the configuration and pairing procedure.
Video 2: How to configure a Harmony wireless receiver.
Parting thoughts
This was a fun product to explore. The system was very easy to wire and configure. I would be very interested to hear your success stories with this product. Also, if you are looking for a challenge, see what you can do with the PLC to user feedback mentioned in the tech tip.
Best Wishes,
APDahlen
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About the Author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this. LinkedIn | Aaron Dahlen - Application Engineer - DigiKey