Optimizing Your Control Panel Using Sensor/Actuator Terminal Blocks

What is a sensor/actuator block?

The sensor/actuator block, also known as the initiator/actuator block, is a specialized type of terminal block used in industrial control panels. They are especially useful when used as the demarcation point between the panel’s internal wiring and the external field devices such as sensors and actuators. They are designed to save panel space by incorporating power as well as signal connections into a 2-tier (3-wire) or 3-tier (4-wire) slice as shown in Figures 1 and 2. The terminal blocks facilitate simple and clean panel construction, while simultaneously providing an easy to identify location for troubleshooting the panel top field device interface. Features such as small circular holes to accept a multimeter probe further aid in troubleshooting to determine if the problem exists inside or outside of the control panel.

Figure 1: The Weidmüller sensor/actuator terminal blocks are located on the right-hand side of the DIN rail. The red and blue four position jumpers for power distribution are visible on the front of the unit.

Tech Tip: One slice of the sensor/actuator block is reserved for the power connections. For example, the unit in Figure 1 features 4 terminal blocks. The leftmost block is for power while the three on the right are available for field devices.

Figure 2: Image of the Weidmüller sensor/actuator components before assembly.

How is a sensor/actuator terminal block used?

The sensor/actuator block is best understood by comparing it to the traditional terminal block. This is shown in Figure 3 where we see a 4-wire M12 connector in the foreground with traditional terminal blocks (blue) in the background. The traditional block consumes a considerable amount of space on the DIN rail as shown in Figure 4, as each wire requires its own block. By comparison, the sensor/actuator blocks are real estate efficient. Note that the situation in Figure 4 is better than suggested as there is an unused sensor/actuator block ready to accept the third sensor.

Figure 3: Picture of a 4-pin M12 connector in the foreground with traditional (blue) terminal blocks in the background.

Figure 4: comparison of the DIN real estate between the conventional (left) and the sensor/actuator terminal block(right). Note that the sensor/actuator system (right) has an open slot ready for a third sensor.

Wire standards for the sensors and actuators

Following the standard wiring convention, recall the color code for a 4-wire. As shown in Figures 3 and 4, each of these four wires requires its own terminal block.

  • Brown: +24 VDC
  • Blue: Return
  • Black: Signal
  • White: Teach

Wiring for the sensor/actuator

The wiring for the sensor/actuator block is shown in Figure 5. The four wires on the right are dedicated to the sensor while the 2 wires on the left allow the sensor to be connected to the control panel. The concept of a control panel “demarcation point” is implicit in this image, as the four closely spaced wire connections provide an ideal termination point for the 4-wire sensor cable.

Figure 5: Image of a 4-wire 3-tier sensor/actuator terminal blocks. The 4 sensor connections are shown on the left while the connections to the PLC are shown on the right.

Family of sensor/actuator terminal block

Like most industrial automation and control devices, the sensor/actuator terminal blocks are offered as a collection of family members. Figures 2 and 6 show a few of the family members. The 3-wire 2-tier block is shown in Figure 3 (bottom) while 4-wire 3-tier blocks are shown in Figures 1, 2, and 3(top).

Accessories

Figure 2 shows some of the family accessories that complete the terminal block including the end plate, red 4-position jumper, and blue jumper.

A complete description of the accessories may be found on a Weidmüller catalog page such as this 4-wire 3-tier block. Click on the accessories arrow in the general ordering data. There you will find test point, markers, and jumpers with length from 2 to 50 positions.

Figure 6: Image of Weidmüller 4-wire 3-tier (top) and a 3 wire 2-tier (bottom) sensor/actuator terminal blocks.

Conclusion

The sensor/actuator terminal block serves as the demarcation point between your industrial control panel and the external field devices. On a single sensor or single actuator basis, they provide convenient, easy to see, and easy to troubleshoot interface. The real estate saving provides an additional benefit to reduce the total footprint of the panel.

Be sure to test your knowledge by answering the questions and critical thinking questions at the end of this note. Also, we would love to hear from you. Please leave your comments and suggestions in the space below. Finally, please give a thumbs up if you learned something from this article.

Best wishes,

APDahlen

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About this 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 (partially interwoven with military experience). 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 educational articles about electronics and automation.

Highlighted Experience

Dahlen is an active contributor to the DigiKey TechForum. At the time of this writing, he has created over 160 unique posts and provided an additional 550 forum posts. Dahlen shares his insights on a wide variety of topics including microcontrollers, FPGA programming in Verilog, and a large body of work on industrial controls.

Questions

The following questions will help reinforce the content of the article.

  1. What is meant by the term “demarcation” as used in this article?

  2. Sketch the side view of a single, 2-tier, and a 3-tier terminal block.

  3. What is the purpose of the jumpers as used in a sensor/actuator terminal block?

  4. A sensor/actuator is configured using 5 position jumpers. How many sensors may be connected to the resulting block?

  5. Sketch the top (birds eye view) of a 3-tier sensor/actuator terminal block linked with 4-position jumpers. To this sketch identify the purpose of each tier.

  6. What is the purpose of the teach signal associated with a 4-wire sensor?

  7. Estimate the total length of a Weidmüller sensor/actuator terminal block featuring a 50-position jumper. For full credit, include the width of the end plate and a pair of Weidmüller end stops.

  8. Assuming we use white with blue stripe wire for the 24 VDC return, what is the color of the corresponding sensor wire?

  9. The previous question identifies a source of confusion regarding wire color. Describe a mnemonic or other memory technique to avoid this problem.

  10. With regards to relays and polarity, what is the designation for the positive and for the negative terminals?

  11. Locate a 3-foot replacement for the M12 to flying leads cable shown in Figure 3.

  12. Use the DigiKey search tools to locate two unique sensors and two unique actuators compatible with the sensor/actuator terminal blocks.

Critical thinking questions

These critical thinking questions expand the article’s content allowing you to develop a big picture understanding the material and its relationship to adjacent topics. They are often open ended, require research, and are best answered in essay form.

  1. Figure 5 describes the wiring for a 3-tier sensor/actuator terminal block. Which connection(s) are fixed, and which are subject to interpretation?

  2. This article features the Weidmüller sensor / actuator terminal block. Use DigiKey’s search engine to locate similar offerings from other manufacturers.

  3. Sketch an image identifying the pins of a M12 connector. To this image, add the color for each pin.

  4. Contrast and compare the high-level implications of a sensor/actuator terminal block system with an IO-Link system. For full credit, address aspects of installation, cost, and maintenance. Disregard the inherent data transfer benefits of IO-Link. Be sure to provide scope with consideration of small standalone machines vs larger industrial settings. Finally, comment on the future of sensor / actuator terminal block-based installations.