Electro Magnetic Interference of DC Motor over Controller Board

Introduction:

This case study involves how we designed an industrial automation system involving 4 DC motors and an associated printed circuit board for controlling the motor system, and also how we overcame the problem of Electro Magnetic Interference (EMI) of the motors over the system display unit.

Background:

At the time of approaching us, our client was already in the market with the system in question. Their current requirement was for the system to be upgraded in terms of features and performance.

Proposed Solution

After thorough analysis, the following proposals were suggested from our end:

  • Addition of custom-made electronics circuit board with digital control of the DC motors instead of electromechanical.
  • Addition of a display to monitor the system status.

As these features were currently not available in any of their competitors’ systems the clients agreed upon its implementation.

Journey And Learnings

The application had to be designed to control four DC motors by a time sequence, which was generated using a micro-controller, and also to keep on updating the system sequence status on a liquid crystal display (LCD) attached to the controller in parallel mode.  Out of the four DC motors three were simple low rpm, 12V geared DC motors and the remaining one was a high torque SHY-RS-775-12V brushed DC motor operating at 3500 rpm.

We selected the controllers and other circuit components and started designing the required schematics. We then proceeded to test the application on general-purpose as well as bread-board level prototypes before starting work on the printed circuit board design. The testing of the fabricated PCB with complete system integration was a success. Hence, we migrated to the production phase. Finally, it was time for delivering the first manufactured batch of 100 units.

The assembly team started integrating the controller into their systems; but after testing we were faced with the problem of LCD garbage display in 90% of units.

After detailed analysis we found that it was the third DC motor which was generating EMI noise. EMI from the motor was random in both frequency and amplitude but this was not the major issue. Instead, it was the motor feedback to the DC power adapter that was the real cause for concern, as this led to the display of garbage values on the LCD (as shown in Figure 1 below).

Figure1: LCD garbage display

Among the various possible solutions to the problem, we started searching for the most promising technique. We tried multiple options such as adding diode, snubber circuit, twisted pair supply cable, including a capacitor between supply and ground rail, as shown in figure 2.

Figure 2: Implemented hardware solution

Finally, we decided to use an amalgamation of hardware and software techniques. The combination that solved the problem permanently was the circuit shown in figure 3, along with periodically refreshing the LCD via a controller.

Figure 3: EMI noise suppression circuit for brushed dc motors.

Summary

This has proved to be a great learning experience for us. Not only did we succeed in delivering the automation system required, the design of the EMI noise suppression circuit was a significant achievement. Now we can say with confidence that all our products are safe from the harmful effects of Electro Magnetic Interference.  Are you facing any such EMI issues with your system? We can help you solve your issues and design a robust EMI immune control system. Click here to get in touch with us.