Reducing Motor Noise

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DC motors will almost inevitably produce some electrical noise. And then there is the fact that at start up, and when under heavy load, a motor can cause the power supply voltage to drop. Either of these effects can cause the controlling circuit to behave erratically. For this reason steps should be taken to minimize these conditions.


[edit] Filtering Motor Noise

Below are three simple ways that capacitors can be added to a motor in order to reduce any noise that motor may be producing.

[edit] Single Capacitor Filtering

Figure 1. A simple one-capacitor noise filter
Figure 1. A simple one-capacitor noise filter

The simplest filter involves adding a capacitor across the motor terminals as shown in figure 1.

Since a capacitor will conduct only currents that are changing at a high frequency, a single capacitor (typically 0.1µF) wired across the motor terminals (as shown in Figure 1) will act as a short circuit for high-frequency electrical noise, while not affecting the DC current to the motor at all. This reduces conduction of noise along the motor wiring.

Also see: Ben Hitchcock's tutorial Isolating Motors

[edit] Two Capacitor Filtering

Figure 2. a simple two-capacitor noise filter
Figure 2. a simple two-capacitor noise filter

Sometimes a motor is particularly noisy. In this case it may be necessary to use two-capacitors to reduce the motor noise. As shown in Figure 2, one side of each capacitor (0.047µF usually works well) is soldered to one of the motor terminals, and other side is soldered to the motor’s case.

This configuration makes the motor's case act as a shield, thus also reducing radiated noise. This makes it more effective than the one-capacitor technique described above.

[edit] Three Capacitor Filtering

Figure 3 - A simple three-capacitor motor noise filter
Figure 3 - A simple three-capacitor motor noise filter

The one- and two-capacitor filters can be used in combination making a three-capacitor filter. As shown in figure 3, one capacitor (again 0.1µF) is connected across the motor terminals, and one capacitor (0.047µF) is connected to each of motor terminals and the motor casing.

This last configuration is the most effective, and will all but guarantee that motor noise will not be the source any odd circuit behavior you may run into.

Note there is absolutly no 'guarantee' that this configuration will eliminate electrical motor noise.

[edit] Motor Isolation

Figure 4: A simple isolated power supply for robot motors and circuits
Figure 4: A simple isolated power supply for robot motors and circuits

At Start up, or whenever a motor is placed under a heavy load, it will draw an increased amount of current form the power supply. This can cause the voltage from the power supply to drop, sometime significantly. As mention previously, this can cause any electronic circuit that shares the power supply to act erratically. This is not something that is normally desired.

The best way to prevent this problem is to isolate the motor from the electronics by using a separate power supplies for each. Typically the negative side of each power supply is connected to a common or ground rail as seen in Figure 4. Using this configuration provides a common reference while insuring that the electronics power supply rail will be unaffected reguardless of how much current the motor power supply rail may be required to provide.

Unfortunately this approach is not always feasable. Extra batteries means extra weight tha motors have to deal with, and more space taken up by batteries, that might be better used by additional circuitry. Fortunately the is an alternative. It is not perfect, but generally work very well.

Figure 5: A pseudo-isolated power supply for robot motors and circuits
Figure 5: A pseudo-isolated power supply for robot motors and circuits

As shown in Figure 5 a couple of diodes and capacitor can be used to create pair of pseudo-isolated power supply rails. The capacitors should each be at least 1uf, but large would be better.

Using this configuration, capacitor C1, which is connected between the common negative rail, and the positive motor power supply will act like a power reservoir, providing bit of extra current when the motor first starts up, or for a short term when the motor is under a heavy load. Diode D1 helps to isolate the positive motor power supply rail from the positive electronics power supply rail.

Meanwhile diode D2 prevents the motor from drawing off any of the current stored in capacitor C2, which is connected across the common negative rail and the positive electronics power supply rail. It too will act like a reservoir, helping to keep the power going to the electronics stable during a short term increase in motor power demand. If the demand is last for more that a moment ot two, C2 will slow a drop in power to the electronics.

The one drawback to isolating the motor power from the power going to the [[electronics is that the effect of what has been called Implex or BEAM feedback will all but disappear. But that is probably a small price to pay to guarantee that that motor(s) in a robot will not cause the control circuit to behave in an erratic manner.

[edit] Conclusion

In closing, it is relatively simple to protect a robot's electronics from the problems that can be caused by the robot's motors. Using a few filter capacitors, and one of the simple isolation techniques describe above, BEAMer can minimize, if not eliminate, a common problem that many robots suffer from. The effect of motors upon the power going to the circuits controling the robot's proper function and behavior.

In some cases all the solutions presented here will not be able to solve motor noise issues. Sometimes the best solution is to obtain new motors, preferably brushless.

[edit] External References

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