Motor Suppression in Robot Wars
Robot wars is a hostile place for motor controllers, and motor suppression should be given careful thought.
The main areas for concern are;
- Shock and vibration caused by collisions and weapon strikes.
- Electrical noise spikes caused by motors being subjected to rapid physical stops or shock loads whilst under full power [aka radio frequency interference or RFI].
Rotating weapons will be particularly susceptible to number 2. The aim of these is to have the rotor spinning as fast as possible and then to transfer that energy instantly to the victim. When that happens there is a lot going on in the system not least of which is oscillation of the armature and possibly bouncing of the brushes in their holders. The physical energy hopefully goes into the victim but the built up electrical and magnetic energy has to go somewhere too. You also have a controller that is trying to drive full current into an armature that could be going forwards, backwards or is stationary, and with the brushes across one winding or bridged across two.
This can lead to some pretty hefty transients being developed across the motor. To protect against these you should consider fitting a fast acting varistor transient suppressor across the motor terminals as well as the capacitors and ferrites mentioned elsewhere.
Varistors come in a wide variety of ratings but we would recommend one with a varistor voltage above the battery voltage + any regen braking surge, but below the Vmax of the controller you are using, and with a current rating of at least 10x the maximum motor current.
We are also looking at using transient voltage suppressors [TVS], we will post more after our tests are finished.
The same is also true of main drive motors, that may get engaged in a pushing contest or are rammed hard.
This picture shows the protection fitted to the motor on one of our test rigs, in this case there is only a capacitor fitted but if this was for a robot I would fit a transient suppressor in parallel with it. Note also the twisted motor leads, twisting them this way prevents them from acting like a loop antenna and radiating noise signals outwards.
Additional RFI hardening techniques include painting the inside of the case with nickel paint such as this from Farnell , and fitting screened control cables.