POT Dead Band
With all controllers, operating from a standard potentiometer, there will be a ‘dead band’ at the bottom of the pot, where turning the speed pot does nothing at all.
This page describes the many factors that contribute to this dead band. We refer here specifically to the Pro-120 Mk1 and NCC series controllers, but most of these factors apply to any controller (of whatever manufacture) and any potentiometer.
- Pot’s dead band
- Controller’s dead band
- Controller’s gain
- Motor stiction
Pot’s dead band
All rotary pots have a mechanical ‘dead band’ at top and bottom of the pot, where movement of the pot has no electrical effect: this is to do with the pot’s manufacture and the way current is fed to the potentiometer at the end of the track. You will find in the specification for any potentiometer two figures:
- Mechanical travel. A typical figure might be 300°
- Electrical travel. A typical figure might be 280°
So there is a difference between the two of 20° – which implies that the top and bottom 10° of mechanical movement have absolutely no electrical effect.
Another cause of excessive deadband can be the wrong law of pot: Potentiometers come not only in linear but also log law and antilog law. A linear pot will be marked 10KA – 10K resistance, and the A meaning ‘linear’. Log pots have G, not A and will cause excessive dead band.
Controller’s dead band
Because operations (such as reversing in the Pro-120 and NCC) have to take place at zero speed, there has to be a dead band in the controller where such actions could be done before the controller starts to feed any voltage at all top the motor.
Mark 2 controllers
In the VTX, Mark 2 NCC and Mk 2 Pro, this dead band is reduced (and it is also adjustable) from the value in the Mark 1 controllers. If you are inquiring for information on the dead band – you probably think it’s too big, so probably have a Mark 1 controller. We will explain then the situation, with these earlier controllers:
Full speed occurs at around 3 volts input to the modulator. However the modulation starts at around 0.5 volts. So there is a dead band built into the modulator of .5 in 3 or about 17%. So you will have to turn the pot so that the input is 17% of the full speed voltage before the modulator starts to deliver voltage to the motor.
The VTX initially had
- adjustable RAT (Relay Acceptance Threshold) – the point where the relay makes as the demand speed in increased from zero.
- Adjustable MST (Modulation start threshold) – the point where the modulator start to apply voltage to the motor.
However we found these adjustments caused confusion so on later issue VTX they are replaced with fixed resistors such that RAT is about 140mV and MST is about 215mV. Both value measured on pin B/C of the expansion connector, i.e. after the ramp circuit. Full speed is approximately 2.95v above MST
If you have the gain control adjusted so that you do not use full speed, then the top part of the control range is lost to you, and the bottom part is expanded pro-rata. So if you set the gain such that you can only go to half speed on the motor, the controller’s electrical dead band doubles from 17% to 33%.
If you feed a small voltage to a motor, even an unloaded motor, the motor will not move. It requires a finite voltage before it does anything. Typically an unloaded motor may require about 10% of its full speed voltage before it starts to move. This figure varies greatly from motor to motor – and is worsened significantly by any load on the motor.
All of the above effects – pot dead band, controller dead band and stiction, are all cumulative. But you will see that the major contribution, with the Mark 1 controllers, is the controller’s own internal deadband, multiplied by the effects of any top speed reduction you have made on the gain preset.
The Mark 2 Pro-120 and NCC and VTX controllers have a much reduced deadband (which you can further adjust out).