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Relationship of Output and Input of Thruster Commander

Hello everyone,
I want to know the relationship between output signal (PWM value) and input (resistances) of thruster commander.

The reason is that I want to rotate the thruster (T200) with ECS at arbitrary rotation speed,.

So, I recently purchased the thruster commander,
and I connected Fixed resistors to the pins named “SPEED” on the Thruster Commander.
( 7 k ohm resistance was connected between the right pin and the center pin,
and 3 k ohm resistance was connected between the center pin and the left pin)

In this way, I could rotate it at a constant rotation speed.
However I could not measure the rotation speed.

if you have some datasheet of thruster commander, could you give me it?

Hi @T.K.jp, welcome to the forum! :slight_smile:

The input channels of the thruster commander work using voltage dividers. A potentiometer is effectively two resistors with inversely proportionate resistances - when you turn it fully to one side the resistance on one half is approximately zero, and the other half is the full resistance of the potentiometer (in the case of the default ones we provide, 10 kΩ). When the knob is in the middle the resistance on either side is roughly equal, so a constant voltage applied across the full potentiometer is split in half by the equal resistances, and measuring the voltage on the ‘wiper’ (the middle pin) gives a voltage of roughly half the applied voltage. The voltage drop across one side tracks along with the resistance ratio \left(V_{top-middle} = \frac{R_{top-middle}}{R_{top-middle}+R_{middle-bottom}}\right), so you get a linear input signal from the potentiometer position (effectively a signal from 0% to 100%). 10 kΩ is a good total resistance to aim for, so when you’re choosing your resistance values try to have them add up to that or at least close to it :slight_smile:

Our ESCs work with an input PWM signal with 1100 to 1900 microsecond ‘on’ regions, so for the LEFT and RIGHT input channels (and SPEED if STEERING is disconnected or set to even) the thruster commander takes the voltage divider input signal and outputs a corresponding PWM signal in the valid range (e.g. 0% → 1100us PWM, 100% → 1900us PWM). Since our ESCs are bidirectional, the mid-point of 1500us (equal resistances) is the stopped point, and 1100us is full-speed reverse, while 1900us is full-speed forward.

The actual speed that the thruster rotates at depends on the power supply you’re using to power it. We have a chart of thruster RPM vs ESC PWM input in our T200 Technical Details which you can use to determine the approximate rotation speed at a given voltage and PWM.

Note that the thrusters use quite high current (see the Current Draw plot in the same section), so if your power supply/battery can’t provide the required current for the speed you’re trying to operate at it will lose voltage and accordingly run at a lower speed than planned. Note also that the thrusters shouldn’t be run in air for extended periods or at high speeds, so when you’re testing this it’s best to do so in some water so the bearings stay correctly lubricated and the thruster motor doesn’t overheat.

Thank you for your prompt and kind reply.

I think that I got all I wanted to know.

If I could understand correctly,
when I put resistors as below,
・R_top-middle = 7 k Ω
・R_middle-bottom = 3 k Ω
then the output will be 70%, and the PWM signal will be 1660us

And I also refer your additional advice.
I will test the thruster with sufficient capacity power supply in a pool.

Thank you again for your support!

1 Like

Yes, that should be correct although depending on which way you define top and bottom that could also be 1340 PWM (30% instead of 70%). Note that the thruster windings aren’t consistent, so 1660 PWM input to one thruster (via an ESC) could go clockwise and then the same signal with the same ESC to another thruster could go anti-clockwise. If direction is important to your application you’ll want to check which direction each thruster goes with the same input. If one goes the opposite direction to what you want then you can either swap two of the phase connections (the three wires from the ESC to the thruster), or you can just control that one in the opposite direction by swapping your resistors around :slight_smile:

Good idea :slight_smile:

No problem!