Large scale servo using DE Kangaroo 2x/Sabertooth 2x12

Help & Support Robot Parts
1

Hello,
I am attempting to create a servo using the Dimension Engineering (DE) Kangaroo 2x and Sabertooth 2x12. I’ve been reading up on how to create a feedback loop using a potentiometer, but I can’t find any examples that show what type of potentiometer to use. I’ve gathered that I need a rotary potentiometer, but I don’t know what the Ohm rating should be to be compatible with the Kangaroo. I’ve ordered some 10k Ohm rotaries along with the Kangaroo and Sabertooth, but I haven’t tried those yet. Will those work?

Thanks,
Shea

2

Hi,

The potentiometer resistance rating is not really important when working with the Self-tuning Kangaroo X2 Motion Controller and Sabertooth Dual 12A 6V-24V Regenerative Motor Driver (or any other DC Motor Controller with analog input), as long as you connect it as shown in page 10 of the Kangaroo datasheet. Since the 2 potentiometer’s fixed pins are connected to 0V (GND) and 5V, the variable (wiper) pin will always provide an analog voltage between 0V-5V. The potentiometer acts as a voltage divider providing a voltage output based on the voltage divider’s formula (Vout = Vin*(R2/(R2+R1))). For example, if you have a 10kOhms potentiometer and rotate it until you have 5kOhms between the wiper pin and one of the fixed pins, and if the fixed pins are connected to 0V and 5V, you will have a voltage potential of 2.5V between the variable pin and the 0V.
By default, in speed control with the kangaroo, a 2.5V is zero speed, 0V is full speed reverse and 5V is full speed forward. In position control, 0V is full travel in one direction and 5V is full travel in the other direction.
The input center and range can be configured with tune mode 4 or programmed with the DEScribe Software.

3

Thanks for the reply! I learned a lot from that.

So the resistance rating is how much resistance it takes to drop the output voltage to zero?

I’m using PWM for the control inputs.

I am going to be using the the potentiometers for the feedback inputs.

Do the feedback inputs work the same way, with regards to reading from the potentiometers? I’m guessing they would; but want to be sure.

Thanks again,
Shea

4

Correct, if the outside pins of the potentiometer are connected to a voltage source (one to ground, the other to Vin), the output voltage (Vout at the middle pin) will mimic a voltage divider.
When the pot is rotated all the way in one direction, the voltage will be near zero; turned to the other side the output voltage approaches the input; a wiper in the middle position means the output voltage will be half of the input.

Yes, as indicated in page 6 in the datasheet of the Kangaroo : " Kangaroo also supports 0-5v analog feedback for use with potentiometers. To use analog feedback, connect the potentiometer’s wiper to the A feedback input. Connect one side of the potentiometer to the 5V terminal and the other side to the B terminal."