Giant servo


Im building an r/c gokart and I need to make a servo powerful enough to turn the steering column .

I plan on using a windshield wiper motor with a direct drive from the motor to the steering column.

I want to know if the what motor driver to use and if the motor driver operrate simlilar to the h bridge in a standard servo when used with potentiometer feed back?


I probably spoke to you on the phone about this, but in case that wasn’t you and for the benefit of others: we have some awesome new feedback controllers called the Jrk 21v3 andJrk 12v12, which should be just what you’re looking for. I don’t know how much current your motors need, but the Jrk 12v12 will probably be enough.

We also have a customer who is developing some giant servos around the Jrk controller, and we hope to distribute them once they become available, which I hope will be in the next month or two.

- Jan

I just ordered my JRK 12v12 and plan to use it exactly as described above. It will be controlling something different, but I will be using a windshield wiper motor hooked to a 5K potentiometer.

I have 2 questions:

  1. Is the 5K Pot the correct value?

  2. I will be using a lynxmotion SSC-32 servo controller card which will be receiving commands from VSA software. I will be sending the Pulse Output from the SSC-32 board to the RX input or the JRK 12v12. The potentiometer will be attached across the 5VDC regulated Voltage output with the POT wiper attached to the Feedback pin. Do I need to hook up the JRK 12v12 to my computer and run the configuration utility on it before using it in this manner? During operation, the JRK 12v12 will NOT be connected to the computer via USB.

My application would look like this:

Is this set up correctly?

Thank you


That’s generally right, but don’t forget you need ground connected to the servo controller. 5k for the pot is fine.

You definitely do need to set up your controller for your system. Once you have it set up, it will store all of its settings so that your final application does not need to be connected via USB.

- Jan

I am afraid I am a bit dense about this thing.

I have the board hooked up as shown in my diagram.

I have installed the drivers and ran the configuration Utility, but I don’t understand it very well.

To do what I need to do with this, what settings will I have to set within the configuration Utility?

I tried the following:

Under Input, I selected “Pulse Width” and clicked “Apply Settings to device”. I can’t say that I am sure what I am supposed to put in the Scaling section. I am completely lost there.

In the Feedback Tab, I selected, “Frequency (digital)”. Is that correct?

Again, I have no idea what else need to set within that tab.

The PID tab is like another language. Do I need to do anything there?

Also, do I need to change any settings in the Motor or Errors tabs?

I click Run Motor and nothing happens. No hum, no motion, nothing.

I do have the ground from my power source running back to the SSC-32 servo controller board.

While I am connected via USB, the Green LED on the board flickers constantly while the yellow LED flashes slowly.

When I disconnect the USB, the green LED goes out and the Yellow one continues to flash.

What am I missing?


Hello Bob,
I think I can answer some of your questions. It doesn’t sound like you are being “dense”, it just sounds like you have not read the Jrk User’s Guide. Most of the options will not make sense until you read about them, and the controller will not do what you want it to do unless you understand what options you are selecting.

As for you specific questions, “Frequency” is the wrong choice for the feedback mode. Your potentiometer outputs an analog signal, so you will use the “Analog” mode. And yes, PID is an important part of setting up a servo system with the jrk, so you are going to have to do something on that tab. The slowly flashing yellow LED means that you have not told the motor to actually do anything yet.

To get started with your system, I recommend that you read over the entire manual until you understand the basic options, then follow the step-by-step guide we provide. The steps on that page are designed to make sure that you have the motor and pot connected correctly and to prevent you from accidentally breaking something while you are setting it up.


I apologize for not catching on very fast here.

The following is where I am at now. I have read through the manual and have the step by step guide in front of me.

I still get no motion on the servo (wiper motor). I have many errors. The red error light is constantly on.

In the input tab, I have the following choices entered:
For Input Mode, I have “Pulse Width” selected because I will be controlling this from my servo controller card.

In the Feedback Tab, I have “Analog Voltage” selected and I have checked the Detect Disconnect with AUX box in the Analog to Digital conversion box.

In the PID tab, I have the Proportional Coefficioent set to 1.

In the Motor Tab, I have set the Max Duty Cycle to 200 and the Max Current to 1.5 amps.

In the Errors Tab I have the following set as “enabled and Latched”:
Awaiting Command
No Power
Motor Driver Error
Feedback Disconnect
Max Current Exceeded

I have “Input Invalid” checked as just “Enabled” and everything else is set as Disabled.

I have 3 errors that I can not clear. They are:
Awaiting Command
Input Invaild
Feedback Disconnect

My potentiometer is connectedwith one end at ground, the other end at 5VDC and the wiper arm in the Aux pin on the JRK.

I have 12 VDC connected with proper polarity.

The motor power is hooked up.

I can not perform the learn function in the “Input” Tab.

In fact, I can not clear the errors or run the motor in any fashion at all so far.

Please note that if I allow the motor to run continuously in any direction, it WILL destroy my entire setup.

Where do I go from here?

Try to do a dry run with out feed back just to get the motor running. set the feed back to none.

:question:I dont understand what u mean by “wiper arm in the Aux pin on the JRK” :question:

-you shouldn’t really have anything connected to aux.

  1. you should have 3 wires coming off your pot to GND-5v-FB.

  2. Wiper motor should be connected to to the A-B outputs.

  3. Power to GND and Vin just next to the motor outputs.

  4. Finaly the 3 wire off of your servo controller to the 5v-GND(unless the servo controller is already powered by another source) with the signal conected to control input or RX on the Jrk.

  5. With feed back set to none,Input set to Pwm(pulse width) and apply the settings to the jrk.

  6. Now, click learn in the input tab and it should ask you for the max and min input values.

  7. Apply settings to Jrk and confirm that the input has been set by looking for proportional changes on on the graph in accordance to the position of your servo controller.

  8. When you get your motor moving you can set the feedback to analog.

  9. Apply settings to Jrk

  10. Have the Jrk learn your desired min-max servo positions

  11. Apply settings to Jrk

  12. test to make sure the servo isn’t reversed

  13. Then tune to make movements smooth with out overshooting or undershooting with the quickest transit times
    (remember to apply the setting to the Jrk after every change in the config)


Hello. Please note that running the motor with feedback off will cause the motor to run continuously in a single direction, which in turn will probably destroy your system. This is a good idea if you can disconnect your motor from the system so that it can turn freely without damaging anything (or if you have another motor around that you can play with, or if you disconnect the motor entirely and monitor the driver outputs with a multimeter or oscilloscope), but it isn’t a good idea if you’re relying on feedback to keep the motor from turning far enough to break something.

- Ben

Ben is right. I can not disconnect this from the system at this point, and letting the motor run freely in either direction will destroy everything.

To clarify what I mean by wiper arm:

In the potentiometer, there are 3 connections. One at each end of the resistance coil and the third is connected to a “wiper” that slides along that resistance coil creating different amounts of resistance between one of the ends and itself. I refer to that connection (the one that moves and wipes over the resistance) as the wiper arm. Please tell me the correct term so I can avoid further confusion.

I do have the voltage (12VDC) connected with correct polarity to the input of the JRK (GND and VIN)

I do have the wiper motor connected to the A and B outputs although I am unsure of Polarity there.

Originally, I had the potentiometer connected as shown in the picture in my post several up. It was connected to the FB pin. Since then, I read the step by step guide which says the following:

“4.Connect your feedback sensor to the FB input. If you are using a potentiometer as a feedback sensor, use the AUX pin to power it, and enable Detect disconnect with AUX in the Feedback tab.”

After reading that, I moved the connection from my potentiometer from the FB pin to the AUX pin. It does seem to be what it is telling me to do.

In reading Francisco’s post, I think there is something I may have missed. It sounds like he is saying that I am supposed to provide the 5VDC for the potentiometer. I was under the assumption that the 5VDC was coming FROM the JRK board. I thought perhaps that there was an onboard regulator converting the input voltage to 5 VDC. It does say in the description of the JRK board that that output is a 5v (regulated output). If I do have to provide the 5VDC myself, where do I connect it? I do have the GND from the 12 VDC Power input on the JRK tied to the GND of the 5 VDC power supply running my servo controller board.

Perhaps this is one major problem.

Please Clarify that for me as well as confirm the correct position for the potentiometer wire (FB or AUX).

Thank you again for all your help.

Thank you for clarifying what you meant by the “wiper arm”.

I was thinking of a wiper motor arm when you posted it earlier.

For your question about the pot:

5v dc is regulated from the jrk

My setup in my go kart is working just as it should with my feed back pot connected the way you described in your diagram with the your pots “wiper arm” connected to Fb on the Jrk

You say that the Ground on your Jrk is also connected to the ground on your servo controller.

Try to isolate the power sources of the servo controller and the Jrk by only connecting the servo controllers signal wire to the Rx pin on the Jrk

Francisco1, you are posting misleading information about the jrk here. To clarify:

  • Running your motor without feedback is likely to destroy a servo system. I always recommend the step-by-step guide, which aims to avoid this outcome, as a starting point.

  • The AUX pin is used when the Detect disconnect with AUX feature is enabled. This is a feature that allows the jrk to identify the removal of the feedback sensor. The idea is that instead of powering it with the 5V line, you use AUX as a power source. The output of the feedback sensor (middle pin of a potentiometer) should always be connected to FB. Note the FB is an input, not an output, so it never “powers” the feedback sensor. In fact, Bob, the error message “Feedback Disconnect” indicates your exact problem - nothing is connected to FB! Your connections to the pot should be AUX-FB-GND.

  • GND on the jrk absolutely must be connected to GND on the servo controller. In general, whenever you do any project, unless you are doing something really special, all grounds should be connected together. Voltage is always relative, so without the connection, there is no way for the jrk to measure the signals coming from your controller.

Bob, the step-by-step guide does not cover setting up the input - this is quite easy once you have the feedback working correctly under serial control, and incorrect input settings are very unlikely to damage a properly configured system, so we just don’t talk about it here. Nowhere in the guide does it say to set the input mode to pulse width, so you must have stopped following the instructions somewhere. Can you try going step-by-step again and see if you can get the feedback part working, now that you know how to plug in your potentiometer? Once you can move the motor back and forth nicely with the slider on the input tab, you can just switch your input to pulse width, have the jrk learn the min and max values, and you’ll be ready to go!


Thank you Paul.

Your information, combined with the help of a friend from the UK on MSN and about 4 hours and I do have it working.

Yes, I was aware of the GND issue keeping everything together.

The wiring I show in my diagram for the POT was wrong and you are the first to point that out.

It does work well the way you stated.

It took some tweaking in the PID, but it seems to be working well now.

Very smooth and quiet by the way.

I will respond again after more intensive testing if it acts up, but so far so good.

That’s great to hear! I could tell you understood about the grounds, but I wanted to clarify that in case someone else is reading this thread in the future and gets the wrong idea. So, everything is working, including the PWM input? When are you taking your car for a drive? If you have a digital camera handy, we would love to see some pictures or a video!

Also, if you could save your parameters as a .txt file and post them here, it would be interesting for us to see what you ended up with.