Linear servo solution

[masterhapero]

Hello,

nice solution you have. Thank you for developing this.

I have a question for you. I am attempting to improve planter that has linear servo, a motor with resistor based feedback.

Would the Rate Controller have functionality to control this kind of solution easily? That is, use the control value to control the linear servo position with the Cytron motor controller.

Thanks.

[SK21]

Possibly could work. How do you know when the linear servo is in the right position? Do you measure rpm?

[masterhapero]

The linear servo would give closed loop feedback via variable resistor (like sliding potentiometer) giving the absolute position. When the value is correct, the servo motor has reached target position. Similar functionality as RC hobby servos have but implemented with ADC and motor controller. Most likely would need a PID controller that would handle motor controlling. Can you suggest what would be easiest way to add such control loop to work with Rate Control. I guess this might be just micro controller side solution? Thanks.

[SK21]

How do you determine the target position?

[masterhapero]

If there is 100k ohm potentiometer as feedback. And say min value is 5k and max 95k, depending 0-100 range would be 5000+900value ohm. And that would translate to range on ADC pin depending how the voltage split circuit is made. But lets say other end of resistor is grounded and other is 3.3v.. That might bring us to linear range of 10-250 on 8 bit ADC. So 0-100 range would translate to value2.5 + 10 on ADC. And maybe a bit of dead band around the target, so any value +-5 around target ADC value would give 0 error to PID so it would not oscillate unnecessary when reaching the target value. The linear servo could change seed planter mechanical ratio between wheels and seed drive train or just control mechanical eye hatch. In short, target position would be the ADC value, in my case it would be up to user to give meaningful set point target. Determining the positions would need a bit trial and error to find the values when doing calibration for the tool. For the far end points of linear servo range, the linear servo should have limit switches or the user should be aware of this when dialing the stuff in (maybe limit PID output / PWM).

[SK21]

If the code was changed you could measure the voltage and return to the same position. But what does that mean? How do you account for changing ground speed? Is it only a manual adjustment? Could a rpm sensor be used?

[masterhapero]

There are more than one application for this linear servo stuff planned. But for the use cases in mind, if we have a setting is for changing mechanical gear ratio between the wheel and planter, the UPM count could change dynamic given from the planting map. But one of the planters planned is multi function, so it also distributes fertiliser and for this we would use another map. For depth of planting, this would be more static setting .. except it would be great if it would follow work switch and would rise up when not planting. And the soil can also contribute, if the soil is softer and tool sinks lower it needs to be adjusted upwards. So the dynamics of these settings in the use case I highlighted is more related to position or work switch than ground speed. The agricultural devices I explained are actual devices we would like to convert and build a bit more remote control into, currently these require operator to step out from cabin and change the mechanical settings manually. RPM sensor would give speed of change for the linear sensor but would not bring it to planned setpoint. I am more of techinical guy helping farmer friend, trying to understand what might be best approach to improve the solution towards the goal. And yes, we have use for the existing functionality, too. But would need additional boards to control this kind of linear feedback servo motion.

[SK21]

Please contact me on Agopen discourse SK21 to continue the discussion.

[masterhapero]

Where is this discourse? Thanks

[SK21]

https://discourse.agopengps.com/