Electronic gearing hardware selection

[geneM]

Hi - I have a new application where I'm trying to slave 1 axis to another axis. The application is an autonomous winch. The master axis drives the winch drum, the slave axis drives a level wind that makes sure the winch line is approaching the drum at a 90 degree angle as the winch line moves back and forth across the winch drum. The slave rotation rate needs to run at a fixed ratio to the master rotation rate, somewhere around 0.1X the master speed. When the line reaches the end of the drum, the slave needs to dwell (slave speed = 0) for a fraction of a rotation and then reverse direction at the same speed ratio. I can sense the line reaching the end of the drum very precisely with an external sensor. Both motors will be BLDC with encoders.

I've been using the STEvalServo1 for a different application and have learned a lot. I have some questions about which eval board would be suitable for this application.

There are 3 complications.

1. In some situations, the master will be running in constant torque mode so its rotation rate will vary by up to 50% or so. The slave needs to follow the master as it changes rotation rate at the same fixed ratio.

2. The steady state current requirement for the master is about 10 amps max.  The steady state current requirement for the slave is <<1 amp and the systems is battery powered.

3. The master winch motor needs to run over a large range of speeds. Something like 100:1. My initial calculations for a Maxon EC60 motor are 3500 RPM to 35 RPM. 

Questions:

Q1. Does this seem like something I can program into a G4 or similar evaluation board?

Q2. The EVSPIN32G4-DUAL seems like an obvious choice but it uses single shunt current sense. Is that a problem for a constant torque, low speed application? Do I needs to be thinking about the ZeST or HSO algorithms?

Q3. Given this is a battery powered application, energy efficiency is important.  Will I lose much efficiency with a 10 amp RMS motor controller driving a << 1Amp load?

Q4. If anyone has eval board recommendations, I'd be happy to hear them.

Thanks - Gene 

[GMA]

Hello @geneM,

Have you considered using the Position Control algorithm for the slave motor? Refer to Application Note document for more details.

[geneM]

@GMA- In my application, the slave motor needs to spin at a velocity that is a fixed fraction of the master motor velocity. The master motor will be in torque control mode so it's velocity will be a function of it's load torque. If I'm understanding your suggestion correctly, I'd have to read the master velocity, calculate how far the winch line has moved along the drum then calculate the new desired slave position, and send that desired position to the slave.  I'll have high resolution rotary encoders on the both the master and slave motors so I'll have a good velocity measurement. I'll have an absolute linear position encoder on the slave axis but measuring and controlling position is harder than measuring and controlling velocity. Wouldn't it be easier just read the master speed multiply by the master/slave ratio and send that new velocity to the slave? I'm guessing that doing that 10-100 times per second will give pretty good results. It will be interesting to see how wrong that guess might be.

PS: If you're familiar with any electronic gearing applications using ST motor control products, I'd be happy to hear about them.  And, if you have some recommendations about what eval boards would be suitable for accurate torque control at low speeds in a sensored application, I'd be happy to hear them.

[GMA · Accepted solution]

Hello @geneM,

If both motors have high resolution rotary encoders, you can get the master motor position angle and apply it to the slave motor thanks to position control Trajectory control mode.