Current motor controllers

[Friedrich Kegel]

Hello,

I'm an Industrial Design Student interested in kin-aesthetic Interfaces of lower Degree. To get into haptics i started with the online course from Stanford "Introduction to Haptics" / ME327 ( https://lagunita.stanford.edu/courses/SelfPaced/Haptics/2014/about )

I made my own little application (turnknob/slider) to design simple haptic interfaces. I found cheap Maxon RE35 with Encoders and controlled them by a VNH2SP30 Motordriver. Except some damping problems this setup seem to work well so far.

( https://www.youtube.com/watch?v=NlaF00GmT28&t=70s )

Im interested in building the Woodenhaptics device. Due I'm in Bachelor i have to pay for all parts myself. So buying three Maxon Escon 50/5 controllers is super expensive to me and I'm looking for alternatives.

Jonas told me that the controllers use current control for the motors which is the main difference to normal DC motordrivers.

I got confused due until now i controlled the motors always by the voltage, like they did int the Stanford course. They used a L298 without any Current Sense (those Pins are grounded in their PCB layout).

To get even more confused they seem to be unable to explain the difference:

by Alsion M. Okamura - Course ME 327

As I understand on motors:

• I=M  
  
• for each voltage/speed there is a specific max. torque which is used for the control by voltage/speed according to 
  
• Current controllers loop back the current to the input signal and increasing the output until the target current is reached
  

So what is the difference between a current and a voltage based motor amplifier? My guess so far there are nonlinearities in the speed-toruque relation?I found those nice IC's which also offer current-control:
http://www.ti.com/lit/ds/symlink/drv8848.pdf (12V 2A 3€)
http://www.semicon.sanken-ele.co.jp/sk_content/sta6940m_ds_en.pdf (40V 4A ~5€)

Does anyone know other current-control capable motor drivers? In theory you could also solve it by software or seperate comperator logic - as long as it's fast enough?  Sincerly Friedrich 

[Jonas Forsslund]

Hello Friedrich,

Thanks for your interesting question, and sharing your cool projects with the group.

I am not the best to answer the question, I have only investigated current controllers on a high level, e.g. what you have already seen from Allison's slides, that current -> torque = good (i.e. it is easy to command a certain torque - which is what we ultimately want - since it is direct proportional to the current). I think that one reason we don't see more of (cheap) ready-made current controllers is that most people want to control for speed, not torque, so there is a limited market - but we should of course make one.

I know my (very busy) colleagues have looked into some IC's that might do it or be part of a circuit for easy current control, and they might be able to provide some input to this later on. I just wanted to pick up your question :)

Jonas