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I am as a beginner in electronics contemplating how to build a motor driver for a stepper motor.

From what I have read, the more sophisticated drivers will have a feature known as micro stepping which basically means that instead of switching winding current on and of per phase (full step) or adding a layer by alsow reversing polarity (half step), the voltage is adjusted in levels where up to 8 levels is common.

Further the state of the art with this technology talks of how an even and artifact-free sinusoidal voltage signal is the most important to get a smooth running and efficient motor with high precision movement.

It occurred to me that this resembles the goals of audio equipment as well. So my question is thus: Would it be feasible to use a speaker driver to drive a stepper motor? Why/why not? What are potential caveats that I must be aware of?

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The most common problem I can see is that audio amplifiers almost never have response to DC. This would make it impossible to use for anything other than constant rotation, and that's not usually what steppers are used for.

The other possible difficulty is the fact that audio amps are voltage devices, while (unless you're very careful with amplitudes) you really prefer current control for driving stepper motors.

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  • \$\begingroup\$ surely you could do some high power current/voltage followers using discrete transistors? So really just using the audio output as a control voltage, which goes to a power-amp stage \$\endgroup\$ – KyranF Apr 17 '15 at 4:33
  • \$\begingroup\$ @kyran : no, the audio amp may be powerful enough already. What you DO need to do is identify and remove all the high pass filters (coupling capacitors) in it and ensure it's still correctly biassed. By the time you've done that, it would have been faster to build a DC coupled amp from scratch. \$\endgroup\$ – Brian Drummond Apr 17 '15 at 10:11
  • \$\begingroup\$ @KyranF - No, that won't work. Let's say you have an amp with a voltage gain of 10, and you want a constant 5 volts out (corresponding to the stepper drive for a particular shaft angle you want to hold). You put in 0.5 volts. What do you get out? The output will step up to 5 volts, then rapidly go back to zero, and there's nothing you can do about it. That's what "no response to DC" means. \$\endgroup\$ – WhatRoughBeast Apr 17 '15 at 10:51
  • \$\begingroup\$ @BrianDrummond and WhatRoughBeast - okay thanks fellas! Indeed it looks like only a constant rotation would be possible in that case. I forgot about the DC blocking capacitors in the way \$\endgroup\$ – KyranF Apr 17 '15 at 15:32
  • \$\begingroup\$ You might be interested to know that feeding a stepper with two sine waves essentially converts it into synchronous induction motor, with the two inputs driven directly out of phase, rather than using a capacitor to produce a phase shift in one winding. \$\endgroup\$ – WhatRoughBeast Apr 17 '15 at 20:26
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You can check out this application note : http://users.ece.utexas.edu/~valvano/Datasheets/Stepper_ST.pdf

enter image description here

Figure 20 shows a way of injecting sinusoidal currents into the stepper motor using an L298 which is basically a powerful opamp. It consists of generating two sine ware reference voltages with a DAC. These reference will be used by an L6506 to control the current flowing though each phases of the motor.

I have personally not tried yet this solution but I'm interested into that in order to drive a stepper with an almost continuous torque at all angle and in order to minimize vibrations and noise into the support. Microstepping drivers are going to right direction but the sin wave current they are outputin still has a lots of squares, harmonics and generates lots of vibrations/noise.

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  • \$\begingroup\$ It would be best if you included a relevant schematic in your answer, because that application note may disappear one day. \$\endgroup\$ – Dmitry Grigoryev Sep 5 '17 at 15:54
  • \$\begingroup\$ Good idea thanks, I attached the schematic, however due to my newbieness, I had to remove the second link (the image counts as a link and I'm not allowed to insert more than two links in a post) \$\endgroup\$ – user1892410 Sep 10 '17 at 15:59
  • \$\begingroup\$ The second one was This application note will help as well: st.com/content/ccc/resource/technical/document/application_note/… It shows a practical example of microstepping and approaching a pure sin/cosin wave for driving a stepper. \$\endgroup\$ – user1892410 Sep 10 '17 at 15:59

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