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I have an Harmonic drive and would like to design a PLL (phase-locked loop) controller for speed control of it. Here is a block diagram of a motor-speed control system based on PLL techniques:

Pll motor speed control

My Question is about power amplifier (servo amplifier) (see image above).

  • What is best circuit for servo amplifier of DC motors?

Desired circuit must have poles at much higher frequencies than the poles of the motor. Any ideas would be appreciated.

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  • \$\begingroup\$ Why do you need the amplifier to have its poles at much higher frequencies than motor's poles? \$\endgroup\$ – Vasiliy Aug 21 '13 at 10:19
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    \$\begingroup\$ What power does the amplifier need to supply? What voltage and current ratings are required? What gain does the amplifier need to have? What is the lowest demanded speed and how many notches in the opto disc are there? \$\endgroup\$ – Andy aka Aug 21 '13 at 10:37
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    \$\begingroup\$ I don't know what Harmonic Drive is, but you wrote that it is a kind of DC motor. The function of this amplifier will be to supply (relatively) slow varying DC voltage to the motor, right? Two questions: 1. if the voltage is DC, then why do you care about the poles of the amplifier? 2. if the motor is DC, and the only input to the amplifier is a voltage signal from the filter, then why do you consider to use servo amplifiers? \$\endgroup\$ – Vasiliy Aug 21 '13 at 10:54
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    \$\begingroup\$ I seem to lack some understanding of what you're doing. If 1. your motor is equivalent to a brushed DC motor; 2. the driving signal is DC (not PWM); 3. the rate of change in motor's speed is low; then I don't understand what is the importance of amp's frequency response. Furthermore, if you do not have any direct feedback from the motor to the amp, then why do you need a servo amp? Aren't these amps designed to take advantage of various feedbacks form a servo motors? \$\endgroup\$ – Vasiliy Aug 21 '13 at 12:45
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    \$\begingroup\$ Then you want to take the output of Loop Filter and convert it to PWM? Why? \$\endgroup\$ – Vasiliy Aug 21 '13 at 12:55
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What is best circuit for servo amplifier of DC motors?

I'd use a PWM converter that feeds to a push-pull MOSFET output stage. You could probably use a power op-amp circuit but you'd be running it hot although it would still work. Your power supply should be OK if it can supply 30V at 6A. For a circuit I'd consider looking at the LT1160 data sheet. It has the following circuit for a 200W class D amplifier.

enter image description here

It needs checking what components other than the one I marked that needs bypassing to allow proper DC control but this would be a good starting point I reckon.

A pointer about terminology - a servo amplifier is more than a simple amplifier like what you have in your diagram. A servo amplifier takes the demand input and processes the feedback signal to ensure the motor is kept in closed-loop control

Wiki has an article.

I also asked in my comment about the lowest speed demand and the number of notches in the disk. I asked to make sure you were properly accounting for low speed running in the loop filter.

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  • \$\begingroup\$ Thanks, but could you give a schematic of circuit? \$\endgroup\$ – Zia Aug 21 '13 at 17:25
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    \$\begingroup\$ @Zia the LT1160 data sheet gives a decent circuit that would be convertable to your needs I think. Don't ask how to convert it - that would be asking too much - also try googling "Half bridge class D motor controllers". Circuit added to answer. It's also worth looking at this device if you can feed PWM directly www1.futureelectronics.com/doc/ANALOGICTECH%20-%20AATI/… \$\endgroup\$ – Andy aka Aug 21 '13 at 18:17
  • \$\begingroup\$ Some experts believe we can't use class D for this case. What is your opinion about this : Using a trigger based controllable device like 6-phase or 12-phase rectifier(using SCR or IGBT) \$\endgroup\$ – Zia Aug 22 '13 at 12:12
  • \$\begingroup\$ @Zia show a circuit of what you mean but I'm not an expert on IGBTs or SCRs. Class D = PWM control. \$\endgroup\$ – Andy aka Aug 22 '13 at 12:48

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