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I want to design a circuit with an H-bridge to control a brushed DC motor. In order to control its speed, I want to measure the back-EMF of the motor and make a PID control loop in software. I do not want to use external sensors for a number of reasons.

I want to build a circuit around a DRV8220 chip. It is a small chip, the specifications suffice, it is affordable and most important, it is in stock at my PCB supplier.

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The motor has to be able to turn in both directions and may experience different loads.

Can I build a circuit around this chip in order to measure the back-EMF of a DC motor?

If so, what would it look like?

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  • \$\begingroup\$ Do you actually mean the inductive back-emf kick-back or, the natural voltage produced by the motor when it is freewheeling? \$\endgroup\$
    – Andy aka
    Jun 15 at 11:22
  • \$\begingroup\$ It would use some current sensing and detect the switching noise which is very dynamic and needs to be specified after measuring with min and max load to create a tach voltage with a one shot,, so one can use DC with a filter or use PFM. Noise pulses will inverse with direction unless FW bridged. \$\endgroup\$
    – Tony Stewart EE75
    Jun 15 at 11:22
  • \$\begingroup\$ Andy. I am not sure. The one which can tell you how fast the motor turns. I believe that is the natural voltage of a running motor. This website refers to it as back EMF precisionmicrodrives.com/ab-021. General idea is to take ADC samples during the off-period of the dutycycle. and calculate an average voltage level which correlates to a certain speed. \$\endgroup\$
    – bask185
    Jun 15 at 11:27
  • \$\begingroup\$ @bask185 if all MOSFETs in the chip are off (thus allowing freewheeling of the motor) then yes, you can measure it using a differential amplifier based around an op-amp. But, the devil will be in the detail. \$\endgroup\$
    – Andy aka
    Jun 15 at 11:32
  • \$\begingroup\$ I assume you want to control the speed. Is that with PWM? What else can you say about the motor datasheet and acceleration expected. Do you have any error tolerances in mind? \$\endgroup\$
    – Tony Stewart EE75
    Jun 15 at 11:42

1 Answer 1

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The EMF of a powered motor is not directly measurable due to the series resistance of the motor. It can only be evaluated indirectly by:

  • Knowing the value of the internal resistance
  • Measuring the terminal voltage
  • Measuring the motor current
  • Calculating the voltage drop across the internal resistance
  • Subtracting out the voltage drop across the internal resistance from the terminal voltage

Of course, if the motor is not powered (is spinning freely), then the current is zero and the motor EMF is the same as the terminal voltage.

The instantaneous terminal voltage of a motor powered by an H-bridge is the same as the power supply voltage, minus any voltage drop across the transistors in the H-bridge (which should be minimal).

You have two options to evaluate the motor EMF:

  • Stop driving the motor for a new ms, wait for the inductive kickback current to die off, measure the motor terminal voltage with an differential amplifier, which is the motor EMF. Or,
  • Without removing power to the motor, measure the instantaneous power supply voltage and current and calculate the motor EMF. (You will need to know the motor resistance and the transistor resistance.)

It is much easier to read the voltage and current of the power supply than to use differential amplifiers to read the terminal voltage of the motor and motor current directly.

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  • \$\begingroup\$ I forgot to post a comment, my bad. I am aware of how the principle of measuring back EMF works with PWM. But I am specifically in search of a circuit in order to measure the motor terminal voltage \$\endgroup\$
    – bask185
    Jul 10 at 10:05
  • \$\begingroup\$ "a circuit in order to measure the motor terminal voltage". Well, then, why did yo not ask That question? If you ask question "A" (when you're thinking "B"), don't be surprised if we give answers "A" (not "B"). Start a new thread and ask your actual question. \$\endgroup\$
    – Davide Andrea
    Jul 10 at 12:49
  • \$\begingroup\$ Well.... I did. I spoke of circuits 3 times. My last 2 lines ended with: Can I build a circuit around this chip in order to measure the back-EMF of a DC motor? If so, what would it look like? \$\endgroup\$
    – bask185
    Jul 10 at 14:09
  • \$\begingroup\$ Your title is "Circuit to measure back EMF around an H bridge". It's not "Circuit to measure motor voltage around an H bridge". Start a new thread and ask your actual question. \$\endgroup\$
    – Davide Andrea
    Jul 10 at 15:09

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