Back-EMF speed detection in motors

Yup. That _might_ be adding 2 body diode forwards to the "broth" ;-) ...Jim Thompson

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Yeah, I know jack about motors, but what about this Tim? For your static measurement what happens if you hold the motor in place and run some current through it?

George H.

Because my computed speed is consistently _high_, in both directions.

Oh, it works great in the static case.

Well, first, the dead time is a minuscule fraction of the PWM cycle time

-- fast FETs are cheap these days, and motors seem to prefer PWM speeds in the low 10's of kHz.

And second, just today I made an RC filter so that I could clip a volt meter onto my motor leads and get a nice filtered DC to the meter. It reads within 1% of the voltage I calculate from the duty cycle and motor supply voltage.

(You cannot beat measurements. When all your calculations are going to hell -- measure).

Yup. No field winding.

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(Oh sorry, damn sign error again.) George H.

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OK then my final plan of attack is to try and make it worse! Inductive? Stick more iron around it, Thermal... heat and cool. Add some more cable here... resistance there... George H.

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Could i persuade you to post a waveform of the R-sense voltages and the inputs to the ADCs? Could you also compare the timing of the ADC trigger commands?

?-)

Motor current readings have also been double-checked with a multimeter and verified correct.

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Except that the motor windings are a kind of field winding - the current you drive through the motor winding acts to reduce the magnetic field that the permanent magnets are creating at the motor windings. Not much, but maybe enough to mess up your calculations.

Well you might need a bit of care with the sign but if you consider that the voltage drop across the brushes can only dissipate energy not create it then including brush drop always results in a lower calculated (and real) rotor winding voltage and therefore speed, regardless of direction, as long as you do not regenerate (use the motor as a generator), although sign consistency should get you the correct answer there too. Adjust brush voltage drop factor in your calculation for minimum calculated to real speed error. Tweak it on the fly based on measured error, and have the controller request motor brush service when the calculated brush voltage drop factor goes above 3 volts or so :-).

Is the current sense resistor common to both halves of the H-bridge?

What is its value relative to the motor winding resistance? ...Jim Thompson

It's amazing how long this thread has gone on. And one of the factors that no one is touching on is the field of the motor?

I see in early post it was stated that it has no field(stator) coil, this means it is a PM motor. (Most likely)

In case some one needs a refresher in the effects of variable field densities, it will vary the speed of the motor with a fixed armature voltage applied.

Being it is a PM motor, weaker field will cause more RPMs and of course stronger fields more foot lbs but less RPMS at the same armature voltage.

Motor temperature is going to make the PMs change their density. As they heat up, they'll attempt to spin faster at the same arm voltage but also increase the current slightly.

Then you start putting loads on the shaft, the motor is going to drop back and exert additional current from the drive which will increase the magnetic field in the armature to only increase speed in the hopes of bringing it back to speed per armature voltage, which it never makes it to the speed of unloaded conditions.

There is the function of auto field control to help maintain the motor RPM at vary loads, which reflect back to the drive electronics as armature current. But since we have a PM motor, that just isn't going to work here.

How many of those remember the DC motors used in tape drive systems that had the centrifugal speed control electronics integrated in them? There was a good reason for that of course, mostly due to the facts that similar attempts were made to maintain a controlled speed with DC brushed motors with no feed back..

Simply put, a good many DC drives do have current compensation controls on them when feed back is not in use. They scale the output verse A(i) and must be set at installation time.

That's my rant for this holiday weekend.

Jamie

I don't understand.

At 0 RPM, there is no error?

At all other RPMs , the error = 30 RPM?

so at 1 PRM it read 31?

Is it an offset error or a gain error?

Again, I suggest as a troubleshooting aid, you program the error out by whatever fudge factor it takes. Then analyze the fudge factor.

Mark

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That is not what i asked. I do not so much care if you post, but that you at least make the measurements asked.

?-)

Another test, switching, varying both duty-cycle and VM...

So my latest suspicion is that the voltage across the sense resistor, due to the full-H drive, flips sign and is not accounted for in the math. ...Jim Thompson

I think this is the proper equation...

Vspeed = (2*Duty - 1)*Vcc - Imotor*(Rmotor + Rsense) ...Jim Thompson