Re: SERVOs & EMC

Very good description Jon!

Re Floating current sense resistor:

You might want to consider two grounded current gain resistors, one in each leg of the bridge and an analog switch to monitor
the current going forward or reverse. It would eliminate the difficult floating measurement. I have also seen halleffect
current sensors for this application.
Bertho
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Jon Elson <jmelson@...> wrote:

> Motor torque is proportional to instantaneous current. But, a change
> in pulse width does not instantly change motor current, because the
> inductance of the motor is substantial, and resists current change.
> This is why most if not all servo drives for machine positioning use
> a current control loop as the innermost one.
>
> SO, here's how it works :
>
> The CPU is involved in the outermost loop. It senses position,
> applies whatever control laws are deemed appropriate, and outputs
> a velocity command, which is the best guess of the control laws to
> obtain minimum position error by the next sample.
>
> The servo amp receives this velocity command, and also receives
> a DC tach signal, indicating actual instantaneous velocity. This is
> compared in an error amplifier (with LOTS of gain, at least 10,000)
> and produces a torque command signal. This signal is fed to a
> variable bidirectional clipper, to limit the commanded torque to
> some reasonable value, and fed to the innermost control loop.
> Torque command = current command. So, the innermost loop
> compares commanded current with actual current measured by
> a sensing resistor, and the error signal becomes the modulation
> for a PWM modulator that drives the full-bridge switch.
>
> Some tricky points are that there is no grounded leg on the motor
> for current sensing, when you use a full bridge. Therefore, the
> current sensing resistor is giving a differential signal of tenths of
> a volt, maximum, while it is swinging between ground and whatever
> the DC motor supply is, in common mode. It takes a VERY
> good instrumentation amplifier, and matched attenuator resistors
> and capacitors to make this work. The other problem is that
> very high gain amps tend to get pegged when the circuit goes
> open-loop, even for a moment, during hard acceleration, as
> when the current limit is reached. Also, when coming out of
> E-stop, the control loops have been open-loop for a long while,
> but have to settle quickly, or the machine will jerk so hard that
> the CNC detects an excessive following error, and goes back
> to E-stop.
>
> Jon