Re: Servo Control--Torque vs Velocity

Thank you Mariss; I definitely could not have said it better myself.

--- In CAD_CAM_EDM_DRO@egroups.com, "Mariss Freimanis"
<geckohall@h...> wrote:

> --- In CAD_CAM_EDM_DRO@egroups.com, jmw@c... wrote:
> > Can someone comment briefly on the difference between the torque
> and
> > velocity servo control schemes? I understand that both require
> > encoders but only one must also have a tach. Other things being
> > equal I'd just as soon not bother with tachs--but are other

things

> > equal?
> >
> > Thanks in advance.
> >
> > --Jack W.
>
> Hi,
>
> Of the two, torque mode control is almost universally used in
> servodrives. Tachometers are used in both schemes; their purpose is
> to derive PID information when combined with the +/-10 volt command
> input in analog servodrives. Newer techniques derive the PID info
> directly from the encoder.
>
> In a velocity mode amplifier the control input to the drive

commands

> a velocity, in a torque mode amplifier it commands a torque.
>
> A torque mode amplifier has a wider bandwidth, is easier to tune

and

> is more stable owing to a 90 degree phase margin advantage over a
> velocity mode amplifier.
>
> An excellent analogy is a car and driver. The car is a torque mode
> amplifier (gas pedal modulates torque) and the driver is the PID
> compensated loop.
>
> Let's say your mission is to stay side by side with a target car on
> the freeway, and initially it is some distance ahead of you.
>
> The "P" part of PID is proportional. You judge the distance that
> seperates you and it. The greater the distance, the more gas

(torque)

> you apply.
>
> The "D" part is derivative. This is your rate of closure. The more
> rapidly you are overtaking it, the less gas you apply.
>
> The "I" part is the integral. Once you have pulled even and have
> matched speed, P and D are near zero. The small adjustments made to
> the gas pedal to maintain your exact position relative to the

target

> are made by the integral component. This comensates for any drift.
>
> Now imagine if the gas pedal commanded speed instead. The correct
> speed would now depend on an exact position of the gas pedal. You
> would forever be overshooting and undershooting your target. By the
> time you detected an error in position and reacted to it (with
> exquisite precision), the correction would be in the wrong

direction.

> Your response would be out of phase.
>
> Mariss