Re: Servo Control--Torque vs Velocity

--- 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