servo basics 101 was Re: Question about Tachs and Encoders...

Paul,

There are some posts on this topic in the group archive online at the
group website. Here's a quick recap:

We feed electricity to the motor, it turns. More electricity, it
turns faster. And that is the FEED side of this servo thing. There is
usually a very precise circuit providing specific, accurate amounts
of electricity to the motor. This circuit is even "tuned" to the
specific motor and its environment and use.

Now for precise control we need some information about what is
happening "out there" at the motor so we can adjust the electrical
FEED to get the desired results. This information FROM the motor is
called feedback.

An encoder gives us the "amount" of the motors' movement. Since it is
connected to the motor either directly, or through mechanical linkage
of some sort, it tells us in discrete bits how "far" the motor has
moved/turned. It often arrives to the control in the form of
a "quadrature" pulse train, which allows the Direction of the motor
motion to also be determined, along with the "count" of the amount of
motor movement. The amount can also be called the MAGNITUDE of the
movement.

A tachometer provides the RATE of the motor movement to the control.
Connected to the motor shaft, it gives feedback of the motor speed.
When computers were not as powerful or as fast as today, they had a
hard time just keeping up with counting the bits coming in from the
encoder. The tachometer was/is an easily "read" way to determine the
speed of the motor.

A calculation is carried out, using the information on RATE and
MAGNITUDE received from the two feedback devices. The result of this
calculation is used to adjust the FEED of electricity to the motor.
This is called "closing the loop" of the servo system.

One popular type of calculation is PID, which stands for
Proportional, Integral, and Derivative.

Now this feedback is "sampled" often so it can keep the motor
precisely controlled. For many years, the "typical" feedback loop,
which consists of getting rate and magnitude feedback, performing the
PID calculation, and adjusting the voltage FEED to the servo motor
amplifier, which in turn controls the motor, was performed
or "updated" 500-1000 times per second. This "update rate" is
measured in cycles per second, which has the given name Hertz,
abbreviated Hz. So it is common to see a servo which has an "update
rate" of 1000Hz.

And that's the basics of an 'old' type servo.

Now that computers are faster, they can easily keep up with the
counts coming in from the encoder. In fact, they even have time to do
something else... So, in the interest of simplifying the system (and
reducing the cost), the encoder pulses are used in a calculation to
derive the RATE from the pulses coming in, which lets the control
system know how fast the motor is turning. The tachometer is no
longer needed for many types of servo sytems, since the encoder is
providing BOTH the RATE and MAGNITUDE feedback for the system.
Servo update rates have also increased dramatically. This is good,
because it means better control and/or higher controllable speeds.

And that is a modern servo system.

But there IS still a place where the tachometer is used. During slow
motor movements, the encoder pulses are coming in very slowly.
The "latency" (time between information arriving) increases to the
point that we can no longer accurately determine the RATE from the
encoder pulses. So the tachometer, which provides ANALOG feedback, is
pressed back into service to provide accurate feedback of slow moving
servo systems.

Which is why you still see tachometers on modern cnc machines. Even
though the computer is fast enough to make the calculations, the math
is "data-starved" by the slow encoder pulses. The tachometer is a
constant source of feedback information, so latencey is not an issue.

Many servo systems do not change directions. The servo controlling
the HUGE roll of paper literally flying through a modern newspaper
printing press only needs to go one direction, but its control must
be very precise, and at a very high speed. In modern servo systems
where direction feedback is needed, it is usually provided by the
encoder.

Hope this helps.

Ballendo

--- In CAD_CAM_EDM_DRO@y..., "sprooney797" <prooney797@a...> wrote:
> I understand how DC Motors with encoders can be servo controlled,
> but I am not sure how DC motors with tachs are used.
>
> I also noticed at work that our Bridgeport EZ-Track has SEM DC servo
> motors that have both a Tach and an Encoder on them. Why are both
> needed? And how is this used in a control scheme?
>
> Any hints or info would be appreciated.
>
> Sincerely,
> Paul Rooney