Servo motor

Servo motors are generally DC motors. They are simple devices but can
do incredibly precise movement when used properly. DC motors are used
in servo control because they can have their speed varied in a very
simple way. Simple is good...

How simple is simple, you ask? Well, all we really do is vary the
voltage going to the motor and the speed will change. More voltage
means faster speed. Less voltage means less speed. See, DC motors are
really simple. Controlling them precisely... well that's another story.

Remember our story from last time where we talked about open loop vs
closed loop control. Closed loop control provides feedback to close
the control 'loop'. We tell the motor to move and we use an encoder or
another method to actually verify that we have moved to where we said
we wanted to go.

We 'closed the control loop' by feeding back our actual position to
verify we moved. If we didn't move to where we needed to be because
maybe there was a heavy weight on the conveyor we're turning, for
example, the encoder would let us know. Then we could adjust our
position by sending more pulses to the motor.

A servo motor is an example of closed loop control. The servo motor
typically has an incremental encoder on it's back monitoring the shaft
positions. If the shaft turns then the encoder tells us so. If it
doesn't turn, even though we told it to, we won't get any pulses out of
the encoder. Again, it's a closed loop system.

So, the plc tells the servo controller where to move the motor. The
controller sends the appropriate voltage pulses to the motor and the
motor moves. The encoder on the motor shaft verifies the movement
happened. If we tell it to move at 700rpm (revolutions per minute) and
the encoder only gives us pulses at 650rpm then we send the motor more
voltage to increase the speed to 700rpm.

To get some accurate control of the position and speed of the motor is
where the fun starts. We can't just send it a voltage and expect high
precision in the end. We must constantly monitor and adjust what's
going on in the 'system'.

Generally we are concerned with two situations... starting/stopping
and continuing movement. We refer to the starting/stopping part of the
cycle as the transient state because it's in a... transient state. In
other words the speed of the motor is transitioning (changing) from
stop to movement or movement to stop.

Once the motor is moving along we enter the steady state because we are
no longer transitioning from stop to movement. Rather, we are moving
along at a semi-steady speed.

To work in both 'states' we need to 'compensate' the motor.
Compensating the motor simply means we need to constantly adjust the
motor so it does what we want it to... move correctly.

We compensate the gain and bandwidth of the servo system. The gain is
the ratio of the output signal to the input signal. It's basically
how much the input signal is amplified or increased and effects the
accuracy of our movements.

A large gain allows the servo system to make small movements in a very
accurate way. This is a large feature of servos. The gain let's us get
close to the speed we desire by adjusting the actual speed that we are
moving.

The bandwidth is a measure of it's frequency. By frequency we mean that
it's a measure of how often or fast our system can respond to a change.
The wider or larger the bandwidth of our system is the faster we can
respond to changes.

So, a faster response is a quicker response which, of course will
result in more accurate movement in the end. The more accurate
movement happens because we are able to react to changes quicker. In
this case quicker is better...

In general, more gain and bandwidth are good things for our application
to use. Of course, different servo systems will allow different gains
and bandwidths. They are generally a limitation to the hardware that
we are using and each application will have it's own requirement. But
we only need to note that the system is tuned by the compensation we
provide it.

So, when you're looking for high speed and accuracy in your motion
control consider a servo motor over a stepper motor. You will be
pleased...

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