RE: [CAD_CAM_EDM_DRO] PWM vs. DAC?

Hi Kirk,

A lot of industrial control systems output a +/-10V signal for control.
More often than not, this is created with a D/A chip. There are other
outputs like 4-20mA and sometimes +/-5V is used. That's the signal out of
the controller much like a TTL 5V type output for on/off or if drivers are
on the controller like a Programmable Logic Controller (PLC) then OC (Open
Collector) or PNP (sourcing voltage) transistors or dry contact relays are
used.

Sometimes a controller will also use PWM internally which is integrated
(smoothed) with a filter network to create the 0 to 10V signal. Outside the
box it looks like analog rather than PWM.

Then we get into hydraulic valve control. If the valve driver accepts 0V to
10V as a control signal it may well convert that into PWM to hold the valve
at a specific location or it may use drivers that take the control signal
and amplify it into a 0 to 24V and 1 amp analog drive signal (for example).

Lots of 'IF' and 'AND' and 'OR' so far more information is needed to answer
your question. What type of valves (make, model) of valves are you
controlling? What voltage and current.

Not sure if you meant plain 'brushed' or bushed (as opposed to ball bearing)
motors. And when you use the term servo do you mean the model airplane type
with the 2mS duty cycle or servo motor like a drive for a milling machine
axis?

It's true that PWM control of a brushed or brushless DC Servo motor is more
efficient than an linear amplifier providing say +/-80V at 10A but normally
for industrial control systems you specify three things:

1. What size motor and type such as brushed, brushless and the torque/RPM
needed (might even be a stepper motor).
2. What sort of driver is used for this motor. If brushless it needs to
know where the armature is in order to drive the correct coils; determined
by an encoder or 3 hall effect sensors. If a stepper driver it could be
full step or micro-step.
3. What type of input signal the driver requires to control the motor.

There are drivers that take step and direction inputs to move the motor a
certain distance and they use an encoder on the motor to ensure that the
motor goes where you want. To run at a certain speed the computer driving
the motor takes care of acceleration/deceleration and speed using a control
algorithm like PID. The profile of how they accelerate or decelerate can be
linear or an S curve.

There are also drivers that take a +/-10V input and run the motor at a
certain speed/direction based on that input. If positioning is possible,
the computer driving the motor then usually gets feedback from the motor (or
driven device) as to its position and changes the voltage to hold the motor
there (again using PID). The computer is then also responsible for
acceleration/deceleration and speed control.

It may well be that internally the driver uses PWM control to drive the
motor.

So for more detailed answers you need to provide more detailed
specifications or if you don't know enough contract a consultant to ask and
answer the questions.

John Dammeyer


Automation Artisans Inc.
http://www.autoartisans.com
Ph. 1 250 544 4950

> -----Original Message-----
> From: CAD_CAM_EDM_DRO@yahoogroups.com
> [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of maxnc15
> Sent: Thursday, February 08, 2007 12:42 AM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] PWM vs. DAC?
>
>
> Hi,
>
> I've hired an electronics man in Russia to produce an S-Curve
> controller for a hydraulic robot project. He says PWM (Pulse Width
> Modulation) is much more efficient than DAC (Digital to
> Analog Control)
> which I'm more familiar with to run the valves. The recent
> failure of
> my MaxNC to respond to my computer makes me consider
> integrating a CAM
> retrofit into this project if PWM is also good for servos. Is it?
>
> I've also been told plain bushed DC motors are just as good as servos
> for robot/cnc use but I don't know if they'll also work with PWM or
> need DA. I want to specify hardware that works for everything with
> only software changes. I figure DA could be programmed to do PWM
> through a PIC but I'm trying to think a little over my head here.
> Advice?
>
> Thanks,
> Kirk Fraser
> Clatskanie, OR
>
>
>
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