RE: [CAD_CAM_EDM_DRO] how to safely stall a DC motor

Hi Lance,

In the past I've considered making a tool changer for my Sherline or EMCO 5"
lathe. I got a peek at the EMCO tool changer.

Sounds like you need a small circuit with a PIC (or BASIC stamp). You will
have rotate forward at full power (24v) until the position is sensed, then
rotate backwards a short time (again at 24v), then drop the power (2v) and
hold it.

I don't think you need PWM, although it COULD be done that way. Low duty
cycle would give you some holding torque. How did you measure the 2V?
'Scope or multimeter? They may just be using a series resistor to reduce
the holding current.

I suggest using a circuit similar to that in K158, which will drive a small
bipolar stepper motor. You only need the 1/2 of the transistor section, and
the D-latch. This will control current in one winding, (your motor). This
gives you reversible control of a winding (motor).

http://kitsrus.com/kits2.html

What is still needed is way to reduce the current. Quick and dirty, use a
relay contact to short out a series resistor in the motor supply line for
full current. Holding current would be with the resistor in the circuit.
The opto isolators are a good idea. A simple transistor can drive the
relay. It might also be possible to "parallel" the forward elements of the
H-bridge, and turn on both forward elements to give full power, and only one
for holding (bottom transistor of one side, top transistor of the other
side).

PWM generated by a PIC could control the duty cycle, although this circuit
does not allow for it. Logic would have to be added to "AND" the gate
signals with the PWM signal.

The PIC or Stamp would need a simple (?) program.

OR,

As you suggest, drive a DPDT relay to set direction. Use two additional
relays, (one with a series resistor to limit current) to both turn on and
set the power of the motor. Drive them with the PIC. You'll need an input
to the PIC to tell it to go to the next position, and probably two opto
sensors. One to detect "on station", and another possibly to detect Tool
#1.

Look for a PIC or BASIC stamp experimenter's kit.


Alan KM6VV

> Subject: [CAD_CAM_EDM_DRO] how to safely stall a DC motor
>
> I'm in the process of completely retrofitting an Emco Turn 120 cnc
> lathe. It's got an 8 position tool turret on a ratchet mechanism,
> driven by a small DC gearmotor (24VDC ~15watts) through a worm gear.
>
> In operation, the motor spins the turret forward until the proper
> tool location is found (a small sensor board senses each position)
> then the motor reverses and then stalls out holding the ratchet pawl
> into the ratchet teeth. This in conjuction with the gearmotor and
> worm drive help to hold the turret in postion during machining. Far
> from ideal I know, but at this point I want to make the existing
> setup work rather than redesigning/rebuilding the whole tool turret.
>
> I've found somebody that has my same machine, but in working
> order, and has verified that the motor sees 24VDC while in forward
> operation, and 2VDC while in reverse/stall. From the manual they
> show a basic H-bridge circuit that drives the motor but virtually no
> details. At this point we are guessing that the motor is driven with
> PWM and the 2V we see is about a %10 duty cycle average voltage etc...
>
> So, how do I go about controlling the direction and current of
> this 24V 15watt motor, such that it will see 24V and full current
> during turret indexing, but then be able to reverse and remain
> stalled for LONG periods of time without burning itself out?
>
> Would it be feasible to simply control the motor with a DPDT relay
> that supplies 24VDC for forward operation, and then supply 5VDC and a
> resistor of some value for reverse/stall operation?
>
> I'm not an electronics guru, so designing and building a circuit
> would be very time consuming and difficult for me. I'll be
> controlling everything with Mach 2. Thanks for any advice.
>