Re: how to safely stall a DC motor

One of these days I'm going to have to learn about PIC's and all the
cool things you can do with them. However, for the time being, I
think the relay and resistor idea seems to be the easiest for me.

I'm still open to ideas though...

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Alan Marconett" <KM6VV@a...>
wrote:

> 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.
> >