Gallery, Projects and General > Project Logs

Quick and Dirty X2 Mill feed

<< < (2/4) > >>

CrewCab:
First class write up Klank, as is Darren's so imho they compliment each other by addressing the requirements from a slightly different view point  :thumbup:

Good on ya both  :nrocks:

CC

klank:
Thank you Gentle men for the encouraging and kind remarks.

I too was a little concerned about the "controlability in a crisis" - as Darren quite rightly remarked.
I bench tested this "H" type kit motor control unit on the Bosch motor and was agreeably pleased.
There is a pronounced "dead-spot" at the middle of the pot's rotation. Provided whatever control knob you use has an easy to see mark, or pointer tip on it (and the knob is mounted with the middle of its travel at exactly 12o'clock), it is fairly quick, positive and obvious to stop the motor. As you approach the centre spot, the speed drops off rapidly, so inadvertantly over controling the pot. beyond 12 o'clock shouldn't result in too much trouble as it takes quite a fair turn to start movement and pick up speed in the other direction. 
I may change the style of my control "knob" to a pointer type one.
This particular Bosch motor runs fairly slowly at "full power" anyway (probably wound for a hoist type application), which should be an advantage in a situation where things get a bit hairy by mistake.
The pot can be turned down to make the motor barely turn over (like one rev in 2 seconds) with no apparent overheating or distress and the ammeter rock steady. I tried to grip the output shaft at this low speed but couldn't check it at all, and still the ammeter barely moved. Very torquey.


I hadn't thought about re-finding the "sweet spot" after stopping - good point. I suppose a simple calibrated dial behind the knob/pointer might help.

There could be a problem though, if the motor is directly switch reversed WITHOUT STOPPING IT FIRST (if you try to keep that sweet spot setting).
I will try and explain - although I am not an electronics expert by any means and please tell me if I am talking rubbish, or this point has been covered and resolved elsewhere:-

Previous to this "H" (kit) controller, I had made one on veroboard, using a simple single chip quad op-amp (rather than a 555 timer) to provide the variable pulses to a single power mosfet. (Used a simple circuit recipe found on the net).
The problem was finding a dpdt. reversing switch rated at 10 amps (to be safe) in order to change motor direction. I tried a relay with a small panel dpdt switch, but life was getting too complicated with the extra wiring and space needed, and instead used a chunky dpdt. "stomp switch" from an old guitar effects unit I had made for one of my sons (in their garage - thrash - band era). I linked the output to a pair of resistor protected led's, back to back (red/green) so as to give a visual indication and avoid confusion over which way the motor was running.
All worked fine, but the Bosch motor quietly screamed with the pulses (?windings/resonance?) at some frequencies.
Not a problem I think. No overheating or anything bad - just a bit weird to hear.
In the "power out" part of the circuit, I had fitted a hefty Schottky diode to protect the Mosfet.
I tested this circuit on the bench, with an ammeter on the power - in - line from my trusty 13v 30amp psu.

I tried the stomp switch first after stopping the motor - no problems, the amps stayed well below 3 or so on restart in the opposite direction as power was fed in, even with loading the output shaft.
I then tried hitting (!) the stomp switch with the motor running at speed (keeping the "sweet spot" setting on the power control pot.). The needle on the ammeter jumped to over 10amps and the schottky diode got somewhat hot under the collar (I had mounted it on a heat sink with the mosfet).
Not good.

I did some background reading and although I do not fully understand it, changing the direction of output of the motor without stopping it first can be BAD!! - Unless the circuitry can withstand the extra amps/heat - even momentarily, and your protection diode has to be good, or you can blow the mosfet and then full power runs straight into the motor - which could be quite exciting if machining at the time. Something to do with ?back emf and inductance as the armature fields collapse/change direction? if the motor polarity is reversed suddenly whilst running at speed.

Maybe someone out there knows more about this.

Anyway, since it seemed good practice to stop the motor before changing polarity (from what I had read), I felt that the "H" type drive and single pot control would be a much better and possibly safer solution - the motor automatically having to be stopped as the pot passes through 12 o'clock, no amp. jump, no heat. It does mean, in either case though, that the "sweet spot setting" cannot be retained at the time of switch over.
If I have got this wrong, please do say so.


John, I think there is a thread re. using a scrapped cordless drill (with the pistol grip sawn off) to directly power the X leadscrew on the MECH forum (I think) - can't find it at present.

Will post a bit more tomorrow if I can couple the motor to the shaft and tidy up the loose ends.

Respects and best wishes.

Darren:
Oops yes, reversing a motor without stopping it can cause some problems, mostly I think because if the motor is still turning without power input then it becomes a generator. (EMF) Then you go and whack voltage in the opposite direction = lots of stress.

I did think about a pulse circuit for mine, but they are not good with inductive loads unless the motor is designed for it. Basically the windings do a dance to the pulses, ie they move. This is not good as the winding insulation won't take long to wear through.

That's as I understand it...?

Glad to hear the voltage knob has a positive center  :thumbup:

For the reverse switch I used a DPDT center off type.

 :thumbup:

klank:
Thanks Darren,

Now I'm being stupid - yes of course, so simple - with a dpdt/centre off switch, no problemo - you switch off automatically as you reverse, just allow a second or so in the "off" position.
 :doh:
I didn't have a toggle switch rated for 10amps - a bit 'spensive to buy new, so used what I had. Got a dead spot in the middle of my brain!

All of my motors are DC magnetic - not field coils - hence (generally) seem o.k. with pwm control?
I suppose cheapo motors only have enamel insulation on the armature windings, but the better ones have some kind of ?epoxy? to stop the wires moving.
I have used that kind of (pulse) control for many years on "Dremel" type low voltage kit and my old model railway.
A (very) few railway motors sang a bit - but generally the motor control was excellent.

sbwhart:
Hi Klank

Very well done  :thumbup: you're clutch system is similar to what I'm intending to do when I fit power feed to my X3, I have difficulties with the elctronicy parts but I have gleaned a bit of understanding from these threads, so keep them comming.

Have fun

Stew

Navigation

[0] Message Index

[#] Next page

[*] Previous page