closed loop, PWM motor speed controller

[darrylp]

hi all unlucky ( or is that lucky ?) SIP/Cosmo owners,

am wondering if their would be much interest in a closed loop PWM motor speed controller ?

i've been thinking of doing one using a PIC, a small LCD display, telling you what speed you've got selected. and of course setting its calibration to the supply voltage, and speed monitoring circuit. to keep it running at your desired speed ( assuming the drive roller is not slipping on the wire... which i cant see a way of monitoring cheaply )

right now, i'd be thinking of using the a fixed voltage supply ( ie. separate power supply - thats what i've got on my cosmo, and its a good worthwhile mod in my opinion. )
i could have a go (later) using the main transformer supplying voltage to this controller, but would need some way to know what voltage that is running so the PWM can do its magic.

i'd use PWM, as its better for the wire speed motor's torque output, would allow a good range from a stall to full voltage of the external power supply to run your wire at some blistering rate if you really wanted to.

I'd build in an output to run a brake relay ( which i've got already fitted ) and the LCD would show the desired speed. I think a light beam being broken by a chopper wheel should be good enough for the speed monitoring, thoughts ? on any other way to do this ?

i'd expect this done on a strip board, not to cost much more than 25 quid ( depending on the PIC i'd end up using ) you would need your own PSU, and relay, and wires from your welder to the various connection points.

simple push buttons to set the calibration speed, and then once in running mode the same buttons to alter speed.

if i got enough inputs spare, then i guess we could impliment a memory for the last used speed on each power setting.

so, seriously who would be up for one ?


this of course could be tailored to run any wire speed motor, with possibly a little bit of work attaching the speed sensor and the wires to the drive motor. and power supply. thinking other makes of welders

this isnt a post to make money, just wondering if / how to go about doing it.

note, I'm not a guy to rush things.... i'd expect realistically to have this done by end of the year, so dont think next week or month....

 

[shenion]

I'd just use a switching controller for a regulated voltage output. Much simpler and good enough for a small MIG.

You can add current feedback like the old tape players used to use. Would need to be calibrated though.

National.com has easy design info on their switching controllers.

I was thinking of doing a PCB with one. Would use on my wire feeder and the power feed screw drive for my lathe.

 

[Luke]

I've done closed loop PMDC motor PID speed control with PWM, using a slotted wheel and IR emitter/detector, with relative coarse, but adequate, resolution. It wasn't for a wire feed, but the performance was fine for the application.

 

[arcpaws]

I've built something pretty much like that for my welder to replace the crappy circuit it came with.

I also control the main contactor with the PIC micro, which allows for a stitch/tack timer and a variable burn-back timer. I don't have anything as complex as an LCD display or buttons though, and they could end up being annoying. I just have normal pots on the front of the welder (including the original wire feed speed adjust) which the micro samples a voltage level from to set it's timers and feed speed. This also means it doesn't need to remember the settings when you turn it off, since they will still be set unless you adjust the knobs. Plus, when I am welding, it's easy and intuitive to grab the feed speed knob and twiddle with it (not that I always know which way to turn it...).

 

[Macleesh]

I've built something pretty much like that for my welder to replace the crappy circuit it came with.

I also control the main contactor with the PIC micro, which allows for a stitch/tack timer and a variable burn-back timer. I don't have anything as complex as an LCD display or buttons though, and they could end up being annoying. I just have normal pots on the front of the welder (including the original wire feed speed adjust) which the micro samples a voltage level from to set it's timers and feed speed. This also means it doesn't need to remember the settings when you turn it off, since they will still be set unless you adjust the knobs. Plus, when I am welding, it's easy and intuitive to grab the feed speed knob and twiddle with it (not that I always know which way to turn it...).

Just found this when searching. This is interesting, it's something I've been pondering over the last few days. There seems to be an opportunity for a generic wire feed controller that could be used to replace faulty PCBs, avoiding the manufacturer's expensive replacements, or to upgrade those machines with well known shortcomings (coughs SIP) in their wire feed systems. On the wish list I had wondered about adding stich/spot timers. Sounds like you're most of the way there already Could be a business opportunity for you !