speed issues

[sixpence]

The machine I have managed to cobble together out of mdf(project log "She Lives") Has developed a death wish! It moves sooooo slowly. Probably due to a bad combo of electronics, motors software and me!
Help, what do I need to change to improve performance?
1. Master 5 ( motors set at max any more and they don't move the gantry)
2. lead screws :10tpi- 1/2" dia
x=800mm
y=500mm
z=300mm
3. Guide rails:thompson 1/2" diam +linear bearings for x+y
draw slides for z
4.motors: 12v .48A 1.8 deg
from rs components no:191 8340
5. drivers:built from scratch using L298 and L297 chips
from Geoff Williams book "build your own workshop bot(I don't want to do that again , ever) . Using 2 computer power supplies. 5v for the boards and 12v for the motors
6. Running pentium 3 computer.

Accuracy seems excellent but I can cut faster by hand!

[High Seas]

Could be in the setup of the Master 5. I now that MACH2 has a few things that confused me when I was setting up and a similar result - different gear though.
I'm no electronics wizzard others could comment on that aspect.
Jim

[arvidb]

Originally Posted by sixpence *snip* 4.motors: 12v .48A 1.8 deg from rs components no:191 8340 5. drivers:built from scratch using L298 and L297 chips from Geoff Williams book "build your own workshop bot(I don't want to do that again , ever) . Using 2 computer power supplies. 5v for the boards and 12v for the motors *snip*

I'm quite sure the motor/power supply combo is your problem. What's your maximum step rate before the motors stalls?

If you power the windings with the motor's rated voltage, the rated current will flow through the winding, which will give full torque, which is great. This is when the motor is stalled.

But the motor windings have an electric property called inductance, which resists the flow of current; that is, it takes a short moment for the current to build up after you (or rather the driver board) apply the voltage. So as soon as you start to switch the windings (as when the motor starts to turn), because of inductance less current will flow. The faster the motor spins, the less current, which means less torque, and the motor misses steps or even stalls.

If you increase the voltage, the current will rise quicker, and so the motor will give more torque. But now the problem is that when the motor is stalled, too much current flows and the motor overheats.

The solution is to regulate current. Two ways are common: using series connected power resistors or transistors (very inefficient and the resistors are expensive and get very hot), or using a chopper drive.

The chopper drive is fed with many times the motor's rated voltage. It monitors current and when it is lower than optimum, it applies more voltage to make it rise faster. When the current is at the rated value for the motor, it regulates the voltage so that the current doesn't rise any more.

So you probably need different motors, different power supply, and/or different motor drivers.

Arvid

[Graham S]

The motor drivers are choppers (L298/L297) so an increase in drive voltage and preferably a decrease in motor voltage would probably do the trick.

Graham

[ger21]

You ideally (at least on a low budget) want 2-3V motors and at least 24V power supply.

[arvidb]

I didn't know the L297/L298 combination makes a chopper driver. I haven't looked much at steppers. Thanks for the info!

I just took a quick look in the L298's data sheet. Looks like it can take voltages up to 46V. Perhaps a good start would be to switch to a 36V power supply? Then if you are still not happy with the speed, upgrade the motors to lower voltage/higher current ones.

Arvid

[Graham S]

You may find this site useful:

http://hans-w.com/cnc.htm

I made my drivers from his designs and they work well, he now has Eagle files for the PCBs as well.

Graham

You can see my boards at: www.indoor.flyer.co.uk/millingmachine.htm

[sixpence]

thanks for all the info. I just happen to have 7v motors and a 24v power supply that I have to assemble. I'll see how that works. I used the 7 v motors with the 12v power supply and speed has improved but still slow. Next step the 24v power supply and loading windows xp so that I can install Mach 2. I have 3 seperate boards one for each motor.My electronics knowledge is verrry basic at best. How do I wire up the power supply to my boards? Do I just replace the 12v connections with the 24 v? Won't I fry the chips.I am trying to make myself believe that the only stupid Question is the one not asked.

[sixpence]

hope this is clear

[sixpence]

oops! try again, story of my life

[Graham S]

too small to see.

With my drivers you just use a different supply, yours should be the same. Check out the circuit on the above link and compare if you can't upload a bigger version of your schematic.

Graham

p.s. I am using 28v