Hi all, I have just built a L297/L298 stepper motor driver to drive 2 Size 11 6-wire motors (really small ones!). These are rated at: 3.75V 0.67A per phase. I wired them up in bipolar parallel. I have 2 issues on hand:
1.
The drivers are working OK, however I have problem trying to control the current via the chopper feature via the L297/L298. When I drive the motors with 9V DC, the motors each would easily draw > 2A which would be above the rating. I had to limit the current via my power supply which I do not wish to. (the motors should be powered by batteries in future)
I measured a 0.2V drop across the sense resistors (0.5ohms 5W wirewound). I understand that I should not be using wirewound type, however these are the only ones i have on hand right now.
Adjusting the VREF does not seem to affect the chopping either.
2.
The schematic of my design is attached. I am currently not using the LM1084 adj 5A linear regulator, simply because it cannot regulate the voltage for the motors.
If I set the LM1084 to regulate at 9v, it will drop to 2-3V the moment I plug the motors on the board, and heats up rapidly. I think that I am doing something very wrong here. (Current control vs voltage control?) Do i need larger capacitors for the regulator to work properly?
Thanks for your time in reading my post!
Cannib
Perhaps its too lengthy a post for anyone to reply =( . I will shorten the questions as per below:
I cannot successfully control the motor current via the L297/298 chopper combination.
I really have no idea whats wrong with my circuit (its the sample circuit taken out of the spec sheets.)
Is there anything drastically wrong with using a wirewound sense resistor?
----------------------
Ok I have figured out the VREF after a bit more reading. thx for reading my thread.
Last edited by cannib682; 01-21-2009 at 11:08 PM.
Well you should avoid using Wire wound resistors. Their inductance can mess with the Current sense circuitry. You could use normal 1 Ohm 1 Watt resistor in parallel to obtain the desired sense resistor power rating. Check out www.pminmo.com . There is a working version of the L297/298 Stepper driver complete with Schematic and Board design. It works flawless.
Hope this information helps.
Joe
This question has come up many times. Most of the time the problem is because of the physicalization of the circuit.
Phil, Still too many interests, too many projects, and not enough time!!!!!!!!
Vist my websites - http://pminmo.com & http://millpcbs.com
How have you wired a 6-wire motor up to be bipolar parallel? 6 wire motors are normally used unipolar. they can be used bipolar by using 1/2 of each winding only - this is not bipolar parallel though.Originally Posted by cannib682
define OK. If the circuit isn't working correctly including the current limit then they are not, by definition, working OK.
As already stated, I suspect the problem here is layout + the wirewounds causing some sort of latchup effect. You really need to 'scope the sense lines to see what they look like.
Possibly, it depends on the regulation of the external supply, but it sounds like its going into overheat current limit. What heatsink have you got on the regulator? The regulator will dissipate (Vin - 9) * Imotor watts, with only 12v in thats 6W @ 2A motor current. With no heastsink the thermal resistance to air is about 35degC/Watt so the junction will be at its limit of 150degC at about 4W. You ned a decent heatsink of better then 4degC/W for this regulator.
Last edited by irving2008; 01-24-2009 at 04:18 PM. Reason: tidying up
Thanks for the link. I am currently using his circuit as my reference.
I think I used the wrong term here. It should be called Bipolar serial instead.
The stepper motor has 6 wires - A /A B /B GndA and GndB
I hooked up the Phase lines of the L298 :
Out 1->wire A
Out 2->wire /A
Out 3 -> wire B
Out 4 -> wire /B
the 2 grounds are left floating.
Sorry I did not clearly state my 'OK'. I can control the motor to step correctly, with direction control and enabled or disabled.
After using Joe's recommendation on using Pminmo's circuit, the Vref seems to be working. There is a slight issue with the stepper here:
Lets say when I run a stepper with 9V, without current limit they are happy drawing 1.5A constantly. When I use the chopper to limit the current down to 2 x 0.67A ~= 1.4A, the motor starts giving a squeaky sound while turning.
Does it mean that I should let the motor draw 1.5A instead of 1.4A? The squeaking seems to suggest I am damaging my motors.
Im not too good with my electronics to understand the effects that the additional inductance can contribute, but I had since changed the resistors to non wire wound ones.
I am using an Agilent bench power supply. I think these can regulate pretty well hence I think we can safely assume the issue does not arise from external supply.
My regulator has a smallish T0220 type heatsink. It gets hot to touch but I can leave my fingers on it a couple of seconds. I have not ran the motors with the regulator for extended period of time, as in the 2-3V range the steppers cant provide useful torque.
I experimented with the regulator a little and in order to obtain a range of 9v on the outputs of the LM1084, i need an input of about ~20V-24V. I ran the regulator that way just long enough to find the range. However this range means that it would be dissipating an approx of (15v x 1.4A) = 21W.
I had assumed that the low dropout feature means that I should be able to use a Vout = Vin - Vdropout(1.250) , in which I should be able to use a 12V supply to obtain an output for 9V.
Does the resistance of my voltage divider for the reference pin affect the maximum current I can get out of my regulator?
Thank you guys for all your inputs! I really appreciate your help alot!
Look at the data sheet for the lm1084. There are two thermal considerations - 150degC max junction temp for the output stage and 125degC for the control stage.
A typical small TO220 heatsink is 19degC/W. Assuming you aren't using thermal paste the regulator to heatsink connection is about 2degC/W and the case to junction from the datasheet is 2.7degC/W so total heatsink thermal resistance is 23.7degC/W. Dissipating 4.5W (3v drop x 1.5A) at 20degC ambient gives a junction temperature of 126.6degC. It is likely therefore that the control circuits are going into thermal overload limit and not driving the output stage properly. You need a much bigger heatsink - 3 to 4 times the size you have. If all three motors are running then you will be dissipating 3 x 3 x 1.5 = 13.5W worst case. Assuming junction temperature no more than 80% max = 100degC then the required thermal path is (100 - 20)/13.5degC/W = 6degC/W. Given you already have 3degC/W from the junction to heatsink (assuming the use of thermal paste at 0.3DegC/W) you need a heatsink better than 3degC/W - typically that will be 20sq in or so ( a multivaned one is about a 3" cube).
Your circuit doesnt show the resistor values for the adjustment resistor. The 'adj' terminal is 1.25v below the output and the adj current is 120uA. Assuming a nominal current of 1mA through the divider the upper resistor should be 1.25/1 kohm = 1.25kohm and the lower resistor must be (9-1.25)/(1 + 0.120)kohm = 6.9k. I'd fix the top resistor at 1.2k and use a 5k variable in series with a 3.3k for the bottom one. This will give a finer control than the arrangement you have used.
Personally I dont see why you need the regulator. The voltage to the motors is irrelevant as long as it doesnt exceed the driver voltage capability and thats up to 45v with the L298, what matters is the current control through the chopper. If the chopper is working right then there's no need for an external voltage regulator.
Originally Posted by cannib682
Hi all, I have just built a L297/L298 stepper motor driver to drive 2 Size 11 6-wire motors (really small ones!). These are rated at: 3.75V 0.67A per phase. I wired them up in bipolar parallel.
Originally Posted by irving2008
How have you wired a 6-wire motor up to be bipolar parallel? 6 wire motors are normally used unipolar. they can be used bipolar by using 1/2 of each winding only - this is not bipolar parallel though.
Unipolar 6 wire motors are wound so current flows in opposite directions to reverse direction of the motor. If you hook up the ends and let the center tap float wouldn't the winding fight itself?
That was my point, althought I'd not picked up how he'd actually wired them. Yet he says they work
Hi Irving,
You are right that I do not need the regulator now that the chopper is working. However it bugs me that the regulator doesnt work as it should have, so I am trying to figure out what is the reason. I will try to pick up a larger heatsink to see if it helps.
Actually I am not really sure how the motor works in a bipolar wiring, but its one of the wiring configuration the stepper motor specs listed.That was my point, althought I'd not picked up how he'd actually wired them. Yet he says they work
Some examples can be found here:
http://www.probotix.com/stepper_moto...polar_bipolar/
Anyone has an idea on why my motors screech/scratch when I chop its power? Do I just tune the Vref until it stops screeching or should i clamp the Vref directly at 0.7V (1.4Amp with 0.5 ohms sense resistors) for my motor?
The former method usually allows more than 1.4Amps through the circuit (At least that is what I read off my power supply's current panel)
What you are seeing (hearing) here I suspect is some form of mid-band resonance. Does it occur all the time or at a specific speeds? Chopper driven motors always make some noise even when stationary.
At higher speeds (> 1KHz), the noise is still present, though not as noticeable when its slow (60 odd Hz). The sound goes away once i tune up the Vref though, for all speeds which I tested my steppers with.
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