Motor "pulsing" but not stepping continuously in same direction

[cnczo2629]

Ok, ignoramus here. Let me go on the record and say I know very little about controlling stepper motors. I have a Centent CNO142 (CN0142 & CN0143 Microstep Drive) microstep drive controller and a Vexta 1.8 deg 1.4v 3.8A 2-phase stepper motor that I'm trying to get to work together in a standalone application. I pulled these two items from a piece of working equipment, so I know they are compatible. To generate a step input for the CNO142 I built a simple 555-based pulse generator (The 555 Astable Circuit - More Detail) The pulse generator is powered from a 6v DC supply, the CNO142 from a separate 24v DC supply.

When I power everything up, the motor "pulses" almost in place, not moving in one direction. It'll take a step or two one way, then back the other way, then vibrate in place and not seem to move at all. If I adjust the speed of the pulse output on the 555 circuit, the speed of the motor pulse varies accordingly, so I think the CNO142 is correctly seeing the step input and the 555 pulse generator is working OK. But why does the motor not turn in the same direction continuously? I suspect it has something to do with the fact that I have nothing connected to the DIRECTION input of the CNO142, but I'm not sure what to connect it to. I've tried connecting it to both + and - of the pulse generator power supply, but neither made any difference. What am I doing wrong? Thanks!

[doorknob]

Can you put together a diagram showing all of the connections that you wired up to the driver (including power, motor leads, and 555 circuit)?

What component values are you using for the 555 pulse generator?

For initial testing you can connect the Direction input pin to the ground/negative terminal of the power supply used for the 555 circuit.

[cnczo2629]

Sure, here's my hookup diagram. The component values I used are as stated in the schematic. I did try hooking the direction input to the pulse circuit ground, but that didn't seem to make any difference. Thanks!

[rowbare]

Just a WAG. Make sure you set some value for the direction signal according to spec. If it isn't tied internally, it is floating and could lead to strange behaviour.

bob

[doorknob]

A few comments and questions:

1) Your circuit does not show a ground connection between the 0v terminal of the 555 and the Ground pin of the drive, but it should have one.

2) How many wires do your motors have? How are your motors wired?

3) How certain are you of the resistor and capacitor values that you are using on the 555?

4) Are you really using 6 volts on the 555, or 5 volts, or something else?

[cnczo2629]

Thanks doorknob. To answer your questions:

1) You're correct, there is no connection between the 555 power supply ground and the drive GROUND terminal. In reading the CNO142 manual it was not clear that this was necessary, and I thought that terminal was only used in conjunction with the CURRENT SET terminal to set the output current (by placing a resistor across the two)

2) The motor has 4 wires. I pulled both the motor and the drive from a working piece of equipment, and hooked them up exactly as I found them in the original setup. I don't have them in front of me so I can't tell you the wire colors.

3) I'm very certain of the component values. I bought them new just for this project.

4) Yes, really using 6v. A lead-acid 6v, 4A battery, to be exact.

[doorknob]

The documentation for the drive is poor at best.

It is not clear from the documentation which ground pin you should connect to the - terminal of your 6-volt battery. My guess would be pin 12, but maybe they intend it to be pin 1.

It is also not clear what the rated maximum input voltage applied to pin 10 should be. You have 6 volts there, which is probably OK, but 5 volts might make the drive "happier".

My suggestions are:

a) try connecting the - terminal of the 6-volt battery to pin 12.

b) also connect the Direction terminal (pin 8) to pin 12 (for initial testing - later on you can connect that to some kind of direction switch).

c) The documentation states:

Both the DIRECTION and STEP inputs require 16 mA current sink capability.

I do not think that you will be able to sink 16 mA through the 1K resistor R3. Try changing that value to 100 ohms.

d) The documentation makes a special point (at least twice, in fact) of the following:

The logic transition time for the STEP input must be 200 nanoseconds or less.

It is possible that the 555 output pulse does not meet that requirement, but you would need to look at that signal with an oscilloscope to be sure. One way to ensure that you get a fast transition would be to use a "schmitt trigger" circuit between the output of the 555 and the STEP input of the driver - for example, by using one section of a 74HC14 inverter to condition that signal.

e) If you're still not having good results, try putting a current set resistor between terminals 11 and 12 (even though the documentation implies that you can get away with not using a current set resistor there). The documentation is a bit ambiguous in this area as well - I'd try something in the range of 50K to 500K.

[cnczo2629]

Thanks for all the great suggestions. I'll try them tomorrow. FYI, regarding your point e), I did try a current set resistor. There's a chart that indicates 3.3M resistor for a 3.5A parallel motor, which had no effect.

[cnczo2629]

Well, still no progress. I tried most of your suggestions Doorknob, to no avail. Same results in all cases. Except I did not investigate item d) as I have no O-scope. I'm assuming the pulse is being picked up ok by the driver, since the motor does vibrate in sync with the step pulse. Hmmm. Not sure what I'm going to try next.

[doorknob]

Have you tried it with the potentiometer cranked all the way towards the low-resistance end of the scale? Do you have a piece of tape on the motor shaft to monitor the behavior of the motor? If you're microstepping, it takes a lot of microsteps to make a noticeable amount of shaft movement, and the step pulse rate from your 555 is pretty slow.

I'm not sure what else I can recommend (other than trying the 74HC14 schmitt trigger inverter). Without an oscilloscope you're really shooting in the dark...

[wildwestpat]

Sorry you are having problems. Here is my take on what you need to consider:-

1. The drive should not need any external set current resistor. It has an internal limit set and adding external resistors between pins 11 and 12 will increase the current pulsing in the coils. The current rating is that of the motor winding and should not be exceeded as you risk burning out the coils or over heating them which depends on the current and how long they remain connected.

2. It can damage the electronics if the coils become disconnected with the power applied to the driver or if the connections are making doggy contact between winding and the driver module. Always disconnect the power before changing motor winding connections. Also be aware there are some high voltages on the coils which can inflict pain or worse.

3. The CN driver is opto buffered so there is no need for the input power source to be connected to the power supply that drives either the amplifier or the motor.

4. The opto isolator requires 20 ma of current to be sourced for both step and direction. The circuit you have for the 555 timer the input to the CN driver is via a 1K resistor. At best your input current is limited to less than 6 ma. and will be much less due to the volt drop across the diode in the opto isolator. However it is not certain from the data sheet if this is enough to operate the driver but is below the recommended current.

5. IMPORTANT please note that a stepper motor that is sitting on the bench and is not connected to its mechanical load does not necessarily rotate. It may just sit there and twitch when stepped. This is because of the resonances associated with pulsing the current to generate a turning moment against the action of the permanent magnetic field are sufficient to make the rotor move but the magnetic force causes the electronically controlled current to cut off prior to the step to be completed. Result is vibrating rotor or twitching depending on the stepping rate. (Micro-stepping can make this worse as well as reducing the out put torque. Nice sets of curves in the data sheet so thanks for the link.)) With the motor held still try pinching the shaft hard but on no account let the motor body turn and pull the leads out of the CN Driver. The driver might be wrecked if a motor could gets disconnected with the power on.

Hope step five provides a solution for you if not keep posting in this thread. Regards - Pat

[cnczo2629]

Well, I finally threw in the towel and sent an e-mail to Centent support, begging for help. Before I knew it I was on the phone with two of their engineers, who graciously filled me in on the finer points of motor control. Kudos to Centent for outstanding customer service! Pretty fine of them to support a 14 year-old device for a random Internet nutjob like myself. After speaking with them, all signs point to weak pulse output from the 555 circuit as the culprit. (and the fact that the 555 can't sink the required current.)

FYI, relative to other comments in this thread, I've got some feedback from Centent to share should anyone else run into a similar situation.

1) The pin 12 ground connection is for current set only. It is not common with the pin 1 ground and should not be used for any other purpose.

3) There should not be a connection between the 555 circuit power supply ground and the CNO142 power supply ground.

4) If not using the DIRECTION input, it should be connected either directly to the 555 circuit +5, or to the 555 circuit ground via a 100 ohm resistor.

5) If the 555 circuit were going to work at all, R3 should be 100 ohms and not 1k.

Thanks to wildwestpat's last post, which was pretty much on the mark. The guys at Centent also recommended their CNO173 pulse generator (CN0173 Step Pulse Generator), which does everything I was trying to accomplish and more. Thanks to all who provided input for this thread.