stepper motor questions

[SeanHatch]

Hello,

I registered way back in the day! This place is huge now!

Anyway, I'm doing some stepper motor stuff, I don't have time at the moment to read all the theory I'd like to, so I'm just gonna take your word for it .

Some of you are more versed than me, but I've never seen a stepper like this one. 200 ohms/coil, and the voltage is rated at 12V.

1.) If I de-energize all the coils, is the stepper guaranteed to stay where it was most recently moved to? Or must I leave the coils energized?

2.) I think I understand the energization orderings, but I'm not sure of the particulars. Should one coil be energized exactly as the other is de-energized? Should there be down-time here?

3.) In the same vein, what pulse length should I apply? I suspect this isn't a simple answer. I get excited when I see the rotor spinning, but I think it may not be reliable.

4.) If I take a scope and watch the pulses at the stepper input, they are UGLY. I suspect this is abnormal, how can I fix it?

5.) These steppers are used to control a spring-loaded XYZ stage for which the controller is long gone. The controller must have had some tricks up its sleeve, because I have to exceed the voltage an current ratings of the stepper to get the thing to move. Any ideas here?

Quite long-winded. Hope someone can help me out with a few of these, thanks a lot!

 

[MikeMl]

...
Some of you are more versed than me, but I've never seen a stepper like this one. 200 ohms/coil, and the voltage is rated at 12V.

1.) If I de-energize all the coils, is the stepper guaranteed to stay where it was most recently moved to? Or must I leave the coils energized?

I have a couple of 12V steppers, but lower voltages are more common. Higher voltage/lower current coils take too long to establish a current, so low voltage is used for faster switching, L=dI/dt.

Normally, if there is any back torque on the shaft, you have to always have one coil (or one pair of coils) energized. An unpowered stepper has some torgue cogging, but it is not normally utilized. They dont exactly freewheel either...

2.) I think I understand the energization orderings, but I'm not sure of the particulars. Should one coil be energized exactly as the other is de-energized? Should there be down-time here?

Most drive schemes overlap to the next coil(s) in the phase sequence, see half-stepping or micro-stepping.

3.) In the same vein, what pulse length should I apply? I suspect this isn't a simple answer. I get excited when I see the rotor spinning, but I think it may not be reliable.

Depends on rotational speed. Long pulses while starting, shorter as the shaft speed increases. Tailoring the pulses to fit the acceleration is an art...

4.) If I take a scope and watch the pulses at the stepper input, they are UGLY. I suspect this is abnormal, how can I fix it?

The current increases more or less linearly due to the inductance of the winding. There is a back EMF effect as the magnetic rotor moves relative to the stator coils, so as observed with a scope, the voltage is complex.

5.) These steppers are used to control a spring-loaded XYZ stage for which the controller is long gone. The controller must have had some tricks up its sleeve, because I have to exceed the voltage an current ratings of the stepper to get the thing to move. Any ideas here?

Quite likely a higher voltage pulse to get the current established quickly, followed by lower voltage (12V?) to sustain the current for the remaining duration of the pulse.

 

[Mr RB]

Run the steppers from a higher voltage source, 24v or 36v with a 200ohm series resistor on each of the 4 leads.

The steppers should be run as "full wave 2 windings on" see google.

For these old low current unipolar steppers I have often used ULN2003 or ULN2804 driver chips, these have 7 or 8 darlington transistors inside and will work with direct logic lvl inputs.

If you want to use a ULN2804 for each motor I developed a system of "high torque half stepping" which is very simple and gives 400steps/rev (not 200) but requires 8 resistors. This iwll give you doubel the positioning accuracy and less resonances. If you want to do that I can post more info.

 

[AlainB]

Hi,

Are you sure the coils are 200 Ohms per phase? 12 volts at 200 Ohms would give 0,06 Amp. and 0,72 watts. Seem almost impossible to me. But then, I could be proven wrong.

Alain

 

[Sceadwian]

Mr RB what would be the point of using 200ohm resistors and double the voltage? The only thing that's gonna happens is you're gonna waste half the power as heat in the resistors the coils won't 'see' the higher voltage so it won't do anything.

I have a stepper sitting right here on my desk that's 540ohms per coil so I don't think 200 is too much. This one doesn't have a voltage rating on it but I think it's from a printer so it's probably 24 volts.

 

[Mr RB]

Hi Sceadwian, this is a standard "old timers" way fo improving stepper performance. Many of the big industrial drives were built this way.

When the stepper is stopped it still gets the same current as normal, but when it needs to move fast there is twice the voltage available to the inductor so the magnetic field can change much faster giving more power at higher speeds and more power to move from one step to another. It's all because of the inductance, which is critical for high speed stepper poerformance.

The 12v 200 ohm unipolar stepper is typical of the little size17 ones you pull out of old 5 1/4 inch floppy drives. It does seem very low powered for a "XYZ stage".

 

[AlainB]

Please Sceadwian provide the part number of your motor. We can try to find datasheet for it.

I have many dozens of stepper motors, mainly unipolars of all kinds and frankly, I have never seen such figures. It baffles me that a printer stepper motor could be run with 0,05 amp. They must be extremely weak.

I have some small nema 17 rated at 12 volts 0,150 Amp. They are 70 Ohms per phase. I even have one comming from an old floppy drive. It is 73 Ohms per phase.

Alain

 

[Sceadwian]

I think the poster may be measuring the resistance of two coils. I double checked some of mine, it seems I have two basic types, low resistance coils of 1-3 ohms and some between 50 and 100 ohms. That seems a little more reasonable, I'm not sure where I got that 540ohm figure from I think my meter leads were lose =O So now 200 ohms does seem high.

200ohm resistors on each lead with a higher voltage would work to stabilize the current with a higher voltage but it seems like a criminal waste of power and would be better used on a very low resistance stepper coil. I think the poster needs to determine exactly what the resistance of his coils are. I'm guessing if he double checks for the lowest reading he can find it's going to be closer to 100ohms.

If his coils really are 200ohms then 200ohm resistors are going to waste 50% of the power to the stepper and not provide anymore of a kick, it forms a voltage divider, so 24 volts would be effectively 12 volts. It would work better on lower resistance stepper coils where the resistors allow large but not fatal currents to flow at startup and as the impedance of the stepper coils increases the voltage divider effect takes over and lowers the effective voltage the motor is getting.

 

[dougy83]

If his coils really are 200ohms then 200ohm resistors are going to waste 50% of the power to the stepper and not provide anymore of a kick, it forms a voltage divider, so 24 volts would be effectively 12 volts.

You are mistaken. The current increases faster because of the higher voltage across the inductor; thus you will get more 'kick' (torque) when you try to run your stepper faster. If you don't understand why the current increases at a faster rate, try simulating the coil at 12V, and a coil at 24V with the series resistor.

 

[Sceadwian]

What inductance should I use?

 

[dougy83]

Whatever you feel like.

Attached is a sim using a stepper coil modeled as 10mH+200ohms. Supplies are 12V, and 24V + 200ohm series resistor.