I?m trying to use some stepper motors in a project but I?m concerned with the speed it is very slow.
My set up is has follows a timer 555 gives clock signal to a GAL16V8 and it feeds 4 2N3055 I don?t think there is the problem, what do you all think? Some guy told me that was a very primitive driver I don?t really understand why I would look for something more complicated if all I want is to switch between coils? Am I wrong?
Regarding the motor, it was salvaged from an old printer, it works with 24 v and has 48 steps there is no more info on it. From some sites I have found I learned they have a pull out rate and as far I understood it won?t go faster than that, is it true?
What I have done is to supply 24v and increase the pulse rate until it just shakes, is there a why to cheat that max speed I have reached? If I supply 50v would I have double speed? Or would I be able to move a heavier load with tha same speed??
I also read they have a resonance frequency but I don?t think I have hit it because once it loose steps it don?t come back to the working behavior...
Are you supplying two phases? For most steppers, the simplest usable driving waveforms look like
¯¯__¯¯__¯¯__¯¯ and
¯__¯¯__¯¯__¯¯_ . Invert one of the waveforms to reverse direction.
Both of those waveforms are bipolar; they switch between +V and -V. Some motors have center-tapped coils, often with a common center tap for both coils. With the center tap(s) grounded, current is applied to one half of a coil. Switching means applying current to the other half of the same winding, with the other side always open. 48 steps per turn usually means 24 steps of each phase. It is usually true that 4 unipolar phases are simpler to generate than two bipolar phases, so the motors are wound accordingly. There is a simple circuit that uses two flipflops. which accepts a single clock and delivers four outputs to drive 4 transistors. I'll show it if you need it.
That depends on the driver. With simple drivers it is true. The drop-out speed depends on the nature of the load. By ramping up the speed instead of trying to start at the highest speed, you can go fairly fast even with a simple driver.
The motor is rated at 24 V and draws some current there. The winding has inductance and resistance, so the current builds up according to the L/R time constant whenever the current is switched. If you increase the voltage and add external resistance to limit the current to its 24V amount, the motor won't overheat, but the time constant will be shortened. For moderately high performance drives of this type, the external resistor is often double the motor's resistance, so one would use triple the voltage. I would try double the voltage with a resistor equal to the motor to start with.
You are driving an inductive load, so of course you provide damper diodes to absorb the kickback. There are ways to connect those diodes that, with a few extra parts, tend to damp the resonance. The motor is much less likely to shake out of lock as the speed ramps through the resonance region.
You can build a chopper drive. Basically, you use a larger voltage and "chop" it with a 555 and a MOSFET or power bipolar transistor. This is then used as the power source for the driver you have. The effect is an increase in current, which increases speed.
Also, as Jerry said, ramping the speed up and down is very effective in reaching higher speeds. What are you using to control the driver?
Some motors have center-tapped coils, often with a common center tap for both coils. With the center tap(s) grounded, current is applied to one half of a coil. Switching means applying current to the other half of the same winding, with the other side always open. 48 steps per turn usually
phases are simpler to generate than two bipolar phases, so the motors are wound accordingly. There is a simple circuit that uses two flipflops. which accepts a single clock and delivers four outputs to drive 4 transistors.
After some braining I think I understood let see if I got it right. My motor have 6 wires I found 2 of them are common for 2 coils (2 commons
4 coils), so what I have is centered tap coils and I can either make 2 square waves delayed 90° from each other or 4 unipolar phases. OK, what I did was to write a program in a GAL16V8 and the outputs (Q1,Q2,Q3,Q4) polarize transistors (T1, T2, T3, T4). The program makes Q1=V+,Q2=V+,Q3=0,Q4=0, then 0V+V+0, 00V+V+,V+00V+, and so on... (that is because I read that would give more torque). Then I?m making 4 unipolar phases, right? Are the 4 unipolar phases less efficient than 2 phases going form V+ to V-?
speed, you can go fairly fast even with a simple driver Do I have to come up with some kind of acceleration routine? Can I do it by hand? If I do it by hand would I reach a significative lower speed? I ask this questions because as soon I saw it was stuck I slow down and try to speed up, it didn?t make a difference...
resistor is often double the motor's resistance, so one would use triple the voltage. I would try double the voltage with a resistor equal to the motor to start with. I will try it!!
ramps through the resonance region.
Noted.
and "chop" it with a 555 and a MOSFET or power bipolar transistor. This is then used as the power source for the driver you have. The effect is an increase in current, which increases speed.
What I understood is apply more juice during less time to each coil, is that correct? If so, do I still need the resistant that Jerry was talking about? I think I do.
I made a clock with a 555 and it gives the transitions to the gal so it switches the outputs. I have some switches to turn it on-off and direction... very simple.
By the way, I found a similar motor and the label says Stepper motor ID27
4304 171 90101 made by Philips I have guesstimated it will be pulling at max 5 kg... and I'm thinking whether it will be capable... have you worked with this motor?
There is something odd about the way that the high bar (¯) reproduced, so I'll use 1 1nd 0 instead of ¯ and _.
To get the same flux (hence torque) using 4 unipolar phases as with 2 bipolar ones, you use twice the current through half the number of turns. If the bipolar power is I^2*R, the 4-phase power is (2I)^2*R/2; twice as much. So no, it's not as efficient, but the drive is simpler and the motor is usually specified that way.
As to your sequence, I'm not completely sure I follow (especially the "and so on" part, so I'll write it down. Before I do, the wiring is worth discussing. Assuming you use a 24V supply for the 24V motor, connect the center taps to +24, and each coil end through a power transistor to ground. Remember to connect a diode from each transistor's collector to +24V, cathode to + and anode to the collector. The diode will be back biased when the transistor is on, and provide a path for the coil current (it's an inductor, right?) when the transistor turns off. For faster switching, use a higher supply voltage, with a separate resistor in each center tap to limit the current. Both the current build-up and decay will be faster.
I label the phases in sets. A and A' are one coil and B and B' are the other. For the regime called "full stepping" (half stepping is more complicated, has less holding torque, but is smother) this is the sequence table:
Phase A A' B B' 1 1 0 1 0 2 1 0 0 1 3 0 1 0 1 4 0 1 1 0 repeat
Half stepping at double the single-step clock speed may avoid problems with resonance. Stopping on a phase that is also in the full-step sequence maintains holding torque.
In regards to the chopper drive, the point that if you want higher speeds, you need to get the coils to charge faster. The increased voltage gets this to happen. The power must then be switched off in order to avoid applying too much current and blowing out the motor.
The resistors Jerry mentioned are to get the "impedance" of the electronic part of the motor to act more like a resistor and less like a coil. In this way, the coils can be switched faster. This is a much simpler way to get similar results. The only downside is the increased power consumption. If this is not an issue, go with this. If you do use this, just make sure the resistors you use are rated for the power you are going to draw.
I'm sure it will help eventually, but I feel that it's premature now. First, lets get the motor running smoothly with a simple drive.
A straight chopper drive -- essentially a class-D current source -- presents a high impedance to the load. When the motor is driven from a low impedance source, resonance effects are greatly reduced. In troubling cases, I have used a negative-resistance source to offset most of the motor's internal resistance; then resonance goes away. (Thump the cone of a loudspeaker while you listen; you can hear the motional resonance. Thump it again with the terminals shorted, you hear only the paper.) With sophisticated drivers like these, you can expect to achieve speeds at least five times the manufacturer's spec and drive inertial loads considered impossible. "Stutter stepping" is another technique for dealing with mechanical inertia. It's nice to know all this, but foolish to use it just because it's there.
About the resistors: the windings have resistance and inductance. When connected to a voltage source, the current rises asymptotically ti its final value with an L/R time constant. An external series resistor Rext changes that to L/(R+Rext). The larger denominator means a smaller time constant and a faster rise. You pay for this two ways: the supply voltage has to be increased to compensate for the voltage drop in Rext, and the larger resistance undamps the resonance even further.
I used to think I knew everything to be known about stepper motor and I know now that I don?t so yep let?s review the wiring thing...
taps to +24, and each coil end through a power transistor to ground. Remember to connect a diode from each transistor's collector to +24V, cathode to + and anode to the collector. The diode will be back biased when the transistor is on, and provide a path for the coil current (it's an inductor, right?) when the transistor turns off. For faster switching, use a higher supply voltage, with a separate resistor in each center tap to limit the current.
I follow the instruction and it worked!!! My connection was quite different I had collector to V+, coil to emitter, center tap to ground no diode at all... let me chew the differences and if I can't grasp them firmly I'll come back... (before I was very confused about the transistors getting real hot jejeje)
In the sequence thing what you mention as full stepping I used to know it as "high torque stepping" and in the full stepping the order will be
A B A' B
1 1 0 0 0
2 0 1 0 0
3 0 0 1 0
4 0 0 0 1
and it repeats, I don?t think it matters but I just wanted to mention it... I am using "high torque" anyway...
what I am using it for is to move a platform in a x-axis that was originally designed to be operated by hand and it has some points with lots of friction... the motor gets stuck there... if there is enough money I?m going for a more powerful source (right now I have
24v and 1 A) or a gear box... but of course first I?m going to take a part the whole thing and put some oil and stuff ... then surf the net and get some info to see how to make a "chopper".
Do you think it would be more expensive to make my own driver or to buy it? What do you recommend? I need to modify 4 units.
The idea is to use them as switches, so either the current is zero, or they're in saturation (less than half a volt across them).
There must be typos above. In this mode, there is always one side of each coil that is turned on.
I don't use choppers. Instead, I use two supplies and the same current sensor that a chopper would need. I use PNP transistors at power supplies to turn the power on and off, and NPNs to ground to complete an H-bridge. (Using the entire winding produces more torque for the same heat, as I explained earlier.) The low-voltage supply connects to the motor through a diode that is back biased when the high-voltage supply is on. When the motor current rises to the design value, the high-voltage transistor is turned off, and the coil voltage is sustained through the diode and low-voltage transistor. This is a low-impudence connection (a bit more elaboration can remove the diode's impedance, but it doesn't seem to be worth it) that applies strong one-sided damping to the motor. I've never had resonance problems with this drive even driving a fly wheel. 100 volts on a 5-volt motor ramps the current up real quick, and 5 volts at low impedance damps it real good. You shouldn't need anything like that. The motor smokes if the current sensor fails to shut off the high-voltage transistor.
Do you know anything about lapping? Use a little 400-grit emery or norbide or carborundum in the oil, then work the slide back and forth past the tight spot until it loosens. When it's good, take it apart and wash it with a brush and plenty of soap and water, then dry it (a hair dryer is good, spray it with WD-40 (sold as KantRust before WW II) to be sure, wipe that down, re-oil and that's it. (I just did that with a cheap pan-tilt head for my tripod. Now it's inexpensive, but not cheap.)
Ho do you value your time? It might be best to buy surplus
200-step-per-turn motors that do an adequate job with the kind of driver you just built. Way back when, I made a circuit-board driller that way.
Taking care of the mechanical problems will definitley make your job easier. As Jerry suggested, lapping the ways is a good idea, also, if they are too irregular, you can read up on scraping them, but this is not nearly as easy.
Are you modifying the units for sale or personal use? If you're selling them, I'd be sure your design will last. If it's for home use, then you'll probably be making improvements over time anyway. :)
It's about time I scrape the bed of my lathe. If I take up all the play with the gibs near the headstock, the carriage binds as I approach the other end. The tailstock, which bears inside rather than outside the bed, has a similar problem. Taking off a few mils is easy enough. Keeping it straight is hard.
Good advice.
Jerry
-- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Right. I visualize them like 4 coils being A-B-A'-B'--A-B-A'-B'--.... In this way I turn on one at the time, that is why it is
1000-0100-0010-0001-1000-0100-...
current
complete an
supply
sustained
low-impudence
damping to
It useful to know these things... but probably I am gonna review the
200 step-per-turn and the driver kit you suggested before I?m move take a decision...
selling them, I'd be sure your design will last. If it's for home use, then
I work as a teaching assistant in a lab at tecnologico de monterrey in mexico. I talked to the guy who built them and he mentioned a lot of short cuts that were taken to built the units because the tight budget the school has... so the professor assign the units to me to see whether I could improve them getting the stepper motor form old printers and building a driver form thing we already have... before I post the question I just follow one of the practices we teach the students and it worked fine the problems started when I connected the shaft to the all-tread that moves the platform... I didn?t take the unit apart because even in the parts with "normal friction" it performed poorly and I decided to post the question and start looking for another solutions... now the motor performs very good and there are few spots where it just can not do the job...
I think I can do the lapping without having to get everything disassembled... actually I will put it in practice in 2 weeks since the lab will be closed due to summer vacation. I will post the end of the story by the 20-th :)
Not right. There are always two coils on, one associated with each center tap. Putting it differently, each center tap always carries current and except for brief transients, the current never varies and is double what you show. Your sequence develops 71% of the sequence I described. If total motor dissipation were the limiting factor, you could raise the voltage to get the torque back, but with your way, the dissipation is concentrated in 1/4 of the copper instead of 1/2. Since the windings are bifilar, the heat distribution is the same at all and
1/2, but there's no way to get back all of the loss from going to 1/4.
There's certainly no point to going a high-performance drive before you squeeze all you can from an L/R drive.
...
You can start lapping by putting the right grit into the lubricant; that's easy. The hard part is stopping before things get worn out. You have to remove every last trace of grit, and that usually requires disassembly.
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.