Motoring on with the A3977

Previously I have blogged about how to set up the Allegro A3977 driver chip to suit a particular motor: -

hydraraptor.blogspot.com/2009/07/lessons-from-a3977
hydraraptor.blogspot.com/2009/08/motor-math
hydraraptor.blogspot.com/2009/08/mixed-decay-mixed-blessing

Most boards I have seen using the A3977 and similar chips just have a current adjustment, with all the other values fixed. Unless you strike lucky this is not going to allow accurate microstepping because the off time and PFD need to be adjusted to suit the motor and supply voltage.

A while ago Zach sent me samples of the prototype V3 stepper controller kits and the NEMA17 motors used on the MakerBot. I made up the board using my SMT oven (pizza oven controlled by HydraRaptor, more on that later).



It works well, but the initial component values are not optimum for the motor, so I decided to make a test bench from the older prototype board that I have been experimenting with. I RepRapped a chassis for it with a panel to mount some switches to vary the timing components.



The chassis is one of the biggest parts I have made, not in volume, but in overall expanse. It warped a little, despite being PLA, heated bed coming soon!



The switch on the left must be at least 20 years old and the one on the right more than 40 but they both still work fine. I save all this junk and eventually it comes in handy.

I also have potentiometers on V_ref and PFD, so together with a bench PSU and a signal generator I can vary every parameter.

I knocked up a label on a 2D printer, it's so much easier to make this sort of thing than it was when the switches were born!



Zach has updated the board to have four preset potentiometers to make it fully adjustable. There are test points to allow the pots to be set to prescribed values with a multi-meter.

Vref and PFD can be measured as a voltage, but the two RT values have to be set by measuring resistance with the power off. My multimeter seems to give accurate readings of these despite them being in circuit. A good tip is to measure the resistance with both polarities and if it reads the same either way round then it is most likely the chip is not affecting the reading.

So here is a list of motors and optimised settings: -

MakerBot Kysan SKU1123029 NEMA17

This is the motor that MakerBot use for the axis drive on the Cupcake, details here. It is actually a 14V motor, so is not ideally suited to being driven from a 12V chopper drive. You normally want the motor voltage to be substantially lower than the supply.

You can't run it at its full current because the duty cycle would tend to 100%. With a fixed off-time, the on-time tends towards infinity and the frequency drops into the audio range. In practice I found the maximum current at 12V was 0.3A, any higher and the microstepping waveform was distorted on the leading edge due to the current not being able to rise fast enough.



To maintain the sinusoidal waveform at faster step rates requires the current to be lowered further, 0.25A gives a good compromise. It is not a bad idea to under run steppers anyway, otherwise they can get too hot for contact with plastic.

I used the minimum values for CT and RT, i.e. 470pF and 12K to keep the chopping frequency as high as possible, so that it is outside of the audio range. Not only is this a good idea to keep it quiet when idling, but also you want it much higher than your stepping frequency, otherwise they beat with each other.

The values give a minimum frequency of ~17kHz @ 0.3A and a maximum of ~150kHz on the lowest microstep value. 17kHz is not audible to me, but younger people might be able to hear it. There is still some audible noise at the point in the cycle when both coils have similar currents and so similar high frequencies. The beat frequency, which is the difference of the two, is then in the audio range. It isn't anywhere near as loud as when the chopping is in the audio range though.

I can't see any spec for the maximum switching frequency although a couple of parameters are given at less than 50kHz. I suspect 150kHz is a bit on the high side, which would increase switching losses, but with such a low current compared to the rating of the chip I don't think it is a problem.

One problem I had initially was that the switching waveform was unstable. It had cycles with a shorter on-time than required, which let the current fall until it then did a long cycle to catch up. The long cycle gave a low frequency that was back in the audio range.



I think it was a consequence of the motor needing a very short off-time in order to be able to have the duty cycle nearly 100%. The current hardly falls during the off period, so a little noise due to ringing can trigger it to turn off too early. It is not helped by using the minimum blank time. I fixed it by putting 1uF capacitors across the sense resistors.

The PFD value is best set to 100% fast decay with this motor.

It works better with a 24V supply. The full 0.4A current can be achieved (but it gets much hotter of course) and it maintains microstepping accuracy at higher step rates than it does on 12V.

MakerBot Lin SKU4118S-62-07 NEMA17

This is the NEMA17 that MakerBot used to supply. It is at the opposite extreme compared to the one above, i.e. it is a very low voltage motor, only 2V @ 2.5A. As mentioned before, this causes a couple of issues: -

1. The inductance is so low that the ripple current is significant compared to the lowest current microstep, causing positional errors. OK at 2A, but gets worse with lower currents.
2. It is difficult to get 2.5A from the A3977 without it overheating. The PCB layout has to be very good. The datasheet recommends 2oz copper and four layers. 2A is no problem and that is the maximum with the 0.25Ω sense resistors fitted to the board.

At 2A the motor runs at about 40°C, so just about OK for use with PLA. The chip gets a lot hotter, about 77°C measured on the ground pins.

I used a value of 56K for RT and 2.1V on PFD. To some extent the optimum PFD value depends on how fast you want it to go.

Motion Control FL42STH47-1684A-01 NEMA17

This is the recommended motor for the Mendel extruder, details here. After buying a couple of these a friend pointed out that Zapp Automation do the same motor with dual shafts for about half the price!

This is a high torque motor so it is longer and heavier than the previous two NEMA17s. Electrically it is in the sweet spot for the A3977 with a 12V supply. The A3977 can easily provide the full current and the switching frequency doesn't have wild fluctuations or drop into the audio range.

When microstepped at 1.7A it gets to about 43°C but the chip only gets to 56°C.

I used 39K for RT and 0V on PFD, i.e. 100% fast decay.

I have high hopes for this motor as a replacement for the one above that is in my extruder currently. It should give me almost twice the torque and has the correct sized shaft, i.e. 5mm. The Lin and Kysan motors both have imperial shaft sizes which caught me out as I drilled the worm gear for 5mm thinking NEMA17 specified that, but it must just be the frame dimensions.

MakerBot Keling KL23H251-24-8B NEMA23

This is the motor I used on my Darwin. It has 8 wires so it can be connected in bipolar serial or parallel. Series has the advantage that the full torque can be achieved with 1.7A which is easily within the range of the A3977. Parallel has one quarter of the inductance so torque will fall off with speed four times slower. To get full torque 3.4A is needed but I found 1A was enough for the X and Y axes. I think Z needs more torque but my z-axis uses different motors so I don't know how much.

An RT value of 56K is fine for currents in the range 1-2A. PFD is best at 0v, i.e. 100% fast decay.

Summary

Here is a summary of the motor specifications :-

Motor	Resistance	Max Current	Voltage	Max Power	Holding Torque	Inductance
LIN 4118S-62-07	0.8 Ohm	2.5 A	2.0 V	10.0 W	0.30 Nm
Kysan SKU 1123029	35.0 Ohm	0.4 A	14.0 V	11.2 W	0.26 Nm	44.0 mH
Motion Control FL42STH47-1684A-01	1.7 Ohm	1.7 A	2.8 V	9.5 W	0.43 Nm	2.8 mH
Keling KL23H251-24-8B Series	3.6 Ohm	1.7 A	6.1 V	20.8 W	1.10 Nm	13.2 mH
MakerBot Keling KL23H251-24-8B Parallel	0.9 Ohm	3.4 A	3.1 V	20.8 W	1.10 Nm	3.3 mH

Here are my suggested settings :-

Motor	Current	Vref	CT	RT	PFD
Kysan SKU 1123029	0.25 – 0.3A	0.5 – 0.6V	470pF	12K	0
LIN 4118S-62-07	1 – 2A	2 – 4V	470pF	56K	2.1V
Motion Control FL42STH47-1684A-01	1 – 1.7A	2 – 3.4V	470pF	39K	0
Keling KL23H251-24-8B Parallel	1 – 2A	2 – 4V	470pF	56K	0

Tiny stepper torques big

Having calculated that the tiny stepper and GM17 gearbox combination should be able to drive a pinch wheel, I made a lash up to test the theory.

When you have a 3D printer "lash up" is probably not the right term as quite sophisticated parts can be made easily.



Here it is pulling a spring balance with a piece of HDPE filament.



It got to 10 Kg and then the coupling from the GM17 to the 4mm shaft of the pinch wheel let go.



Not surprising given the torque involved and the fact that it was made with 25% fill. I made it again with 100% fill. I can't remember the last time I made a solid part.



It is coupled to the shaft with a hexagonal steel insert drilled out to 4mm and tapped M3 for a set screw onto a flat on the shaft.



With the 100% fill coupler it easily pulled the scale to the end, i.e 12.5Kg. The motor was powered from 8V (to stop it getting too hot) and stepped at 200pps. With a step angle of 15°, the GM17 default gear ratio of 228:1 and a 13mm pinch wheel that gives a feed rate of: -

200 × 15 / 360 × 1 / 228 × 13 × π = 1.5 mm/s.

That would give an output rate of 54mm of 0.5mm filament per second. I think that is comparable to the rates Adrian Bowyer has reported from a NEMA17, but it only weighs about 60g whereas a NEMA17 is about 200g. There are a lot more parts to wear out though, so a NEMA17 may be a better option. Darwin can easily throw 200g about and HydraRaptor is moving table, so the head weight has little relevance.

I have some NEMA17's arriving this week. I tried one from an old disc drive but it didn't have much torque. I don't know if that was because it had aged in the 20+ years I have had it or whether modern motors are much better.

Double chicken and egg

My spiral saw arrived at the weekend. Although it was dispatched the day after I ordered it, it took another 17 days to get here from Yorkshire. For some reason Business Post could not find the address. Have they never heard of Google maps ?!



Although it was only cheap it does seem to have pretty solid bearings so looks promising for routing. At 600W and revving at 30000 RPM it should rip through most things. I just hope my z-axis motor is strong enough to lift it at 1.6Kg. If it can't then I may have to counter-balance it with a weight and a pulley or else use a bigger motor like this one for example.



I don't have any torque data for it but, at 85mm diameter, it looks beefy.

I plan to make the extruder body by milling this material.



I don't know what it is but it seems very hard and rigid. My best guess is that it is some sort of epoxy type resin with possibly a metallic filler - it has a bit of a sparkle to it. I might be completely wrong though. Anybody know what it is?

The chicken and egg problem is that to make the extruder I need to use the router, but to make the router mountings I could do with using the extruder!

Worth the wait

The XY stage turned out to be a really nice piece of kit. It is an XYR-8080 from NEAT, details here. It also came complete with a pair of MDM7 stepper motor drivers. These are bipolar, constant current, micro-stepping with anti-resonant circuitry and opto isolated inputs, i.e. top of the range. Perhaps I should explain each of these terms :-

Bipolar versus Unipolar drive

Stepper motors usually contain two electromagnets which need to be energised in one of two magnetic polarities, giving four combinations or phases. If this is done with a single coil per electromagnet then the electronics must be able to drive a positive or negative current into each coil. This is bipolar drive and requires four transistors per coil, i.e. eight per motor. Alternatively the coils can be centre tapped which effectively creates two coils per electromagnet wound in opposite directions. One of these can be energised at a time to produce opposite magnetic fields. This only requires one transistor per half coil, i.e. four per motor. The advantage of unipolar is that the electronics are cheaper but as only half of the windings are energised at one time the amount of torque available from a given size of motor is less.

The field produced by an electromagnet and hence the torque of the motor is proportional to the number of turns times the current. The maximum current that can be applied is limited by the maximum allowable temperature rise. The heat generated in the windings is proportional to current squared times resistance. This means that if a unipolar motor is operated in bipolar mode then the maximum current is root two times less because the resistance of the full winding is double that of the half windings. However, the number of turns is doubled so the torque is root two greater.

Constant current versus constant voltage

The simplest way to drive a stepper is to apply a constant voltage to the coils. The problem with this is that when a voltage is applied to a coil the current builds up gradually at a rate proportional to the voltage divided by the inductance until it reaches the steady state defined by the voltage divided by the resistance. This causes torque to fall off with speed because at higher step rates the current does not get time to reach its full value before the next step.

A better driver system is to apply a much higher voltage to the coil to get the current to rise quickly and then turn it off when it reaches the correct value. The current then starts to fall at which point the voltage is applied again. This on / off switching occurs at a high enough frequency to avoid producing audible noise.

Micro-stepping

This is a technique to increase the number of steps per revolution by varying the current in the two windings in a sinusoidal fashion. In this case it increases the number of steps from 200 to 2000 per rev. As the screw threads have half an inch travel per rev this gives me a step size of a 4000th of an inch or just over 6 micrometers. The target for RepRap V1.0 aka Darwin is 0.1mm resolution so I am well within that that! The only downside is that the maximum travel is only 150mm in each direction compared to Darwin's 300mm.

Another advantage of micro-stepping is that it produces smoother running at low speeds.

Anti-resonance

Because the force applied by the motor increases as it is displaced from its resting position it behaves like a spring. This together with the mass of the rotor and the load forms a resonant system. If the step rate gets close to the resonant frequency oscillations build up and the motor gets out of step and / or stalls. This is a major problem with high speed operation of stepper motors. An anti-resonant drive monitors the drive waveforms to detect when resonance starts to occur and adjusts the drive current to dampen it down. This allows the motor to be stepped through its resonant band to achieve higher speeds. Clever stuff!

Opto isolated inputs

The step and direction inputs are electrically isolated from the drive electronics by opto couplers. This avoids heavy motor currents sharing the same ground path as the logic signals, which can cause signal corruption.

The XY stage also includes hall effect limit switches and 2000 step shaft encoders. A great find, the challenge now is to build a machine that does it justice.

Here it is being put through its paces with a signal generator on one axis.

It can easily handle step rates up 6kHz which is about 40mm per second. With a bit of ramp up and ramp down I think it would go well above 10kHz. Also I have the full windings connected for maximum torque. The motors are centre tapped so I have the option of using half the winding. This gives root two less torque but one quarter of the inductance, so should be better for higher speeds if needed.