Lessons from the A3977

Having established that I want to move to a stepper driven extruder I set about designing a new extruder controller for HydraRaptor. I fancied using one of the Allegro micro-stepping chopper drivers.

With these chips there are a few things you can adjust by changing component values, like the off time, minimum on time and percentage fast decay. The data sheet explains what they do and gives the formulas but it's not obvious what you should set them to for a particular motor.

Not having any previous experience with Allegro drivers I decided I needed to knock up an evaluation circuit. Fortuitously Zach had sent me some PCBs a long time ago that were his first version of the Stepper Motor Driver v2.0. They used the PLCC version of the A3977.

PLCC packages were a bit of a halfway house between through hole and surface mount. They have leads which come out of the side and then curl underneath.

They are handy for programmable devices because you can either surface mount them or put them in sockets (which can be either SMT or through hole). The problem with them in this application is that using a socket is not recommended for current and heat dissipation reasons.

That makes the package a worst of both worlds solution. It is big and bulky like through hole parts but still difficult to hand solder because the pins are underneath. The surface mount version of the A3977 is a fine pitch (0.65mm) TSSOP with a heat slug underneath, so again not easy to solder by hand, it really needs to be done by the solder paste and oven / hotplate method.

Zach moved to the A3982 on subsequent versions, which is easy to hand solder because it is in a SOIC package with 1.27mm pitch. It also has a lower external component count. The down side is that it does not do micro stepping and is only 2A rather than 2.5A. I will probably use the A3983 (which is like the A3982 plus micro stepping and in a TSSOP package).

I managed to hand solder the PLCC at my second attempt. My first attempt had a short, which damaged the chip. I damaged the board removing it (with a cutting disk), so I had to start again on a second PCB. Lots of cursing! The lesson is always to meter a PLCC for sorts before powering up as you can't see shorts underneath it.

Here is my test lash up: -



I can set the step rate with a signal generator, vary the supply voltage from 8 to 35V, see the temperature of the chip and look at the current waveform on a scope .

The initial results were disappointing due to a couple of problems: -

The first was that the chopping occasionally had glitches in it. With the motor stationary I could hear it clicking, and with a scope I could see some cycles shorter than they should be. It got worse with higher supply voltages. At low speeds it did not make much difference, but it did lower the maximum speed. I tracked it down to a lack of high frequency decoupling on the 12V rail. I added a 220nF de-coupler close to the chip and the problem went away. Adding it further from the chip actually made it worse.

The next problem was that the microstepping was very uneven. I had noticed that same effect with the z-axis of my Darwin using the $800 microstepping drivers (that I got cheap) that I use on HydraRaptor. At the time I put it down to the small, large step angle tin can motors I was using at the time not being very linear. When I moved to larger 7.5° tin can motors I still had the same problem, and even with the Keling NEMA23 1.8° motors it did not seem right. This puzzled me because they are very similar to the NEMA23 motors on HydraRaptor, which work well with the same drivers. The shaft encoders have the same resolution as the ×10 microstepping and they are always spot on or one count out, so pretty linear.

With the A3977 it is easy to get an idea of the current waveform of the motor by measuring the voltage on the sense resistors. It should be a stepped sine wave like this: -



Regardless of which way the coil is energised, the current flows to ground through the sense resistor, so the waveform looks like a full wave rectified sine wave. The current only flows in the sense resistor when the chopper is in the on state though. In the off state the current is circulating through the coil and the bottom two transistors of the H-bridge, so the current in the resistor is zero. That is why there is a bright line along the X-axis. On the falling edge of the wave you can see the sense current goes negative. That is because the chip switches to fast decay mode. When the chopper is in the off state, instead of short circuiting the coil, it reverses the voltage on it, causing the current to flow backwards through the sense resistor onto the supply rail. It only spends part of the switching cycle in fast decay so you see positive current, a lot of zero and some negative current, hence the relative brightness of the lines. This is a case where an analogue scope gives you more information than a digital one.

Initially the waveform looked like this, it was somewhat distorted: -



The current rises too quickly at the start of the waveform. The chopper has a constant off time (20uS in this case) and varies the current by changing the on time simply by turning it on until it reaches the target value. But, there is a minimum on time of about 1.4uS, called the blanking period. During that time it ignores the current sense signal to avoid false readings due to ringing on the switching waveform. That means there is a minimum mark space ratio of 1.4 : 21.4 in this case. That sets a minimum current, which also depends on the ratio of the supply voltage to the motor voltage. If this minimum current is more than the lowest microstep value (19.5% of the peak for 1/8 steps) then you get a distorted waveform as above, and the steps are uneven.

To fix it you can lower the supply voltage, raise the current setting or increase the off time. The latter reduces the chopping frequency. If it is below about 15 kHz it will be audible when the motor is stationary. It can also start to beat with the stepping frequency when running at high speeds, particularly when micro stepping, as the step rate is n times faster.

This form of distortion is analogous to crossover distortion on a class B audio amp. You can also get the equivalent of clipping if you use a high voltage motor on a low supply voltage. If the current setting is set to a value which is more than the motor will draw when connected to the supply, then the top of the waveform is flattened off and again the microsteps will be uneven.



Yet another form of distortion occurs when running at high speed: -



Here the back EMF from the motor acting as a generator is preventing the current from falling fast enough to follow the sine wave. This can be fixed by increasing the Percentage of Fast Decay, set by the voltage on the PFD pin. If there is too much you get excessive ripple as shown here: -



For a particular speed and motor there is a sweet spot which sounds audibly quieter: -



So setting up a microstepping drive is not straight forward unless you have an oscilloscope. You can tune the PFD by ear though, as this video demonstrates: -

Tuning PFD from Nop Head on Vimeo.

You can also see the other forms of distortion if you attach a long pointer and step it round slowly.

Another lesson is that you cannot simply just set the current to accommodate different types of motor. You really need to be able change the off time and the PFD as well, especially if you use different supply voltages.

So I solved the mystery of why microstepping does not work well with the expensive drives on my Darwin. They are rated at 7A but I am only using them at 1A, I am also using low voltage motors on a 36V supply. I bet it is a constant off time chopper and the minimum current is too high.