Now let's examine the closed-loop step response. Add the following commands to the end of your m-file and run it in the command window. You should generate the plot shown below. You can view some of the system's characteristics by right-clicking on the figure and choosing Characteristics from the resulting menu. In the figure below, annotations have specifically been added for Settling Time, Peak Response, and Steady State.

From the plot above we see that both the steady-state error and the overshoot are too large. Recall from the Introduction: PID Controller Design page that increasing the proportional gain will reduce the steady-state error. However, also recall that increasing often results in increased overshoot, therefore, it appears that not all of the design requirements can be met with a simple proportional controller.

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This fact can be verified by experimenting with different values of . Specifically, you can employ the Control System Designer by entering the command controlSystemDesigner(P_motor) or by going to the APPS tab and clicking on the app icon under Control System Design and Analysis and then opening a closed-loop step response plot from the New Plot tab of the Control System Designer window as shown below.

After that you can right-click on the plot and select Edit Compensator. You can then vary the control gain in the Compensator Editor window and see the resulting effect on the closed-loop step response as shown below.

A little experimentation verifies what we anticipated, a proportional controller is insufficient for meeting the given design requirements; derivative and/or integral terms must be added to the controller.

Recall from the Introduction: PID Controller Design page, adding an integral term will eliminate the steady-state error to a step reference and a derivative term will often reduce the overshoot. Let's try a PID controller with small and . Modify your m-file so that the lines defining your control are as follows. Running this new m-file gives you the plot shown below.

Inspection of the above indicates that the steady-state error does indeed go to zero for a step input. However, the time it takes to reach steady-state is far larger than the required settling time of 2 seconds.

In this case, the long tail on the step response graph is due to the fact that the integral gain is small and, therefore, it takes a long time for the integral action to build up and eliminate the steady-state error. This process can be sped up by increasing the value of . Go back to your m-file and change to 200 as in the following. Rerun the file and you should get the plot shown below. Again the annotations are added by right-clicking on the figure and choosing Characteristics from the resulting menu.

As expected, the steady-state error is now eliminated much more quickly than before. However, the large has greatly increased the overshoot. Let's increase in an attempt to reduce the overshoot. Go back to the m-file and change to 10 as shown in the following. Rerun your m-file and the plot shown below should be generated.

Any suggestions what I should be looking for to try and fault-find this? My gut feeling is that the drive isn't well tuned at this speed and this is perfectly hitting a resonant frequency in the motor itself.

I should say I've got zero experience with this product or chip, the person who designed the circuit is no longer with the company and I'm fairly new here. Documentation is a bit on the sparse side regarding how they came up with the values they have hard coded in to the firmware.

One thing I did try is measuring voltage across the current sense resistors. They are 0.01R for some reason - the CAD files I've found suggest they should be 0.051R. I'm wondering if this is part of the issue - the motor is rated at 2.6A and the gain is set to 20, so at 2.6A the input to the comparator will only be 0.52V. The original value would seem to make more sense as this would give around 2.5V, which seems closer to the numbers I'm seeing in the datasheet.

What I am seeing are a lot of large spikes. Up to around 1V across the resistor, so 10A spikes, but these decay very quickly. Also, despite being set to 16x microstepping, where I do see levels in the current they only seem to be one value positive and one negative. For example: (This is for 100RPM with a 23V power supply)

You mention that increasing speed or voltage helps the problem. Do you happen to have a current probe where we can examine the current in the coil with more precision? I would like to see what it looks like in the "smooth" state vs. the state where you are having trouble.

Is there a particular reason why you chose 100Hz as the switching frequency? Such low frequency can definitely cause noise and vibrations in motor due to the frequency being in the low-end of the audible range. To remove any audible noise and potential motor vibrations, it is recommended to use switching frequencies greater than 20kHz. Increasing the stepping frequency will cause the motor speed to increase. to keep the motor speed the same, you have to adjust the microstepping accordingly to keep the same angular speed. If possible, can you try performing this test?

The waveform looks abnormal which could be the result of not tuning the driver properly. The problem of motor vibration should be fixed after tuning the driver accordingly. I have listed some useful resources for you below for help on tuning stepper motors.

I've been delving in to the firmware and it appears they are setting a torque register value of 30 as standard, with a maximum of 58. This seems low to me, could that also be contributing? (Understand it is linked to the current sense resistor and amplifier gain)

Unfortunately the person who designed this is no longer with the company so I can't ask why they made certain decisions. A 10mR 2W resistor seems overkill for the sort of currents this is handling but maybe they thought they needed to keep it small to reduce power dissipation?

It isn't actually increasing voltage that makes the difference, it is a limited range of voltages that cause this vibration. Lowering it also helps, but I can't understand why only a limited range would cause this. I'm also worried about the current spikes, what could they be a sign of? Or might they just be noise from elsewhere?

Not sure what timeframe you are on with this design, but it might be helpful to purchase an EVM that would allow you to at least rule out hardware issues and the EVM comes with software for easily manipulating the registers. If you are able to change the design, we have integrated parts that you could also try. This would be the DRV8424. It has a STEP/DIR interface like the DRV8711.

The design is actually in production and has been for around two years. I suspect this customer has just hit the sweet spot in terms of pump speed to cause the worst vibration. Unfortunately the person who created the design and his successor who made some changes have both left the company so I have no real reference as to what testing was done.

I did look at the DRV8424 and DRV8434, I'm slightly concerned that they quote a full scale current of 2.5A but the motors we have are 2.6A and other common NEMA23/24 motors are 2.8 or even 3A. I can't spot any devices currently that are rated any higher than these ones, I'm not sure if there are plans in the future to go higher.

Several reasons for this question, main one being the sheer size of a 100uF 50V electrolytic, but also that I wonder if local bulk capacitors near the FETs would be a good idea as they are supplying most of the current to the motor but don't seem to have any capacitance specified to be near them.

You are correct that it makes the most sense to have the capacitance near the drain connections on the external FETs. The DRV8711 itself does not draw much current and a small ceramic of 0.1uF-1uF near the IC + 10uF of ceramic bulk would be sufficient.

One more thing I'm unsure of, am I OK to use one ground for the whole circuit (power and microprocessor logic) or would it be better to have separate planes connected in one place only? I'm wondering if there is scope for switching noise to affect the rest of the circuitry. If so, would you just use a net tie, or would something like a ferrite be better to join them?

Now, I'm trying to tune the driver - it looks like I won't be able to use the same settings for all speed ranges we need to run at, but will need to adjust the settings in software. No big problem, however I can't actually get the drive tuned at higher speeds. It is close enough to a sine wave at slower speeds (up to about 150 RPM), however above about 200 RPM the current is essentially a triangle wave. Nothing I change improves this, some changes make it worse and we end up with something like this, especially towards the top speed of 400 RPM:

Any suggestions what might be causing this and what settings I should be looking to change? Mind you, I've tried all decay modes, a wide range of settings for Decay, Blank and Off times and I can get nowhere near a sine wave at these faster speeds.

I'm wondering if there is something preventing the chip achieving a sine wave, or if how I'm measuring it distorting what is going on. The ACS712 is specified to have 80kHz bandwidth, so I would think this is more than enough to measure the current here? One way in which the circuit varies from the EVM is that it has a form of LC filter on each motor terminal, being a Murata BLM21PG220SH1 and a 47pF capacitor. Would this distort the waveform this much? I'm guessing it is there for EMC reasons but I wonder if it is (a) necessary and (b) optimised for the speed range.

I should also mention that the peak to peak current drops considerably at higher speeds. The 'scope grab above represents less than 2A peak to peak, so 1A chopping current. At say 50Hz this is 5.5A / 2.75A as per design.

(Just to explain by the way, I'm redesigning this product for future production, but also need to try and fix the units already out there. This question relates to the unit that has already been on the market for two years that I've just gained responsibility for.) 152ee80cbc

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