DRV8824: Cannot measure the desired current

Hi Serafino,

Did you start the motor at 50000Hz? Stepper motors typically cannot go from 0 speed to high speed directly.

You can try starting at lower speed (like 12800 or 25600Hz) and increase the STEP rate to 50000Hz.

Please note the motor graphs of output power vs speed is listed at 24V and 48V.

At 12V, less power will be available.

Hi Rick,

I start from 10kHz and increase the frequency woth a step of 1kHz. Across the 50kHz the motor doesn't work well.

Even if 12V should be less than the desired voltage, why there is a different behaviour between the rest situation and moving situation?

Even if I power the driver with 12V, the driver shoul chop the current to 1A, as I set?

Thank you

Hi Serafino,

As the motor is at standstill, the current can be regulated to the desired values. As the motor spins, the back electro motive force (BEMF) acts to create a voltage that reduces the ability to inject the current into the windings. This causes the peak currents to be reduced.

Eventually the motor will stall (stop moving) even though STEP pulses are sent.

If you have a current probe you will see the peak current change as the motor speed increases. 

If you have the ability to increase the VM voltage to 24V, you should see improvements in motor performance.

Hi Rick,

today I switched on my board and I observe that the driver is providing two identical ouput as shown in the figure below:

The signals are AOUT1 and BOUT1.

RESETn, SLEEPn and FAULTn are ok

Should I conclude that the driver is broken?

Hi Serafino,

I am sorry. I missed V3P3OUT is not connected to a capacitor. This is required to maintain the proper voltage for the internal logic.
Please add the recommended 0.47uF, 6.3V capacitor from V3P3OUT to GND.

The outputs are switching, which indicates the device is working. As the speed increases, noise could be affecting operation.

Hi even if I add a capacitor to V3P3OUT, the problem exists anyway...

Hi Serafino,

First we need to get to the proper configuration. Now that the capacitor is added, we can continue debug.

Do you have a current probe? Can you get access to one?

Scope captures of the STEP input and current through the motor coils provides the most information. Can you provide them?

If you do not have a current problem, can you provide scope captures of the STEP, ISENSA, AOUT1, and AOUT2 voltages?

The scope captures should be zoomed out to see a full 360 electrical degrees (128 steps).

Dear Rick,

here attached the picture you asked me.

I used a PWM with 1kHz. The total time sweep should be 128ms but I used a time division of 10ms/div.

I zoomed some pulses to allow you  a better observation.

1. STEP

2.AOUT1

3.AOUT2

4.ISENSE

I fed the pwm signal with a signal generator. I set the pwm signalwithout load with 1kHz and a voltage sweep of 0-3.3V. But when I connect the probe generator to the driver, the voltage sweep changes as you can see.

Hope that these informations are ok for you

Hi Serafino,

The image is a little confusing. The outputs are switching like current is being regulated, but I do not see any voltage on ISENA.

Also can you adjust the STEP so that low voltage is below 0.7V and the high voltage is above 2V (this is per datasheet recommendation).

Can you explain why there is no voltage on ISENA?

As the current is regulating, the peak should reach approximately 680mV (1A).

Ok,

I will try.

About the ISENA, I asked you the same question and you answered me that "As the motor is at standstill, the current can be regulated to the desired values. As the motor spins, the back electro motive force (BEMF) acts to create a voltage that reduces the ability to inject the current into the windings. This causes the peak currents to be reduced.".

I don't know why I can't see anything from the ISENA :)

Hi Serafino,

Without seeing the scope captures, the most likely explanation is the back EMF that was described.

Would you also check the B channel voltages, and provide the scope captures for the B side once the STEP input voltage is changed?

Hi Rick,

I moved on another identical board and I get the following picture

1.Step

2.AOUT1

3.AOUT2

4.ISENSEA

The waveforms on the B-side are identical. and since the coil waveforms haven't a delay of 90°, the motor doesn't move.

Why the driver generate identical waveforms on both coils?

Hi Serafino,

The device does not seem to be responding properly to the STEP input. Please note the outputs reverse direction when the STEP input transitions.

In normal stepper operation, the current should reach the Itrip level and begin regulation. Eventually as the STEP reaches electrical angles 90 and 270 degrees, the outputs should be disabled.

Do you have the 0.47uF V3P3OUT capacitor connected on this new PCB?

Can you provide a photo of the top of the DRV8824 showing all the markings (or provide all the markings)? This will help us identify information about the device.

Hi Rick,

thank you for all the time you spend for me.

unfortunately there was a solder tear that cause a short circuit between step and enable pins.

So explaned that strange behaviour.

Just a last question: I'm using a motor with 400 steps per ring.

I'm sending a STEP signal that is a square wave with 6.4kHz. I can measure the STEP frequency, is there any way to measure the frequency on the motor colis?