Question about Vref in DRV8811

Hi albabalba,

There is definitely something going on with this system, and with your data I feel confident enough to take a swing at what may be the problem.

First data point is IChop should not vary with VM. But it is donig so... So let's see.

IChop is a factor of VREF and SENSE resistor according to the equation you have correctly used. With a SENSE resistor of 0.1 ohms and a VREF of 0.72V, your ICHOP should be equal to 900 mA.

However, it is very important to understand the DRV8811, like every other stepper drivers employing current regulation, is not a current generator. In other words, it will not try to source the current as derived from the ITRIP equation. If the winding current can reach the ITRIP point, then ITRIP events take place. Otherwise, current is not regulated.

As a result, as much as the ITRIP parameter is a function of VREF and RSENSE, it is also a function of the motor internal resistance. For example, if the motor resistance is 48 ohms, then with 24V you can only go to 500 mA, regardless of what RSENSE and VREF are when ITRIP is set to any current larger than this. In other words, you are saturating your winding long before you reach ITRIP, so ITRIP can not occur.

This is the only scenario in which the observed current can vary with VM, which is one of the behaviors you are observing. In fact, if you see 0.2A @ 24V, we should be able to quickly guess the motor internal resistance is 120 ohms. That is a fairly large resistance for a stepper motor, but if it is a unipolar motor being driven as bipolar, I would imagine it is reasonable. Most of the large internal resistance unipolar motors I have seen would have such a resistance close to 80 ohms. So I would imagine 120 Ohms is not off the chart.

A solution to this problem is to increase voltage, although I think it would be best to change the motor. If the motor has such a large internal resistance, then it was designed to be driven with a power drive without current regulation. If the torque response you require can only be acquired through a 900 mA winding current, then you would need the voltage to be 108V and clearly the DRV8811 can not go that high.

I took a fair number of guesses here, so I may be talking about something else entirely. If you can share some more details, hopefully a schematic and motor information, I may be able to get closer to the root cause. Hope the info helps. Best regards,

JIQ

Hi Jose,

thanks for your anwer...

Bipolar stepper motor i've used has these features:

• Nema 17
• Non captive shaft
• Phase current 1.5A
• 4 leads
• Phase resistance 1.3ohm
• Phase inductance 2.1mH
• Step angle 1.8°

DevBoard i've used is AE-MDL-STPR8811 from avayanelectronics (http://www.avayanelectronics.com/Products/AE-STPR8811/AE-MDL-STPR8815_Schematic.PDF).

Any suggestion?

Hi Albabalba,

I have used this module a few times before. Everything I explained before can be pretty much disregarded as with a 1.3 ohm resistance, there is no way you will saturate the inductance at 200 mA. Plus four wires define a bipolar motor, and most of what I wrote was based on unipolar motors being used as bipolar. But the 1.3 ohms per phase data point quite suffice.

Hence I must ask how you are measuring things. Do you have a current probe at one of the stepper terminals? If you are reading 200 mA, how is this measurement taken?

If you are using the power supply current indicator, I can already tell you this current is not to be trusted. I see a lot of people using the Power Supply current meter as an amperage meter and although this gives you a power supply RMS current, it can not be used to determine instantaneous stepper winding current. Easily at least...

Remember the H Bridge is pretty much a transformation. Power IN will be equal to power OUT. When the power supply tells you there are 200 mA at 24V, this is the same as 4.8W being delivered to the load, in this case the chip and the motor.

Now, the motor only sees 24V for brief periods of time as we are chopping the current. Hence, there is a PWM associated with the voltage the motor sees. As a result, the motor voltage is something less than 24V, or namely 24V * Duty Cycle. I have no idea what this PWM Duty Cycle is as it is derived from motor inductance and DRV8811 settings, but it is safe to assume it is not 24V. Thus, it is also safe to say the current is not 200 mA. In fact, if the voltage the stepper sees is less, the current must be more!

If this is the case, then chances are the stepper is actually seeing the 900 mA you configured the device to deliver. You should then be able to obtain the 4.8W that went into the system by taking into consideration how much power is being delivered to the motor (IV), minus how much power is lost on the driver (I^2*R). I am not saying this is a piece of cake computation or that you should do it, but it is pretty much all the power supply current meter is good for.

The only way to see the actual winding current is by looking with a current probe. I realize these are not easy to come by, not to mention they are quite pricey. But if you have access to this kind of equipment, you should be able to see what the current is truly doing.

If you want to use a multimeter (at the motor winding, not at VM), you will need to take into consideration the current will be measured as RMS. If your sine peak current is 900 mA, then you should see something like .64 A on the multi meter.

Again, I am assuming a few things about your setup, so I may be once again off. Other stuff I would check for:

1. The motor is rated at 1.5A. If you feel the torque or the speed you are getting is not correct, it may be because you are operating at 900 mA, in which case the motor could be operating below the rated curve. It is also important to know what this curve is. If you operate outside of the stepper's curve, I can already tell you that is not going to work well. Steppers are "picky" in that sense.

If your setup is working as expected but you just want to understand why motor current seems to be varying with VM the bottom line will be the method you are using to measure said current. I'll be happy to help you get to the bottom of this so feel free to let me know if there is anything else I can help you with.

Best regards,

Jose I Quinones