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Problem with DRV8825 EVM

Other Parts Discussed in Thread: DRV8825, DRV8834, DRV8824

Dear Sirs,

I designed a circuit with DRV8825 chip to drive bipolar stepper motor with U=3.9V and I=0.6A, step=1.8 deg.

For begining I have bought DRV8825 EVM, installed soft.

I used full step regime, reference value Vref=0.48V (Rsen=0.2 Ohm for this EVM), 1000 PPS, Vm=9V. Other parameters

were in the default state.

But when I switched up the motor it sounds very noisy and warms up!

I connected oscilloscope to the output pins of the EVM and see very strange (for my opinion) picture - the base

signal from B-,B+,A-,A+ was modulated by high frequency pulses (near 31 kHz like switching ITRIP frequency).

Moreover the value of the amplitude of the output signal was 9V, but I hope i would be near 4V (as my motor need). I

tested IsenA, IsenB output and saw that voltage on sense resistors is modulated too.

What can it be? How should I use this IC to control my step motor without this modulation?

Below I attached some pictures.

CH1=IsenA, CH2=Aout1

CH1=Aout1, CH2=Aout2

 

I can increase Vref to stop current control, Vref=1.72 V. Modulation disappeared, but in this case my motor will burn! Below I attach picture with Vref=1.72V

 CH1=Aout1, CH2=Aout2

 Alexey

  • Hi Kozlov,

    What you are observing is what you should be observing. The DRV8825 is a current chopper bipolar stepper motor driver and it is based on the idea that we can PWM the input voltage to obtain a lower voltage (the voltage your motor needs) and at the same time regulate the current to a safe level. I recently posted a blog article depicting this confusing matter, here: http://e2e.ti.com/group/motor/b/blog/archive/2012/01/17/the-5-2v-stepper-motor-and-50v-stepper-driver-paradox.aspx

    That being said, even with current chopping, the motor will get warm. This is just part of the game of driving a stepper. Ideally you do not get it too hot, as that could cause damage but if your output current is below or at the motor's rated current, you are in good shape. Believe it or not, if the motor is driving a load, the motor will get less hot. This is because when there is no load, the great majority of the energy being put into the motor will be transformed into heat. When there is a load, however, most of the energy put into the motor is transformed into kinetic energy and the rest is heat. Of course we are dealing with a non ideal system, so transforming all the electrical energy (P=IV) into mechanical energy (P=wT, where w is the angular velocity and T is the torque) is just not possible.

    Stepper motors will always get hot (or at least warm), which is why they are built to radiate heat as much as possible. Do note that this is just how steppers work because you are always putting a current regardless of whether the load needs it or not. On a DC motor, the current is directly proportional to the actual load being driven.

    The audible noise is caused by the current chopping and is also part of the game. There are ways around it and you can find the answer that I posted yesterday on a very similar question, here: http://e2e.ti.com/support/applications/motor_drivers/f/38/t/161463.aspx  Since you are energizing your stepper with full step, slow decay mode should give you the best response.

    Hope the info helps. Best regards,

    Jose Quinones

  • Hello, Jose

    Thank you for your quick answer! And your excursion into the physics of elementary. 

    But the main question was not in why the stepper motor is warm, but why I see 100% amplitude modulation of current. I expected something like 

    But now I understand that in PWM concept it's impossible if I have high different between Vm_stepper_motor and Vm.

    Thank you!

    In your blog (http://e2e.ti.com/group/motor/b/blog/archive/2012/01/17/the-5-2v-stepper-motor-and-50v-stepper-driver-paradox.aspx) you have written about  DRV8834, but I couldn't find alink  on it. Is this new driver? 

    Alexey. 

     

     

     

     

  • Hi Alexey,

    Glad I was able to help. The graph that you show is what you will see with the DRV8825 and your stepper motor if you look at the winding current with a current probe. The captures that you sent on the first post were for the output voltage and the SENSE voltage. Both of this will look like a PWM because in essence that's what they are. Except that we close the loop by modulating the PWM duty cycle according to the desired current and the measured current on the SENSE pin.

    The DRV8834 is a new device which is coming out pretty soon. Its information will be made available on the wen in the upcoming weeks.

    Best regards,

    Jose Quinones

  • Hi folks,

    I am using a drv8824 and have noticed similar modulation, I take that it's charicteristic of the chip? 

    I also notice that the stepper motor  a KH42HM2R023 consequently emits EMI that can induce noise in colocated but electrically isolated circuits. 

    Is there any advice on how to supress this interference?

  • Good day! 

    As I understand now this modulation  is in the principle  of  work of this driver. So I this controller has PWM and you have low voltage (compare to 9 V of min supply) stepper motor you will have long lag of modulation. The stepper motor will be very noisy. And I think - there will be lots of EMI. 

    I  decided this problem using independed keys on the output of this driver. So driver opens this keys and I can supply any engines with different supply voltage. In 32 steps in 8825 there are still some PWM modulation, but for me it's not critical. 

    Alexey 

  • Hi Alexey,

    Quick response :-)

     

    I have a 24v supply and the modulationis there pretty much all the time.  Slow decay mode helps to reduce it but I need to get rid of it.  I don't quite understand what you mean by 'independent keys' as I am not too bright on harware ... could you explain more please?

     

    Stalis

     

  • Maybe I have incorrect translation from native language) The "key" is the switch, I think it's more correct. 

    This is the scheme for example. Some words are on Russian, but I think it's not the problem for understanding.

    I have 4 V supply engines, so I control by the driver the switches. But this is actual for engines that has supply voltage slower than 9 V..as I think. 

    What type of modulation do you mean? Maybe not deep triangle like shown in manual, it depends on current measuring time of the driver. 

     

    Alexey 

  • The modullation is on voltage signal across the output.  The graph resembles your sawtooth but for voltage instead of current.  This modulation seems to be the source of the EMI that is causing grief to my inputs from a magnetic decode head. 

     

    Stalis

     

  •  

    Hi Stalis and Alexey,

    As has been correctly stated, DRV8824/25 are drivers using what is called a Current Chopping topology. In this driving style we use a higher voltage power supply and then regulate current down to a safer value. In essence, this power stage is like a Buck Converter (although we regulate current and not voltage) where the stepper winding is the inductor and the H Bridge is a synchronous switch output. The SENSE resistor and the SENSE circuitry close the loop.

    The first advantage of a current chopper drive is that the increased input voltage allows the current charging mechanism to be faster, which translates into faster possible commutation speeds.

    The second and most notorious advantage of a current chopping driver is that you are not wasting energy. For example, on an LR Drive you would use a bulky resistor to limit the current and then have the winding sink the rated current. The resistor, however, would represent a massive waste of energy, not to mention its bulkiness represents a fairly large added cost.

    The most efficient (as well as cost efficient) way is to regulate current, but while chopping current some EMI can be generated due to the high speed switching. The truth of the matter is that as it is with most engineering problems, efficiency is indirectly proportional to EMI compliance. For example, to make the power FETs on the H bridge as efficient as possible we have to turn them ON and OFF as fast as possible in order to reduce switching losses and let the current flow through reduced RDSON for a longer period of time. As good as all of this sounds, the EMI implications are pretty bad as the faster we make the rise and falling edges of FET enablement/disablement, the better chance of generating some unwanted EMI we get. So there will always be a tradeoff between power distribution efficiency and EMI compliance.

    Not everything is lost. EMI can be reduced by filtering some of this energy. The most typical venue is to add a snubber circuit in between the power stage and the motor winding. This is often done with passives at the board side. There are two aspects to the typical stepper motor drive snubber that I have seen and used before. One is to use a 10 ohm resistor in series with a 470 pf cap which is connected from output to output on a per winding basis. The other aspect is to add a 1000 pf capacitor from each output to GND.

    I have attached a PDF file discussing some snubber circuits you can start with, depending on motor style. Do note that each stepper drive system is unique and these values may not apply for your particular application and set of conditions. But they should work as a starting point.

    Hope the info helps. Best regards,

    Jose Quinones

    3632.SnubberCircuits.pdf

     

  • Hi Jose,

    Thank you big time!

    I'll go away and digest it all.

    Thanks to you too Alexey,

    Best Regards

    Stalis