DRV10987: Using DRV 10987 Evaluation Board but No control over Motor Speed via Analog/PWM input at speed pin

Hey Cole,

Thank you so much for your sincere reply. It helped a lot !

However, I am still concerned about certain issues:

1) I have calculated Kt (= 400 mV/Hz) as per Tuning Guide, however, as I look into the GUI Display Tab, the calculated BEMF constant Kt displayed is 306.4 mV/Hz and also the speed in rpm displayed is several times higher than the motor speed in rpm

2) The motor start-up time has reduced ( owing to disabled current limit for lock detection ) as it was going through multiple On-OFF cycles. Still it displays Lock Code 0 .

Also further as I enter into the closed loop operation, it immediately shows, Lock code 1 and 2 are also activated.

So, my questions are:

a) Whether it is feasible to disable the Lock Code 0 detection  (i.e on Current Limit) or there is any other way out ?

b)  If the calculated Kt is apt, then why Lock Code 1 and 2 are activated in the closed loop operation.

c)  The rated speed of BLDC Fan Motor is 375 rpm. With EVM and GUI, the maximum speed I could achieve was only 330 rpm. How do I achieve rated speed ?

Please kindly suggest suitable remarks for the same.

Thank You.

Regards,

Pratik

Hey Pratik,

I'm glad it helped a bit. Let me address your new questions:

1.  Also the speed in rpm displayed is several times higher than the motor speed in rpm
   1. Unfortunately, this is a known issue with this version of the GUI, we plan to release a newer version to fix the calculation. If you do the calculation by hand you should be able to look at the electrical frequency box and use the equation to calculate it
      RPM = 120*f_electrical/n_poles
2. Whether it is feasible to disable the Lock Code 0 detection  (i.e on Current Limit) or there is any other way out ?
   
   1. It is feasible to disable the Current Limit (Lock 0) and rely on Over Current Protection (4A Typ.) for current protection. Usually, this is used when customer would like to push higher power in our devices.
   2. With that said, whenever a protection is disabled, the user should always check if the remaining locks will catch any specific unwanted behavior (such as a locked rotor condition)
3. The rated speed of BLDC Fan Motor is 375 rpm. With EVM and GUI, the maximum speed I could achieve was only 330 rpm. How do I achieve rated speed ?   If the calculated Kt is apt, then why Lock Code 1 and 2 are activated in the closed loop operation.
   
   1. Just to be clear, we are talking about those locks appearing after spending some time in closed loop, not necessarily right when the device tries to enter closed loop, right? Lock 1 and 2 are both related to the programmed Kt again, I'd like to just confirm that the Kt_calc (shown on the GUI) closely matches the Kt_prog (the value in manually placed in the box on basic settings tab) leading up to the Lock 1 and Lock 2 activation.
   2. If both Kt's match, then we need to make sure Tcontrol is tuned the RPM right before the triggers occur. If they do not, then manually match the Kt_prog to the Kt_calc.

If this keeps happening then we'll need to gather some waveforms and see what the phase current and voltage is doing.

Best,

-Cole

Hey Cole,

I have tried tuning as per your post.

I hereby list down my observations:

1) During both  Open loop and Closed Loop operation, the actual speed measured and the speed calculated using formula

(speed = 120*f_electrical/n_poles) does not match.

2) Supply Voltage read from GUI is less than the actual supply voltage.

3) Max. motor speed is still 330 rpm.(below rated speed even at max speed input)

Also, I am still facing issues as  mentioned in earlier post related to  programming Kt value.

I attempted it as per  your suggestions in previous posts but still it seems to be not optimal (programmed Kt and Kt viewed in GUI  are unequal) I hereby attach the configuration file.

Kindly do suggest suitable means to optimize Kt value.

Thanks,

PratikTI_com_param2.cfg

Hey Pratik,

1.  During both  Open loop and Closed Loop operation, the actual speed measured and the speed calculated using formula (speed = 120*f_electrical/n_poles) does not match.
   1. I should have been given a little bit more context. We expect up to a 7% mismatch been the actual motor speed and the electrical frequency box in the GUI. If the period box is used, the mismatch goes down below 1% (RPM = 120/(n_poles*T_electrical*10^-6)). Alternatively, measuring FG frequency with an oscilloscope should yield the most accurate value for the speed of the motor (because FG is the actual commutation frequency).
   2. Because of delays in GUI calculation, I trust measuring FG and plotting it over time the most, and then the GUI in closed loop only
2. Supply Voltage read from GUI is less than the actual supply voltage.
   
   1. That is very strange, we've only seen that one other time. Essentially, the device on the EVM was pre-production silicon and some internal registers were not correctly set which lead to strange Kt and a lower supply voltage (like you may be seeing)
   2. As a result make sure the top markings on the IC do not have "P" somewhere to indicate "prototype" silicon (e.g. DRV10987P)
3. I attempted it as per  your suggestions in previous posts but still it seems to be not optimal (programmed Kt and Kt viewed in GUI  are unequal) I hereby attach the configuration file. Kindly do suggest suitable means to optimize Kt value.
   1. You've mentioned the way to try and tune it. If the programmed and measured value are different, keep "chasing" the measured value by changing the Kt value until they are the same

1. Also, I attached some very conservative settings you should continue testing with. It has your Kt, Rm, and Lead Time. We want to make sure other settings are not causing the problem.

TI_com_param3.cfg 

Best,

-Cole

Cole,

Your previous post helped a lot ! Thanks. The Motor works fine.  In reply to the previous post I would like to add:

1) I am able to read correct speed via FG pin.

2) The DRV10987 does not indicate "prototype silicon". I simply followed your suggestion : " If the programmed and measured value are different, keep "chasing" the measured value by changing the Kt value until they are the same ". Now the voltage displayed is as per attached file GUI Screenshot.

Now my only concern is that the motor speed control is limited as follows:

Lower Limit  27.6 Hz  i.e 207 RPM for any GUI Speed Command  below 280

Upper Limit 45.2  Hz  i.e 335 RPM for GUI Speed Command 511

My question is :

How do I control motor for lower speeds or at higher speeds because rated motor speed is 375 RPM ?

Thanks,

Pratik