MCF8316AEVM: Low speed and high torque rotation of low power motor

Kobayashi Tetsuya

Part Number: MCF8316AEVM
Other Parts Discussed in Thread: MCF8316A, MCT8316Z

Hello,

I asked and solved the problem that the motor does not rotate well in the closed loop.
But I still have some problems.
(1) Motor does not rotate stably at low speed (30 [fps])
When the speed drops below about 40 [fps], it often stops.
(2) If the torque load fluctuates, the motor cannot maintain its speed.

Especially low speed rotation is difficult with my low power motor.
My motor has an encoder, but it doesn't have A, B, C phase Hall sensors...

regs_220128_temp1.txt

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{
  "signature": "oneui-register-data",
  "data": [
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        "id": "isd_config",
        "value": "0x04638C20"
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        "value": "0x283AF064"
      },
      {
        "idx": 2,
        "id": "motor_startup1",
        "value": "0x0A8407D0"
      },
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

{
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        "idx": 14,
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        "value": "0x00000000"
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        "value": "0x00000000"
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        "idx": 22,
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        "idx": 25,
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        "idx": 26,
        "id": "speed_ref_open_loop",
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        "idx": 28,
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        "idx": 31,
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        "idx": 32,
        "id": "speed_fdbk",
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        "idx": 33,
        "id": "sin_commutation_angle",
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        "idx": 34,
        "id": "cos_commutation_angle",
        "value": "0x00000000"
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    ]
  ]
}

Thanks,
Tetsuya K

over 3 years ago

0 Vishnu Balaraj over 3 years ago

TI__Mastermind 24415 points

Hi Tetsuya-san,

Can you tune the Speed loop PI controller at lower speeds to fix both issues?

Regards,

Vishnu

0 Kobayashi Tetsuya over 3 years ago in reply to Vishnu Balaraj

Prodigy 20 points

Hi Vishnu,

Thank you for your continued support.

When I changed the Speed loop PI value, the motor was able to rotate at low speed.
However, in today's conditions, the rotation was unstable when the motor speed was 30 [fps].
I want to secure a little more margin.
The torque did not change.

Kp	Ki	@30[fps]	Torque	Note
10	1	Stoped	-	Default
30	3	Stoped	No change
6	1	Stoped	No change
5	1	Rotated unstable	No change
5	5	Rotated unstable	No change
5	30	Rotated unstable	No change
5	100	Rotated unstable	No change
10	100	Rotated unstable	No change
10	500	-	-	Not Startuped Over Current
1	1	Rotated unstable	No change

Thanks,
Tetsuya K

0 Vishnu Balaraj over 3 years ago in reply to Kobayashi Tetsuya

TI__Mastermind 24415 points

Hi Tetsuya-san,

What is the current drawn by the motor at 30 fps? Can you disable Dynamic CSA gain and set the CSA gain to 1.2V/A at lower speeds? You should find both parameters under "System level configuration".

Regards,

Vishnu

0 Kobayashi Tetsuya over 3 years ago in reply to Vishnu Balaraj

Prodigy 20 points

Hi Vishnu,

The motor current at 30 [fps] is about 270-350 [mA].

I disabled Dynamic CSA gain and set CSA gain to 1.2 [V / A].
But, it had no effect on the low speed stability and torque of the motor.

Thanks,
Tetsuya K

0 Vishnu Balaraj over 3 years ago in reply to Kobayashi Tetsuya

TI__Mastermind 24415 points

Hi Tetsuya,

It is challenging to tune sensorless drivers at low speeds due to low BEMF. We provided steps to manually tune the speed loop Kp and Ki in the "Guided tuning" section. Can you check the manual tuning (Step A to step H) to tune speed loop Kp and Ki? Check below screenshot.

Regards,

Vishnu

0 Kobayashi Tetsuya over 3 years ago in reply to Vishnu Balaraj

Prodigy 20 points

Hi Vishnu,

I've tried this manual tuning before, but at step F the motor didn't spin in a closed loop.

Then I tried again based on the modified json file, but it still failed in step F.
As a test, I changed [FORCE_IQ_REF_SPEED_LOOP_DIS] in the range of 600 to 1000, but the motor did not transition to the closed loop.

As you say, I find it very difficult to control my low inductance motor without a sensor.
If there is no effective solution, I would like to review the method of motor control.

Thanks,
Tetsuya K

0 Vishnu Balaraj over 3 years ago in reply to Kobayashi Tetsuya

TI__Mastermind 24415 points

Tetsuya,

FORCE_IQ_REF_SPEED_LOOP_DIS in the range of 600 to 1000 is very high. You mentioned that the phase current at 30 fps is around 270-350 mA. Using the below equation, FORCE_IQ_REF_SPEED_LOOP_DIS is 15. Can you try this value?

Iq_ref = (FORCE_IQ_REF_SPEED_LOOP_DIS / 500) * 10

Also, what is the speed variation % seen at lower speeds and how much is the expected variation?

Regards,

Vishnu

0 Kobayashi Tetsuya over 3 years ago in reply to Vishnu Balaraj

Prodigy 20 points

Hi Vishnu,

Oh, I didn't understand the IQ equation.

I can't test today, so I'll try it tomorrow.

By the way, I also don't understand the equation of speed loop Kp in Step H.
・ Pole Pairs: 2
・ Maximum motor speed: 180 [fps]
∴ Maximum electrical speed: 360 [Hz]
Using this value in the equation, Kp is 0 when Iq is 180 [A] or less.

Thanks,
Tetsuya K

0 Vishnu Balaraj over 3 years ago in reply to Kobayashi Tetsuya

TI__Mastermind 24415 points

Hi Tetsuya,

Iq_ref is the actual phase current of the motor. FORCE_IQ_REF_SPEED_LOOP_DIS is an algorithm variable that is scaled to make algorithm computations much easier. In step H, you should be using the FORCE_IQ_REF_SPEED_LOOP_DIS which was calculated in the previous step.

Regards,

Vishnu.

0 Kobayashi Tetsuya over 3 years ago in reply to Vishnu Balaraj

Prodigy 20 points

Hi Vishnu,

I tried manual tuning again but couldn't rotate it well.
The details of the implementation after step E are shown below.

Step E and F
I set [FORCE_IQ_REF_SPEED_LOOP_DIS] to 15, but in today's conditions the value was still high.
I was able to rotate it in a closed loop by lowering it below 15.
Step G
I adjusted the register value so that the motor rotates at the maximum electric speed = 360 [Hz]. Its value is 8.
Step H
Divide this value (: 8) by the maximum speed (: 360 [Hz])
Speed loop Kp = 0.
Step I
Speed loop Ki = 0.
Step J
I enabled the speed loop, but the motor didn't rotate and a large current flowed through it.

Thanks,
Tetsuya K

0 Vishnu Balaraj over 3 years ago in reply to Kobayashi Tetsuya

TI__Mastermind 24415 points

Tetsuya-san,

Small correction. Iq reference that you plug in to the equation 6 should be the actual motor phase current that the motor draws at max speed and not the Force_iq. What is the phase current drawn by the motor at max speed?

Regards,

Vishnu,

0 Kobayashi Tetsuya over 3 years ago in reply to Vishnu Balaraj

Prodigy 20 points

Vishnu,

The phase [A] current drawn by the motor at maximum speed was about 300 [mA] (= 0.3 [A]).
After all, substituting this into equation (6), the Kp value is 0.

Thanks,
Tetsuya K

0 Vishnu Balaraj over 3 years ago in reply to Kobayashi Tetsuya

TI__Mastermind 24415 points

Hi Tetsuya-san,

You mentioned in one of your post that the current drawn by the motor is 270-350 mA at 30 fps. Is 30 fps the max speed? At what speed is the motor speed unsable?

Kobayashi Tetsuya said:
The motor current at 30 [fps] is about 270-350 [mA].

Regards,

Vishnu

0 Kobayashi Tetsuya over 3 years ago in reply to Vishnu Balaraj

Prodigy 20 points

Hi Vishnu,

Sure, I said so, but at step G, the top speed Phase A current was measured less than expected.
I think this is due to manual tuning.

If it is not manual tuning, it was measured as follows
30 [fps]: 270-350 [mA]
180 [fps] (maximum speed): 490-530 [mA]

I also tried when Kp value: 1
The motor did not rotate.

Thanks,
Tetsuya K

0 Vishnu Balaraj over 3 years ago in reply to Kobayashi Tetsuya

TI__Mastermind 24415 points

Hi Tetsuya-san,

I will get back shortly!

0 Vishnu Balaraj over 3 years ago in reply to Vishnu Balaraj

TI__Mastermind 24415 points

Tetsuya-san,

Can you try setting higher PWM output frequency?

Regards,

Vishnu

0 Kobayashi Tetsuya over 3 years ago in reply to Vishnu Balaraj

Prodigy 20 points

Hi Vishnu,

I'm sorry, I said yesterday that manual tuning reduced the phase current, but it seems a mistake.

Last Friday, the motor broke down and I changed it to another one.
I think the decrease in phase current is due to variations in motor characteristics.

And after replacing the motor, it became difficult to rotate.
In today's conditions it is not possible to rotate the motor manually.

So without manual tuning, I increased the frequency of the output PWM.
I raised it from 30 [kHz] to 50 [kHz], but it doesn't seem to affect the stability * of the motor.
At frequencies above 50 [kHz], the motor could not rotate.

* Stability
Normally, the motor rotates at ± 5 [%] of the command speed.
When the speed decreases, it exceeds ± 20 [%].

Thanks,
Tetsuya K

0 Vishnu Balaraj over 3 years ago in reply to Kobayashi Tetsuya

TI__Mastermind 24415 points

Tetsuya,

Can you try setting PWM output frequency to 60 kHz and speed loop Kp and Ki to 1?

The R/L time constant of your motor is very high and the phase current at lower speeds is too low. This makes it really challenging for MCF8316A to spin your motor stably at lower speeds.

Regards,

Vishnu

0 Kobayashi Tetsuya over 3 years ago in reply to Vishnu Balaraj

Prodigy 20 points

Vishnu,

I can't test this week. I will test it early next week.

I would need to consider another way to rotate my motor.

Thanks,
Tetsuya K

0 Vishnu Balaraj over 3 years ago in reply to Kobayashi Tetsuya

TI__Mastermind 24415 points

Tetsuya,

Yes, please check and let us know next week.

Regards,

Vishnu

0 Kobayashi Tetsuya over 3 years ago in reply to Vishnu Balaraj

Prodigy 20 points

Hi Vishnu,

I set the output PWM frequency to 60 [kHz] and the speed loops Kp and Ki to 1.
The motor did not rotate.
When the speed loops Kp and Ki are 1, the motor can rotate if the output PWM frequency is 50 [kHz] or less.
But, low speed stability was not obtained.

Thanks,
Tetsuya K

0 Vishnu Balaraj over 3 years ago in reply to Kobayashi Tetsuya

TI__Mastermind 24415 points

Tetsuya,

How much percentage of the maximum speed is your low speed?

Regards,

Vishnu.

0 Kobayashi Tetsuya over 3 years ago in reply to Vishnu Balaraj

Prodigy 20 points

Vishnu,

My low speed is 16.7 [%] of the maximum speed.
Maximum speed: 180 [fps]
Minimum speed: 30 [fps]

Thanks,
Tetsuya K

0 Vishnu Balaraj over 3 years ago in reply to Kobayashi Tetsuya

TI__Mastermind 24415 points

Tetsuya,

I believe you are working on Pre-production samples. We made significant changes in our algorithm in production samples. Can you try tuning this motor using production samples?

Regards,

Vishnu

0 Kobayashi Tetsuya over 3 years ago in reply to Vishnu Balaraj

Prodigy 20 points

Vishnu,

I found it very difficult to control the low inductance motor without sensors.
My request will be difficult to fulfill, even if you make significant improvements to the algorithm.
My motor has an encoder, so I think the feedback signal from the encoder will facilitate my development.

Thanks,
Tetsuya K

0 Vishnu Balaraj over 3 years ago in reply to Kobayashi Tetsuya

TI__Mastermind 24415 points

Tetsuya,

Thanks for the update! We have MCT8316Z (trap control) which supports Hall sensors. Let me know if you are interested in this device.

Regards,

Vishnu.

0 Kobayashi Tetsuya over 3 years ago in reply to Vishnu Balaraj

Prodigy 20 points

Vishnu,

My motor has encoders, not phase A, B, C Hall sensors.
However, in fact, the method of using the encoder signal as a feedback signal is not common.
I will consider the best solution for my device specifications.

If I come to the following conclusions, I need your help again.
・ Increase the inductance of the motor
・ Equipped with a Hall sensor on the motor

Thank you for your long-term support,
Tetsuya K

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MCF8316AEVM: Low speed and high torque rotation of low power motor