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DRV8834: Issues about driving stepper motors in indexer mode.

Cherry Zhou
Mastermind 22235 points
Part Number: DRV8834
Other Parts Discussed in Thread: DRV8434

Hi team,

Stepper motor specifications: 2-phase, 1.8 degrees, 4.9 V nominal, 0.7 A nominal, 7 ohm phase resistance, Phase inductance of 5 mh.

The customer intends to use 32 microsteps for control, and now has the following questions:

1. Is it necessary to have the 8834 work in indexer mode, if microstepping control is used? Can other modes implement microstepping?

2. Spec 8.4.2 defines the selection of microsteps, and I currently have the monolithic IO pins directly connected to M0 and M1, so what should the monolithic do with the high-impedance state of M0?

3. Full-Range Current I-TRIP from Specification 8.3.1, dose this I-TRIP numeric mean the output current? Is it needed to configure by the customer? For example, the customer can configure the output current to 1A or the maximum 1.5 A, is that right?

4. continued with question 3, if customer configured I-TRIP to 1.5A, is the current flowing through the winding resistance 1.5A when driving stepper motor?

5. And also continuing with question 3, the 8834 has overcurrent protection, and the spec says the threshold is 2 A, so what is the meaning of setting the I-TRIP?

6. continuing with question 3, the xVREF in the equation should be an analogue, and as stated, I-TRIP should be dynamically adjusted with an analogue input, so that's the case. In the 9.2.3 reference design, xVREF is directly connected to VREFO (fixed value of 2V), whether the I-TRIP cannot be changed if the sample resistance parameters are determined.

7. continuing with the question 6, if the customer wants I-TRIP to be variable, would you recommend using a single-chip DAC to drive xVREF directly? Or use the 4051 analogue switch to select different sample resistors? If choose the former, can one DAC output be multiplexed to both AVREF and BVREF inputs?

8. The 8834 has three decay modes, fast, slow, mixed, and these decay modes, what are the pros and cons when the stepper motor is in operation?

9. continuing with question 8, in Specification 8.3.2 Table 1, which 0% and 100% correspond to slow decay? Also reference the 51 kohm recommended in the design, which is the 50% decay mode, is this corresponding to the mixed decay mode?

10. When the stepper motor starts up, the current is large, and specification 10.1 recommends increasing the tolerance of the decoupling capacitance of the VM. However, once the tolerance is too large, there is an overshoot current when the system is powered up, which may pull down the VM bus voltage. The customer would like to know if there are any empirical or recommended values for the selection of capacitance values in general.

Could you please help resolve this case? Thanks a lot.

Best Regards,

Cherry

  • 0
    TI__Guru 56650 points

    Cherry,

    1. Yes. It is necessary to have the 8834 work in indexer mode if microstepping control is used. In indexer mode, the STEP pin can move the current regulation point on each micro-step. Or, customers can do their own sinusoidal waveform on xVREF pin and control xPHASE pin to achieve the microstepping.

    2. Leaving M0 pin floating can give the high-impedance state of M0.

    3. This I-TRIP numeric means the output current. It is needed to configure by the customer according the motor spec and the load torque condition.

    4. if customer configured I-TRIP to 1.5A,  the current flowing through the winding resistance can be found in the datasheet "Table 4. Current and Step Directions".

    5. The 8834 has overcurrent protection is not related to the I-TRIP setting. They are independent.

    6. Yes. In the 9.2.3 reference design, xVREF is directly connected to VREFO (fixed value of 2V), the I-TRIP cannot be changed if the sample resistance parameters are determined. BTW,  customers can drive xVREF if they want to change the setting during  the operating. 

    7. using a single-chip DAC to drive xVREF or use the 4051 analogue switch to select different sample resistors both are OK to me. Just make sure the xVREF stable.

    8. Check the output current waveform first. And then, use https://www.ti.com/lit/an/slva637/slva637.pdf (2.1 and 5.1adjustment) as reference to adjust the decay mode. 

    9. In 8.3.2, 0% is the slow decay; 100% is the fast decay. Others are mixed decay. 

    10. For a stepper motor, the output current is regulated. The start-up current should not be very large. The input capacitors can absorb the energy sending back from the motor to the VM. For example: in fast decay, the current flows back to the VM during TOFF. Or, the motor is studently brake, please check if the input voltage jumps up. Starts with 47uF input capacitor and then check the VM voltage to see if we need increase or decrease the input capacitor setting.

    Regards,

    Wang Li

  • 0 in reply to Wang5577
    TI__Mastermind 22235 points

    Hi Wang,

    Thank you so much for your patient reply. And here is one additional question:

    11. What does the STEP pin mean to be present? After the fine fraction is changed, the frequency of STEP needs to be adjusted accordingly, so how should the STEP frequency be calculated for the different fine fractions?

    And for the point 8, after checking the link you provided, the customer has some follow-up questions:

    a. How to understand the T-BLANK parameter?

    b. For the 8834 chip, T-BLANK is adaptive, does it mean that the hardware and software are not adjustable at this time? If so, how to tune?

    c. Section 2.2 in slva637.pdf, which says PWM OFF can be extended, is PWM here referring to the input PWM of the STEP pin of the 8834?

    d. Continuing with the point c, is the PWM period fixed for microsteps already determined (for example, microsteps 3200)? If fixed, then the time for PWM OFF is extended, which is definitely to reduce the time for PWM ON, so that Is there a risk that the current will not rise below I-TRIP (assuming the I-TRIP value is equal to the rated current of the winding)? In other words: For a given micro step, to what extent can the PWM OFF be extended to solve the current waveform problem while ensuring that the current can reach I-TRIP during PWM ON?

    e. Section 4.2 in .slva637.pdf points that the T-DECAY parameter can be adjusted, is the time parameter adjusted by hardware or software? How to adjust it?

    Thanks a lot!

    Best Regards,

    Cherry

  • 0 in reply to Cherry Zhou
    TI__Guru 56650 points

    Cherry,

    1. At each rising edge of the STEP input, the indexer travels to the next state in the table (the datasheet table 4).  So, the A bridge and B bridge current setting (sinusoidal waveform) frequency is proportional to the STEP frequency. 

    a. after the power FET switching, the motor driver doesn't sense the current in Tblank to avoid switching noise.

    b. DRV8834 Tblank is not adjustable.

    c. DRV8834 has a fixed PWM frequency. PWM OFF time cannot be extended.

    d. DRV8434 PWM frequency is fixed 42.5kHz and cannot be extended.

    e. only section 2.1 and 5.1 are related to DRV8834 discussion here. DRV8834 doesn't have T-decay.

    Regards,

    Wang Li