SLVAFO8A April   2024  – May 2024 DRV8214 , DRV8234

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction: Need for Sensorless Designs
  5. 2Ripple Counting − Concept
    1. 2.1 Ripple Counting Algorithm Details
  6. 3Case Study: Robotic Wheel Drive
    1. 3.1 Robotic Wheel Motor Operating Conditions
    2. 3.2 Tuning Parameters for Ripple Counting
      1. 3.2.1 Resistance Parameters
      2. 3.2.2 KMC and KMC_SCALE
        1. 3.2.2.1 Tuning KMC_SCALE
        2. 3.2.2.2 Tuning KMC
    3. 3.3 Robotic Wheel Motor with Ripple Counting
      1. 3.3.1 Inrush and Steady State Performance
        1. 3.3.1.1 Motor Speed Calculation
      2. 3.3.2 Soft Start
      3. 3.3.3 Loaded Conditions
  7. 4Challenges and Workarounds
    1. 4.1 Low Average Currents
    2. 4.2 Motor Inertia During Stop
    3. 4.3 Inrush
    4. 4.4 High Load Conditions
  8. 5Summary
  9. 6References
  10. 7Revision History

4.2 Motor Inertia During Stop

In this case, the RC_OUT pin does not give out pulses when the current to the motor is turned off either during Hi-z or brake modes. Due to inertia, the motor continues to rotate for some time before coming to a halt, during which ripples fail to be detected. This is due to change in direction of current as observed in Figure 4-4. RC_OUT stops giving out pulses whereas the motor continues to rotate due to inertia.

RC_OUT Stops Pulse Output During Normal Stop Figure 4-4 RC_OUT Stops Pulse Output During Normal Stop

A possible workaround for this case is to use the soft-stop feature (EN_SS = 1b). This allows the motor to come to a halt gradually which enables detection of ripples for a longer time leading to a lesser percentage of missed ripples. Figure 4-5 shows the performance of ripple counting during soft-stop. TINRUSH[15:8] (MSB) was set to 0x08.

RC_OUT Tracking Pulses During Soft Stop Figure 4-5 RC_OUT Tracking Pulses During Soft Stop

As explained in the data sheets of DRV8214 and DRV8234, the soft-start-stop feature is activated only during motor voltage or speed regulation. To use the soft-start-stop feature without speed regulation, use the following steps:

  1. Set REG_CTRL register to 10b (speed regulation mode).
  2. Set W_SCALE to 11b (128 rad/s).
  3. Set WSET_VSET to 0xFF (255).
  4. Set TINRUSH to an appropriate value.

This sets the speed reference value to the highest possible value (255*128 rad/s = 32640 rad/s). Since the upper limit for the duty cycle is 100% at a particular voltage level, this effectively means that the device is unable to regulate speed, thereby allowing the soft-start-stop feature to be used without speed or voltage regulation. If PWMing externally, programming the duty cycle to reduce gradually to 0 resembles soft-stop behavior.