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

3.3.1.1 Motor Speed Calculation

To calculate motor speed and verify that ripple counting gives out accurate motor speed, we need to know the actual motor speed first. This is done with the help of the encoder. From Figure 3-5, the encoder gives out 32 pulses in 80.3ms. Since the encoder also gives out 4 pulses per motor rotation, the actual motor speed, is given by:

Equation 6. Motor Speed rpm=3280.3×14×1000×60=5977.58 rpm

From Figure 3-6, the RC_OUT pin gives out 48 pulses in 80.3ms. Since RC_OUT gives out 6 pulses per motor rotation, the calculated motor speed using RC_OUT is given by:

Equation 7. M o t o r   S p e e d   r p m = 48 80.3 × 1 6 × 1000 × 60 = 5977.58   r p m

Thus, ripple counting output matches encoder output during steady state conditions.

Actual Motor Speed Using Encoder Figure 3-5 Actual Motor Speed Using Encoder
Motor Speed Calculated Using RC_OUT Figure 3-6 Motor Speed Calculated Using RC_OUT