SLVAFF1 January   2023 DRV8452 , DRV8462

PRODUCTION DATA  

  1.   Abstract
  2.   Trademarks
  3. 1Power Efficiency of Stepper Motor Drivers
  4. 2Auto-Torque
    1. 2.1 Auto-Torque: Learning Principle
      1. 2.1.1 Configuring Auto-Torque Learning Routine
    2. 2.2 Current Control
      1. 2.2.1 Setting Current Control Parameters
    3. 2.3 PD Control Loop
    4. 2.4 Impact of Auto-Torque Tuning Parameters
      1. 2.4.1 Impact of Learning Parameters on Load Transient Response
      2. 2.4.2 Impact of ATQ_UL, ATQ_LL Hysteresis
      3. 2.4.3 Impact of Load Profile on Power Saving
      4. 2.4.4 Adaptive ATQ_UL, ATQ_LL
      5. 2.4.5 PD Parameter Dependency Curves
        1. 2.4.5.1 Dependency on KP
        2. 2.4.5.2 Dependency on KD and ATQ_D_THR
        3. 2.4.5.3 Dependency on ATQ_FRZ and ATQ_AVG
        4. 2.4.5.4 Dependency on ATQ_ERROR_TRUNCATE
      6. 2.4.6 ATQ_CNT at Different Motor Speeds
      7. 2.4.7 ATQ_CNT at Different Supply Voltages
      8. 2.4.8 Motor Temperature Estimation
    5. 2.5 Efficiency Improvement With Auto-Torque
  5. 3Case Studies
    1. 3.1 Application 1: ATM Machines
      1. 3.1.1 ATM Motor Operating Conditions
      2. 3.1.2 ATM Motor With Auto-Torque
    2. 3.2 Application 2: Textile Machines
      1. 3.2.1 Textile Motor Operating Conditions
      2. 3.2.2 Textile Motor With Auto-Torque
    3. 3.3 Application 3: Printer
      1. 3.3.1 Printer Motor With Auto-Torque
  6. 4Summary
  7. 5References

2.3 PD Control Loop

This section explains how the internal PD control loop smoothens the response to sudden load torque transients while minimizing the error.

Table 2-3 describes the major parameters associated with the PD control loop -

Table 2-3 Parameters for PD Control Loop
Parameter Description
KP[7:0], KD[3:0] Proportional and differential gain parameters for the PD control loop.
ATQ_AVG[2:0] The ATQ_CNT parameter is a moving average of ATQ_AVG number of half-cycles. Therefore, a high value for ATQ_AVG slows down the loop response time to a sudden peak load demand, but ensures smooth jerk-free transition to higher torque output. A low value causes the loop to respond immediately to a sudden load demand.
  • 010b - 2 cycle average
  • 100b - 4 cycle average
  • 111b - 8 cycle average
  • Other values : no averaging
ATQ_FRZ[2:0] Delay in electrical half-cycles after which current is changed in response to the PD loop. A small value increases the current quickly to meet peak load demand. This parameter has a range of 1 to 7.
001b - Fastest response time, but the loop can become unstable
111b - Slowest response, but the loop will be stable
ATQ_D_THR[7:0] If error change is less then ATQ_D_THR, then KD does not contribute to correction. KD contributes only when error change is greater than ATQ_D_THR.
For example: if ATQ_D_THR = 10,
If error change is 9, u(t) = KP * e(t)
If error change is 12, then u(t) = KP * e(t) + KD * de(t)/dt
ATQ_ERROR_TRUNCATE[3:0] Number of LSB bits truncated from error before used in PD loop equations. A high value reduces any oscillation in the current waveform.

The PD control algorithm is expressed as -

Equation 5. u(t) = KP * e(t) + KD * de(t)/dt

where,

KP and KD = PD loop constants

  • u(t) = output of controller
  • e(t) = error signal

Guidelines to tune the PD loop parameters are as follows:

  • Set KP = 1, KD = 0, all other PD loop parameters should be at their default values
  • Apply load profile specific to the application
  • If the motor stalls, increase KP, KD and decrease ATQ_D_THR till the motor stops stalling
  • Once the motor does not stall any more, observe the current waveform at constant load torques
  • If the current waveform has oscillations, increase ATQ_FRZ, ATQ_AVG and ATQ_ERROR_TRUNCATE
  • Very high values of ATQ_FRZ, ATQ_AVG and ATQ_ERROR_TRUNCATE can deteriorate load transient response, so it is recommended to check load transient response once more to ensure the PD control loop is stable.

Figure 2-7 is the flowchart for selecting PD control loop parameters.

Figure 2-7 Selecting PD Control Loop Parameters