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7.3.12.3 PD Control Loop

Table 7-25 describes the major parameters associated with the PD control loop -

Table 7-25. 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 -

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

(14)

where,

KP and KD = PD loop constants

u(t) = output of controller

e(t) = error signal

• In general, increasing the KP will increase the speed of the control system response.

• However, if KP is too large, the current waveform will start to oscillate.

• If KP is increased further, the oscillations will become larger. The system will become unstable and may even

oscillate out of control.

• Increasing the value of KD will cause the control system to react more strongly to changes in the error term

and will increase the speed of the overall control system response.

• It is recommended to use small value of KD, because the derivative response is highly sensitive to noise.

• When non-zero values of KD is selected, to improve noise immunity of the system, a high value of

ATQ_D_THR should be used.

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

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DRV8461

SLOSE98 – DECEMBER 2022

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