DRV8461SPWPR datasheet(47/117 Pages) TI

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ATQ_CNT

Load Torque

AMAX

AMIN

TMIN

TMAX

ATQ_UL

ATQ_LL

I1 < I2 < I3 < I4 < I5

Figure 7-26. ATQ_CNT as a function of load torque

The auto-torque algorithm confines the ATQ_CNT within the hysteretic band defined by the user programmable

ATQ_UL and ATQ_LL parameters, by modulating the motor current, as shown in Figure 7-26.

• If load torque demand increases (T1 to T2), the ATQ_CNT goes above the ATQ_UL threshold, and in

response, the algorithm brings the ATQ_CNT within the band by increasing the current (I3 to I4).

• When load torque demand drops (T2 to T1) and ATQ_CNT goes below ATQ_LL, the algorithm reduces the

current to bring the ATQ_CNT within the hysteretic band (I5 to I4).

The following methodology explains how user should select the values of the current control parameters -

• ATQ_TRQ_MIN is the minimum motor current needed to support the minimum load torque applied to the

motor. To find this parameter -

– Load the motor with minimum load torque (TMIN) and drive the motor with full-scale current (IFS)

– Set ATQ_UL and ATQ_LL to zero and set KP as 1

– Reduce current till the motor stalls

– Note the current (IA) at which the motor stalls

– Set ATQ_TRQ_MIN = 1.1 x IA

• To find ATQ_TRQ_MAX -

– With the motor current at IA, load the motor with maximum load torque (TMAX). The motor will get stalled.

– Start increasing the motor current

– Note the current (IB) at which the motor comes out of stall

– Set ATQ_TRQ_MAX = 1.1 x IB

– Note the ATQ_CNT (AMAX) with current at ATQ_TRQ_MAX and load torque at TMAX.

• For the ATQ_UL -

– Set an initial value of 0.5 x AMAX.

– Apply the load profile (peak load and idle load) specific to the application.

• If the motor stalls, decrease the value ATQ_UL till the motor is no longer stalled.

• If the motor does not stall after applying the load profile, ATQ_UL can be increased till the motor stalls.

– Higher value of ATQ_UL saves more power at peak load, but in case of a fast load transient, the motor

can stall.

– Lower value of ATQ_UL reduces the power saving at peak load, but also reduces the chances of motor

stall and step loss.

• For most applications, a difference of 2 between ATQ_UL and ATQ_LL is a good starting point.

• VM_SCALE bit should be made 1b only after ATQ_UL and ATQ_LL have been set by the user.

The flowchart for selecting ATQ_UL, ATQ_LL, ATQ_TRQ_MAX and ATQ_TRQ_MIN parameters is shown below.

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DRV8461

SLOSE98 – DECEMBER 2022

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