Delta Tau PMAC2 PCI User Manual download pdf (Page 29)

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101

PMAC2 User Manual

Setting Up PMAC2 for Direct PWM Control 21

PWM Scale Factor: Ix66

Ix66, the PWM Scale Factor, scales the output command values so that they use the PWM circuitry

effectively. The result of the current-loop calculations is a fractional value between -1.0 and +1.0. This

value is multiplied by Ix66 before between written to a PWM command register, where it is digitally

compared to a PWM up/down counter moving between I900+1 and -I900-2 (between I906+1 and -I906-2

for channels 5-8). To utilize the dynamic range of the PWM circuitry well, Ix66 should be set slightly

greater than I900 or I906. Typically, a value 10% greater is used, permitting full-on conditions at

maximum command values over about 1/6 of the commutation cycle.

Ix66 acts as a voltage limit for the motor. If the amplifier is oversized for the motor, exceeding the

maximum permitted voltage for the motor, Ix66 should be set proportionately less than I900 to limit the

maximum possible voltage for the motor. Since Ix66 is a gain, if it its changed, the current loop must be

tuned or retuned afterwards.

Servo Loop Output Limit: Ix69

Ix69 is the limit of the output of the position/velocity servo loop, which is the torque (quadrature) current

command input to the digital current loop. As such, it acts as an instantaneous current limit for the motor.

Open-loop O-commands are expressed as a percent of Ix69. A value of 32,767 (2

-1) for Ix69 for this

parameter is full range, corresponding to a full -1.0 to +1.0 command input range for the current loop,

which corresponds to full-range feedback on the current-loop ADCs.

Current Sampled

at Counter Turnaround

Ix66

-Ix66

-I900-2

I900+1

PWM

Command

Range

PWM

Counter

Range

4xI900+6

PWM_CLK

Cycles

PWM

Up/Down

Counter

New Command

Loaded at

Counter Turnaround

PWM

Command

Deadtime

(I904 or I908)

PWM

Bottom Signal

PWM

Top Signal

Time

PMAC2 Digital PWM Generation (Per Phase)

Figure 10 Digital PWM Generation (Per Phase)

If the ADCs are in the drive, the drive manual should specify the level of current that provides full-range

feedback from the ADCs. The user should then take the instantaneous current limit for the drive or for

the motor, whichever is less, and set Ix69 according to the following relationship:

InstCurrentLimit

FullRangeCurrent

69 32 767 32 768=













min , , * ,

If the drive outputs analog current readings and the ADCs are on the interface board, the full-range

current value must be calculated from the volts-per-amp gain of the current sensing in the drive and the

full-range voltage into the interface board.

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Delta Tau PMAC2 PCI User Manual Page 29

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