AD7864 datasheet(18/19 Pages) AD

18 / 19 page

AD7864

–18–

REV. A

Vector Motor Control

The current drawn by a motor can be split into two compo-

nents: one produces torque and the other produces magnetic

flux. For optimal performance of the motor, these two compo-

nents should be controlled independently. In conventional

methods of controlling a three-phase motor, the current (or

voltage) supplied to the motor and the frequency of the drive are

the basic control variables. However, both the torque and flux

are functions of current (or voltage) and frequency. This cou-

pling effect can reduce the performance of the motor because,

for example, if the torque is increased by increasing the fre-

quency, the flux tends to decrease.

Vector control of an ac motor involves controlling phase in

addition to drive and current frequency. Controlling the phase

of the motor requires feedback information on the position of

the rotor relative to the rotating magnetic field in the motor.

Using this information, a vector controller mathematically trans-

forms the three phase drive currents into separate torque and

flux components. The AD7864, with its four-channel simulta-

neous sampling capability, is ideally suited for use in vector

motor control applications.

A block diagram of a vector motor control application using the

AD7864 is shown in Figure 23. The position of the field is

derived by determining the current in each phase of the motor.

Only two phase currents need to be measured because the third

can be calculated if two phases are known. VIN1 and VIN2 of the

AD7864 are used to digitize this information.

Simultaneous sampling is critical to maintain the relative phase

information between the two channels. A current sensing isola-

tion amplifier, transformer or Hall effect sensor is used between

the motor and the AD7864. Rotor information is obtained by

measuring the voltage from two of the inputs to the motor. VIN3

and VIN4 of the AD7864 are used to obtain this information.

Once again the relative phase of the two channels is important.

A DSP microprocessor is used to perform the mathematical

transformations and control loop calculations on the informa-

tion fed back by the AD7864.

MULTIPLE AD7864s IN A SYSTEM

Figure 24 shows a system where a number of AD7864s can be

configured to handle multiple input channels. This type of con-

figuration is common in applications such as sonar, radar, etc.

The AD7864 is specified with maximum limits on aperture

delay match. This means that the user knows the difference in

the sampling instant between all channels. This allows the user

to maintain relative phase information between the different

channels. The AD7864 has a maximum aperture delay match-

ing of 4 ns.

All AD7864s use the same external SAR clock (5 MHz). There-

fore, the conversion time for all devices will be the same and so

all devices may be read simultaneously. The example shown in

Figure 24 the data outputs of two AD7864s are enabled onto a

32-bit wide data bus when

EOC goes low.

EOC

AD7864

VIN1

VIN2

VIN3

VIN4

ADDRESS

DECODE

VREF

CLKIN

AD7864

VIN1

VIN2

VIN3

VIN4

VREF

CLKIN

ADSP-2106x

REF193

5MHz

Figure 24. Multiple AD7864s in Multichannel System

DAC

TORQUE & FLUX

CONTROL LOOP

CALCULATIONS &

TWO TO THREE

PHASE INFORMATION

DSP MICROPROCESSOR

DAC

DRIVE

CIRCUITRY

PHASE

MOTOR

TRANSFORMATION

TO TORQUE &

FLUX CURRENT

COMPONENTS

–

AD7864*

VIN1

VIN2

VIN3

VIN4

ISOLATION

AMPLIFIERS

VOLTAGE

ATTENUATORS

TORQUE

SETPOINT

FLUX

SETPOINT

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure 23. Vector Motor Control Using the AD7864

AD7864 Datasheet(PDF) 18 Page - Analog Devices

AD7864 Datasheet(HTML) 18 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Datasheet Download

Link URL