AD7863ARS-3 datasheet(19/24 Pages) AD

19 / 24 page

AD7863

Rev. B | Page 19 of 24

Vector control of an ac motor involves controlling the 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

transforms the three phase drive currents into separate torque

and flux components. The AD7863 is ideally suited for use in

vector motor control applications.

A block diagram of a vector motor control application using the

AD7863 is shown in Figure 24. 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. VA1 and VA2 of the

AD7863 are used to digitize this information.

Simultaneous sampling is critical to maintaining the relative

phase information between the two channels. A current sensing

isolation amplifier, transformer, or Hall effect sensor is used

between the motor and the AD7863. Rotor information is

obtained by measuring the voltage from two of the inputs to the

motor. VB1 and VB2 of the AD7863 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 information fed back by the AD7863.

DSP

MICROPROCESSOR

DAC

DRIVE

CIRCUITRY

ISOLATION

AMPLIFIERS

VOLTAGE

ATTENUATORS

TORQUE

SETPOINT

FLUX

SETPOINT

*ADDITIONAL PINS

OMITTED FOR CLARITY.

DAC

AD7863*

VA1

VA2

VB1

VB2

TORQUE AND FLUX

CONTROL LOOP

CALCULATIONS AND

TWO TO THREE

PHASE

INFORMATION

TRANSFORMATION

TO TORQUE AND

FLUX CURRENT

COMPONENTS

THREE

PHASE

MOTOR

Figure 24. Vector Motor Control Using the AD7863

MULTIPLE AD7863S

Figure 25 shows a system where a number of AD7863s can be

configured to handle multiple input channels. This type of

configuration is common in applications such as sonar and

radar. The AD7863 is specified with typical limits on aperture

delay. 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.

VA1

VB1

VA2

VB2

VA1

VB1

VA2

VB2

VA1

VB1

VA2

VB2

AD7863

(1)

VREF

AD7863

(2)

AD7863

(n)

ADDRESS

DECODE

ADDRESS

VREF

Figure 25. Multiple AD7863s in Multichannel System

A common read signal from the microprocessor drives the RD

input of all AD7863s. Each AD7863 is designated a unique

address selected by the address decoder. The reference output of

AD7863 Number 1 is used to drive the reference input of all

other AD7863s in the circuit shown in Figure 25. One VREF can

be used to provide the reference to several other AD7863s.

Alternatively, an external or system reference can be used to

drive all VREF inputs. A common reference ensures good full-

scale tracking between all channels.

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