Datasheet

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. V
A1
and V
A2
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. V
B1
and V
B2
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
I
C
I
B
I
A
V
B
V
A
ISOLATION
AMPLIFIERS
VOLTAGE
ATTENUATORS
TORQUE
SETPOINT
FLUX
SETPOINT
*ADDITIONAL PINS
OMITTED FOR CLARITY.
DAC
DAC
AD7863*
V
A1
V
A2
V
B1
V
B2
06411-024
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.
V
A1
V
B1
V
A2
V
B2
V
A1
V
B1
V
A2
V
B2
06411-025
V
A1
V
B1
V
A2
V
B2
AD7863
(1)
V
REF
RD
CS
AD7863
(2)
AD7863
(n)
ADDRESS
DECODE
ADDRESS
V
REF
V
REF
RD
RD
CS
RD
CS
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 V
REF
can
be used to provide the reference to several other AD7863s.
Alternatively, an external or system reference can be used to
drive all V
REF
inputs. A common reference ensures good full-
scale tracking between all channels.