EVAL-AD7863CB View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-AD7863CB' PDF : 24 Pages View PDF
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
TORQUE AND FLUX
CONTROL LOOP
CALCULATIONS AND
TWO TO THREE
PHASE
INFORMATION
DAC
DAC
DAC
IC
DRIVE IB
CIRCUITRY
IA
VB
THREE
PHASE
MOTOR
VA
TORQUE
SETPOINT
FLUX
SETPOINT
ISOLATION
AMPLIFIERS
VA1
TRANSFORMATION
TO TORQUE AND
FLUX CURRENT
COMPONENTS
VA2
AD7863*
VB1
*ADDITIONAL PINS
OMITTED FOR CLARITY.
VB2
VOLTAGE
ATTENUATORS
Figure 24. Vector Motor Control Using the AD7863
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
RD
VB1
RD
VA2
AD7863
(1)
VB2
CS
VREF
VA1
VB1
RD
VA2
AD7863
VB2
(2)
CS
VREF
ADDRESS
DECODE
ADDRESS
VREF
RD
VA1
VB1
AD7863
VA2
(n)
VB2
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 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.
Rev. B | Page 19 of 24
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