VXI VT1422A - Manual (Page 525)

Acquisition Acquisition
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Algorithmic Closed Loop Controller and
Remote Channel Multifunction DAC
VT1415A/1422A
The on-board 40 MHz pipelined DSP provides highly
deterministic execution, making it easy to accurately predict
cycle times. Engineering unit conversions for temperature,
strain, resistance, and voltage measurements are made
automatically without slowing down the algorithm execution
speed.
Wide Choice of Inputs/outputs
The inputs to the loop algorithm can be measured values
from multiple channels, operator input values, outputs from
other loops, or values from other subsystems. The VT1415A/
VT1422A have a variety of signal conditioning plug-ons for
making measurements of:
• Temperature, strain
• Voltage, current, resistance
• RPM, frequency, totalize
• Discrete levels, TTL, contact closures
In addition, the measured input values and the calculated
output values can be stored in a 64,000-sample FIFO buffer
and efficiently transferred to the controlling computer in
blocks of data. With this feature, it is no longer necessary to
waste resources by dedicating a data acquisition channel to
monitor each control loop input and output. The result of any
algorithm calculation can be an input for use by another loop
or subsystem, or it can be a direct output of several different
types. Among the choices of output are:
• Analog voltage
• Analog current
• Discrete levels (TTL)
• Pulse width modulation (TTL)
As an example of output flexibility, the pulse width
modulation output has several modes. In the PWM free-
run mode, the frequency or pulse width output rate is
independent of the loop update rate and can be changed
once per loop update cycle. The square wave mode provides
a variable frequency, fixed 50% duty cycle output signal.
The pulse-per-update mode provides a variable width pulse
synchronized to the loop update cycle.
Operator Control
Manual control can be implemented through a user software
interface or external hardware, such as a potentiometer.
Seamless transfer from auto to manual mode, or manual to
auto is handled automatically by a set-point-tracking routine
in the default PID algorithm code.
Signal Conditioning Plug-Ons
A Signal Conditioning Plug-on (SCP) is a small daughter
board that mounts on VXI Technology’s VXI scanning
measurement and control modules. These SCPs provide a
number of input and output functions. Several include gain
and filtered analog inputs for measuring electrical and sensor-
based signals, as well as frequency, total event count, pulse-
width modulation, toothed-wheel velocity, and digital state.
Output functions include analog voltage and current D/As, 8-
or 16-bit digital outputs, pulse output with variable frequency
and PWM, and stepper motor control.
Refer to the information on each individual SCP for more
details.
Voltage Measurements
Use any of the following SCPs with the VT1415A/VT1422A to
make voltage measurements: VT1501A, VT1502A, VT1503A,
VT1508A, VT1509A, VT1512A, or VT1513A.
Temperature Measurements
Any of the input SCPs can be used to make temperature
measurements with thermocouples, thermistors, or RTDs, but
the VT1503A/VT1508A/VT1509A SCPs provide higher accuracy
with thermocouples.
Resistance Measurements
Resistance is measured using either the VT1505A 8-channel
Current Source SCP and an input SCP or the VT1518A 4-wire
Resistance Measurement SCP. Measurements are made by
applying a dc current to the unknown and measuring the
voltage drop across the unknown.
Static Strain Measurements
There are two ways to make static strain measurements.
The VT1506A and VT1507A SCPs provide a convenient way
to measure a few channels of static strain. When using the
VT1506A/VT1507A for bridge completion, a second SCP is
required to make the measurement connection. You can use
the following SCPs for this type of static strain measurements:
• VT1503A 8-channel Programmable Filter/Gain
• VT1506A 8-channel 120 Ω Strain Completion & Excitation
• VT1507A 8-channel 350 Ω Strain Completion & Excitation
• VT1508A 8-channel 7 Hz Fixed Filter & x16 Gain
• VT1509A 8-channel 7 Hz Fixed Filter & x64 Gain