Sensoray 118 - Manual (Page 34)

Sensoray Model 118 Smart A/D™ Instruction Manual 32 Theory of Operation
7 Theory of Operation
7.1 Firmware
The Smart A/D’s internal microcomputer executes a
firmware-resident control program. At the heart of this
program is a high-performance, event-driven kernel.
The following paragraphs describe the functions of the
major tasks that are managed by the kernel.
7.1.1 Digitizer
This task performs the fundamental data acquisition
function central to the Smart A/D’s purpose.
The Digitizer begins a conversion by selecting the next
channel to be digitized. The sense and excitation
multiplexers are switched to the desired channel, and, if
the channel is connected to a passive sensor, the pulsed
excitation source is configured and activated. The
scanner then sleeps while the sensor signal stabilizes.
When the sensor signal has stabilized, the Digitizer
initiates an A/D conversion. Other tasks are allowed to
run while the conversion is taking place.
Upon completion of the conversion, the digitized value is
forwarded to the Cruncher task.
7.1.2 Cruncher
The Cruncher task is responsible for polishing raw A/D
data into finished form. Typically, the Cruncher task runs
while the scanner is digitizing the next channel in the
scan loop.
This task begins when it receives a digitized value from
the Digitizer task. The data is normalized to the internal
standards, then is linearized and converted to engineering
units appropriate for the declared sensor type. The
resulting value is passed through a single-pole low-pass
filter.
Next, the filtered data is checked for alarm limit
violations. If a limit has been exceeded, the channel
alarm is disabled and the Alarm status flag is set.
Finally, the processed sensor data is posted to internal
dual-ported RAM for rapid client access.
7.1.3 Command Processor
The Command Processor receives and executes
commands from the host. Top priority is given to this task
in order to minimize communication latency.
This task is run when a command is received from the
client.
7.2 Analog Circuits
The Smart A/D analog circuitry is functionally
partitioned into Measurement and Excitation sections.
7.2.1 Measurement Section
The Measurement section selects and conditions a sensor
input signal for digitizing.
The Measurement section begins sensor signal
processing by routing a pair of channel sense signals
through a differential analog multiplexer. The selected
channel is then conditioned by a programmable gain
amplifier. Finally, the amplified signal is applied to the
input of an integrating A/D converter.
Channel selection, amplifier gain, and A/D functions are
all under control of the onboard microcomputer. None of
these circuits are directly accessible by the client.
7.2.2 Excitation Section
The Excitation section sources a pulsed DC current or
voltage that is used to excite passive sensors.
The Excitation section supplies one of three pulsed
signals to passive sensors, depending on the declared
sensor type. The length of the excitation pulse is equal to
the channel slot time (see Section 8.1 for an explanation
of slot time).
In the case of strain and pressure gages, 10VDC
excitation is applied. This voltage is current limited so
that shorting the excitation signals together or to ground
will not damage the excitation source.
In the case of RTD’s and the 400
Ω
and 3K
Ω
resistance
ranges, a constant current of approximately 1.2mADC is
forced through the sensor.
5VDC is applied, in series with a reference resistor, to
sensors measured on the 600K
Ω
resistance range.