Moog Crossbow VG700MB Series - Manual (Page 23)

VG700M User’s Manual
Doc.# 7430-0280-01 Rev. F Page 17
3.7 Timing
In some applications, using the DMU’s digital output requires a precise
understanding of the internal timing of the device. The processor internal to
the DMU runs in a loop - collecting data from the sensors, processing the
data, then collecting more data. The data is reported to the user through a
parallel process. In continuous mode, the system processor activity is
repeatable and accurate timing information can be derived based purely on
the overall loop rate.
The unit goes through three processes in one data cycle. First, the sensors
are sampled. Second, the unit processes the data for output. After
processing the data, the DMU will make another measurement while
presenting the current measurement for output. Third, the unit actually
transfers the data out; either over the serial port, or onto the analog outputs.
A time tag is attached to each data packet. The time tag is simply the value
of a free running counter at the time the A/D channels are sampled. The
clock counts down from 65,535 to 0, and a single tick corresponds to 0.79
microseconds. The timer rolls over approximately every 50 milliseconds.
You can use this value to track relative sampling time between data packets,
and correlate this with external timing.
3.8 Temperature Sensor
The DMU has an onboard temperature sensor. The temperature sensor is
used to monitor the internal temperature of the DMU to allow for
temperature calibration of the sensors. The sensor is specified to be within
±2% accurate over the DMU temperature operating range. The DMU reads
and outputs the temperature sensor voltage in the digital data packet with
12-bit precision.
The temperature sensor voltage is sent in the data packet scaled as:
V
temp
(V) = data * 5/4096,
where data is the 16-bit unsigned integer sent as the temperature
information in the data packet. (The DMU uses two full bytes to express
the data, but it is really scaled to 12 bits.)
Calculate the temperature with the following calibration:
T (
°C) = 44.4 (°C/V) * (V
temp
(V) – 1.375 V),
where V
temp
is the temperature sensor voltage sent in the DMU data packet.
The DMU temperature sensor is internal to the DMU, and is not intended to
measure the ambient temperature. The internal temperature of the DMU
may be as much as 15
°C higher than the ambient temperature.