Precision Measurement Engineering, Inc.
Precision Measurement Engineering, Inc.
The hardware section presents a description of the electrical operation of the SCAMP, organized by sensor or circuit card location, and roughly in the order that the measured information is processed by the circuits. It is provided to give you insight into how information about the physical environment is transformed into measurements, and to give you a basis for understanding the software morphology
The paramount design criteria in both the SCAMP electrical design and mechanical packaging is to obtain a measurement system designed for spectral analysis of measurements and having the lowest measurement noise possible. All other features are chosen with this criteria in mind. Certain features that may seem odd at first, such as three different battery voltage supplies or the short battery life, make sense when measurement noise is considered.
Processing begins with the sensors shown at the upper left section of the general block diagram of the SCAMP system. The various sensors operate in different ways depending upon the parameter being sensed. In all cases, these together with the sensor electronics, produce an analog voltage that is in some way related to at least one and possibly more sensed parameters.
In some cases, the analog voltage from a sensor electronics is routed to a gradient, programmable gain, and filter processor sequence. These signals are marked 'TO d/dt' on the diagram and appear at the lower left. These analog circuits compute the time rate of change of the sensor electronics output voltage, then amplify and filter the result.
There are housekeeping circuits that provide voltages related to SCAMP internal features such as battery voltages or circuit/ temperature. These are shown in the four boxes at the lower left. Like the environmental sensors, these circuits have voltage outputs.
Voltages from the sensor electronics, from the gradient and filter processors, and from the housekeeping circuits are all routed to the A/D converter through two 16:1 analog multiplexors and one 2:1 multiplexor. The A/D converter is able to produce a 16 bit digital output for each of the voltage inputs.
Multiplexor channels 0 to 15 arrive at the A/D converter after processing by the analog offset and gain circuits. These circuits allow the A/D input voltage range of +/- Vref (+/- 3.000 volts) to be mapped onto sub-ranges of channel 0 to 15 voltages. Multiplexor channels 16 to 31 are directly routed to the A/D.
The digital output of the A/D converter is received by the Central Processor Unit (CPU) where it is stored in the RAM memory. When a profile is ended, the data in the RAM memory is appended to SCAMP parameters and the result moved to the MultiMedia Card (MMC). When SCAMP is connected to a host computer all profiles in MMC are automatically uploaded and stored on the host computer disk drive.
The CPU program is stored within the flash memory and utilizes some of the RAM memory. The CPU provides USB I/O for normal SCAMP operations and RS232 I/O for housekeeping functions. The CPU is always powered by 3.3 Volts but the other voltage supplies are turned off when not required. During sleep time the CPU enters a low power mode, but can respond to magnetic switch generated interrupts.