TrySense™ is a self-contained test bench, designed for the
prototyping and small-scale production of sensor systems. The system has been developed in a joint project involving three Swedish resea rch institutes: Imego, IVF and Acreo. TrySense is a wireless programmable platform that can be adapted to most sensor readouts. Feedback parameters, filter characteristics, signal conditioning, etc. can easily be configured to fit a certain application via a wireless Bluetooth interface. It is also equipped with a memory allowing temporary data storage. These characteristics make Try-Sense well suited to several sensor and measurement applications where a portable wireless system including signal conditioning must be developed in a short time at low cost. Two possible applications are surveillance of patients and measurements on cars during crash tests.
Nature is analogue and therefore most sensing elements give analogue output signals. Further, in more complex sensor systems feedback is often required. Therefore analogue feedback channels are necessary. In high performance systems the sensor output signal usually needs to be corrected for imperfections such as drift, offset and cross-sensitivity. The capacity of digital systems has followed Moore’s law during the last few decades. Therefore, it is usually best to use the digital domain for as much functionality as possible and only to use the analogue domain when necessary due to noise and/or speed requirements.
Sometimes when a prototype is being developed, several re-designs of the signal conditioning are needed before the optimum solution is reached. These re-designs are time consuming if a new ASIC (Application Specific Integrated Circuit) needs to be built each time. Furthermore, if two potential customers want evaluation samples of the sensor system with different configurations, for example with different measurement ranges or output formats, a new batch of ASIC's must be made, which is also time consuming. The electronics industries often use FPGA’s (Field Programmable Gate Array) to replace ASIC’s for prototyping and low volume production because of their low cost and high flexibility. An FPGA functions as a substitute for an ASIC where the digital electronics can be configured or re-configured in a few seconds. The configuration is described in a high level language, VHDL or Verilog, which allows complex digital systems to be designed in short times. This design methodology also takes advantage of using IP’s (Intellectual Property) in the design. An IP can be a VHDL code describing a part of the system. If IP’s are used, the system can be re-designed by just switching IP’s or by changing the source code of the IP. For example, the system output can be changed from 7 segment code to a data bus by simply switching the IP’s generating the output. Using FPGA’s and VHDL in the design of sensor systems ensures optimum flexibility and short development time from idea to a fully working prototype. For low price at high volume, the final design can be translated into an ASIC or into the sensor chip for single chip solutions.
The heart of TrySense™ is the FPGA; a Spartan II from Xilinx. This component controls the A/D and D/A converters and processes the incoming data from the sensors. The FPGA is configured from the SRAM at start-up, but may be reconfigured at any time via the Bluetooth link. This makes the system simple to reconfigure without cables. The data is forwarded from the FPGA via the microcontroller to the Bluetooth module for transmission to a PC. TrySense™ is equipped with two four channel A/D converters for signal readout and a four channel D/A converter for analogue feedback to the sensor element. Apart from the analogue I/Os, there are also 12 digital bits available for interfacing the sensors. TrySense™ uses a 4 Mbit NVSRAM for data and configuration storage. The system is supplied either by a 6 – 10 VDC source or by the 6 V built-in battery pack.
Sensors are connected to TrySense™ via a 0.8 mm printed circuit board (PCB) contact, i.e. sensors are mounted with their analogue electronics (preamplifiers, anti-alias filters...) on an application specific PCB. The contact can be seen at the bottom of TrySense™ in Figure 2. This interface supplies the sensor board with 50 input and output signals including 2.5, 3.3 and 5.0 V digital and 5.0 V analogue supplies. There is also a generic board available that allows for connection of all sensor components via coaxial and flat cables.
TrySense™ is aimed at sensor system development but is also well suited for applications such as:
• Wireless surveillance and control – e.g. in process industry, chemistry or automotive industry.
• Portable sensor systems – e.g. health monitoring in hospitals.
• Mobile data acquisition – e.g. environmental surveys.
• Embedded control and acquisition systems.
• Self-calibrating control systems.
• Test set-ups with moving/rotating parts – e.g. test and verification systems for accelerometers, inclinometers and gyros. In addition to the above, several different sensor systems have been successfully implemented on TrySense™ or its predecessor, including: • Pressure sensors with built-in temperature sensors (resistance output).
• Gas sensors (MOSFET output [5]).
• Accelerometers with preamplifier ASIC (voltage output).
• Light Position-Sensing Detector, PSD (current outputs).
• Temperature sensors (resistance output).
Several IP’s have been developed and verified:
• A/D converter communication
• D/A converter communication
• Microcontroller and Bluetooth communication
• Drift compensation
• Low pass filtering
• Algebraic operations
• Pulse width modulated output
• Parallel digital output
• Analogue output through the D/A converter Windows software allowing wireless reconfiguration of the FPGA, storage of measured data to files and data presentation has also been developed. As far as we know, this is the first time an FPGA has been used in a system that allows wireless reconfiguration through a Bluetooth interface. |