CANopen Digital Device Offers Custom Pressure Measurement Solution in Engine Test Stands

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By Jürgen Reiser (WIKA) and Gerhard Kadenbach (Volkswagen)

Measuring operating pressures is especially important for engineers working with engine test stands, which can benefit from pressure transmitters using digital signal processing and standardized CANopen interfaces. In particular, a digital Bessel low-pass filter has been used in the procedure described below to take the burden off higher-level controls in these settings.

Before any diesel or gasoline engine goes into production, extensive bench testing must be conducted on engine test stands. These tests optimize engines and simulate everyday loads in line with their expected use. Fast collection and analysis of the measured values, particularly pressure and temperature, provide important information on the exhaust, fuel, oil, and water-cooling systems.

In close cooperation with their customers, WIKA and Volkswagen AG have engineered a custom digital pressure-measurement solution. Along with its research and development divisions worldwide, Volkswagen uses about 180 engine test stands, all of which operate a uniform test stand automation system (Venus). High flexibility and quick and simple adjustment to different test configurations are important aspects of system set-up.

Given those needs, Volkswagen has developed a modular configuration of instrument cases. The integrated measuring instruments are all directly connected to a bus system (CANopen, Interbus, and Profibus DP). If necessary, further modules can be switched into the network over the serial bus system; as a result, the thick cables back to the control cabinet are no longer necessary. Internal calibration and diagnostic data stored in the pressure transmitter improve operation; remote diagnostics using the bus system are also possible. With access to the last and next calibration date, overpressure and/or over-temperature conditions make it easier for the operator to administer calibration history and remote diagnostics.

One option for communication interface is the CANopen protocol (CiA 301), standardized through the CAN in Automation (CiA) users’ and manufacturers’ organization.  The organization has defined device profiles for various device types (measuring devices and closed-loop controllers [CiA 404]). This definition ensures compatibility of basic functions for devices from different manufacturers. The CANopen protocol can be operated on the same bus without excessive expenditure. The WIKA model D-10-9 precision pressure transmitters that are used feature accuracy better than 0.1% of the measuring range. Within the temperature range +32°F to +122°F, any additional temperature error is eliminated through integrated, dynamic temperature compensation. This compensation corrects the pressure value in relation to the current temperature measured within the instrument.

These instruments also incorporate a Bessel low-pass filter. If needed, this can be activated via the corresponding objects in the CANopen object dictionary of the device (these are defined in the device profile CiA 404). Cut-off frequencies (fg), defined by Volkswagen, between 0.25Hz and 10 Hz, are adjustable with a sixth-order filter response.

The user transmits the desired cut-off frequency as float32 to the instruments, which is then assigned to the appropriate coefficient set to the next convenient implemented filter cut-off frequency. User-friendly set-up is available via the EasyCom CANopen configuration software, making setting appropriate parameters of the filter stage possible.

One use of the filtering is the RPM-dependent (revolution per minute) pressure measurement of the intake pressure in a six-cylinder gasoline engine, which exhibits strong signal fluctuations. Previously, the pressure values were captured in analog and filtered within the higher-level control system. The disadvantage of this solution is the high processing cost in the PLC control. By decentralizing the computing algorithms using dispersed signal processing, the same filtering result is achievable.

Employing the most modern bus technology substantially reduces wiring costs, allows flexible system configuration, and enables remote maintenance. It also leads to less downtime in individual engine test stands, lowering overall costs. Further, the standardized CANopen communication guarantees compatibility with devices and systems from various manufacturers. Distributed intelligence in the sensors enables digital signal processing for active temperature compensation and decentralized filtering of the measured values. These factors make a worthy case for changing from traditional analog pressure transmitters to digital instruments.

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