Case Study: Flexible COTS Pressure Measurement System

Overview

KineticaRT recently designed and implemented a pressure measurement system for a state-of-the-art aero engine test facility; it uses individual Keller 33X and XEi digital pressure transmitters and a commercial off the shelf data acquisition system. The data acquisition system not only offers high speed RS485 communication for the collection of pressure values at 50Hz but also high accuracy thermocouple measurement as well as miscellaneous voltage, current and frequency inputs.

Also integrated into the system are interfaces for MIL-STD-1553 aircraft buses, a dual redundant throttle controller, and multi-camera pan and tilt CCTV and site wide intercom systems, both over IP.

KineticaRT's customer used the KineticaRT Studio .NET Rapid Application Development (RAD) tool to develop all the control, data acquisition and automated test software.

Design Issues

One of the challenges when designing systems for testing aero engines is the selection of a suitable pressure measurement solution.

The requirements normally include:

• a mix of pressure ranges,
• measurement of a significant number of gas pressures and just a few liquid pressures,
• significant sensor temperature variations,
• operation of some sensors in ATEX explosive environments,
• high accuracy and resolution data acquisition at, typically, at least 50Hz,
• and a customer’s expectations for a cost effective commercial off the shelf (COTS) system.

One key decision is whether to use pressure scanners or individual analog or digital pressure transmitters:

Pressure scanners typically offer digital temperature compensation of individual or groups of sensors. However it is rare not to end up with a number of unused pressure ports, especially when there is a requirement for gases and fluids and several different pressure ranges. With some scanners, failure of a single sensor requires return to factory of the complete scanner; this increases the cost of spares holding. Pressure scanners are easy to integrate into an overall system because they normally use Ethernet TCP/IP for delivery of data to the test cell control and data acquisition system.

Individual analog pressure transmitters typically use analog temperature compensation, offering poorer accuracy than either pressure scanners or individual digital pressure transmitters. Also, the data acquisition system needs to provide as many analog inputs as there are pressure transmitters.

Individual digital pressure transmitters can offer the best of both worlds by providing digital temperature compensation at each transmitter and serial communication with the data acquisition computer via, typically, RS485 or CAN. Either communication method allows multiple transmitters to be interfaced to a single communication port and can use multi-drop cabling. ATEX approved digital transmitters are also available.

Keller