The Rotating Wheel Dynamometer

This article was first published in 2000.

While we tend to think of dynamometers as being power measuring devices, in fact all dynos measure torque and speed. In addition to the torque being developed by the engine, a dyno can also measure the braking torque being developed by a car. The most accurate of way of doing this is with a Rotating Wheel Dynamometer, a dyno that actually replaces one of the car's standard wheels.

Recently released by Kistler Instruments AG is a new Rotating Wheel Dynamometer. Major companies are expected to use it for testing in the areas of:

• Control systems for vehicle dynamics
• ABS anti-lock braking and other braking effort distribution systems
• Investigations of brake fade
• Brake judder
• Residual brake torque
• Power transmission measurements
• Determination of friction values
• Coast down tests
• Government safety tests

The modular dyno system consists of three main components: the torque sensor, the transceiver head for radio signal transmission, and the control unit located in the vehicle. The torquemeter installs in place of a standard rim and is mounted on the car's hub. The special rims are based on the BBS rim series and are easy to modify for a specific vehicle. The wheel offset can be maintained because of the combination of rim and hub adapters that is available.

Additional measurement data can be gathered with the optional Corrsys slip angle sensor. This combination allows the measurement of tyre slip angle with tyre forces and torques.

The Torque Sensor

The torque sensor uses 8 quartz shear force sensors preloaded between two flanges. It is optimised for low mass, high rigidity and low effect of temperature variations. The modular design allows the original rim size and position (positive or negative offset) to be maintained by simply using car-specific rim and hub adapters and rim components from the BBS standard range.

In addition to the torque signal, there is provision on the rotating element of the sensor for installing four K-type thermocouple elements. These can be used to suit individual applications, for instance for monitoring temperatures on the brake disc. The amplifiers for the charges and the thermocouple signals are integrated into the torque sensor. The thermocouple signal conditioning and correction includes a compensation circuit for the cold junction.

The Transceiver Head

The measured data are sent from the transceiver head by telemetric signal transmission to the control unit. The transceiver head includes all components for the data acquisition, telemetry and telecommand, ie processor, signal conditioning circuits, a 16-bit-A/D converter with an 8-channel multiplexer and sender/receiver module with antenna. Batteries supply the power. The sampling rate is 1000 samples/sec for the torque, 8 samples/sec for the temperatures and 1 sample/sec for the battery status. To save battery power, the transceiver head is set to sleep mode - either by a command from the control unit, or, after a selected lapsed time interval, without any commands received from the control unit. Any movement of the head wakes it up again.

The Control Unit

The control unit in the vehicle consists of a 32-bit processor and digital-, serial-, CAN- and analogue interfaces. Radio links are set up between the control unit and the transceiver heads attached to the system. One control unit can serve up to four transceiver heads (ie each wheel can be replaced by a dyno!). The radio link is based on a DECT transceiver module as used in cordless phones. Several such systems can operate simultaneously without interfering with one another.

After switching on, the transceiver heads automatically log on to the control unit. All sensor-specific data are downloaded onto the control unit. A few seconds later, the system is operational without any further user intervention. The synchronisation delay between the control unit and each transceiver head is less than 1ms. The control unit accepts commands - eg "start of measurement" through CAN-bus or P5-2320 interface. A digital input that serves as trigger input (eg for the start of measurement sequence) is also available. Measured data - converted into physical units - are output through the CAN or RS-232C interface. An analogue output (1 per transceiver head) is also available.

Optional Slip Ring Signal Transmission

For applications where angular information is required it is possible to mount a data transfer module, using slip rings and fixing arm, instead of the transceiver head. This option allows also the use of a digital angle encoder (1024 pulses per revolution). Signal outputs from the torque sensor and the thermocouples are then available as analogue signals only.

This graph shows data collected on a front wheel driven vehicle. During acceleration each gearshift can clearly be seen as a drop in driving torque. Additio­nally to the torque data, the brake pressure has been recorded. This allows the correlation between brake pressure and braking torque.

These two graphs show data from brake-torque measurements taken at a Rotating Wheel Dynamometer on the AT&G testing ground in Papenburg, Germany. The first shows a complete braking manoeuvre from 200 km/h to stop (and just check that time! - Ed). During this manoeuvre the vehicle's ABS systems takes control of the process. The second shows in more detail the effects of the ABS.

Fitting the Slip-Ring Dyno

First the standard wheel is removed and a hub adaptor is torqued into place.

The new rim and tyre - with the assembly incorporating the dyno - is then attached.

The cable connecting to the slip rings is fastened.

The encoder is adjusted then...

Data acquisition can begin.

www.kistler.com

Thanks to Kistler Instruments AG for making available the material used in this article.

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