Tech Briefs
Rotating wheel torque meter

Kistler Instrument Corp.'s new rotating wheel torque (RWT) meter. A schematic cross section of the RWT's design and mounting configuration. The RWT has a modular design, featuring rim and hub adapters for easy adaptation to different cars. Kistler's 3-component force calibration press. RWT mounting on a rolling test stand equipped with a FWD as a reference sensor for dynamic tests.

Kistler Instrument Corp. has developed a new system for measuring the driving and braking torques that act on passenger car wheels. Its battery-operated rotating wheel torque meter (RWT) is intended for vehicle dynamics applications, particularly the development of control systems for dynamic stability, traction control, and ABS, as well as investigations of brake fade, transmitted power measurements and determination of friction values, coast down characteristics, and safety tests.

The modular system consists of three main components: a torque sensor, a transceiver head for telemetric signal transmission, and the control unit located in the vehicle. The torque-sensing device can be fitted to the vehicle with the aid of hub and rim adapters that allow rapid installation to different vehicles. Torque measurement is achieved via piezoelectric quartz force sensors. Kistler says the RWT combines a wide measuring range with a high frequency response characteristic, has inherent low crosstalk, and is linear in its signal response, stable and rugged.

Kistler has a long tradition in wheel force measurement equipment and has developed two types of multi-component wheel dynamometers. Its rotating wheel dynamometers (RWD) are used mainly by vehicle manufacturers and tire makers for collecting real-time road data. Second-generation RWDs feature advances in multi-component force measuring techniques. Fixed wheel dynamometers (FWD), introduced more than 30 years ago and continuously improved, are used for measuring wheel loads on rolling roads.

The system's torque sensor is based on the RWD design and uses eight quartz shear force sensors between two flanges. It is optimized for low mass, high rigidity, and resistance to 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.

In addition to the torque signal, there are provisions on the rotating element of the sensor to install four K-type thermocouple elements for applications such as monitoring temperatures on the brake disk. 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 measured data are sent from the transceiver head by telemetric signal transmission to the control unit. The transceiver head includes all components for data acquisition, telemetry, and telecommand (i.e., the processor, signal-conditioning circuits, a 16-bit-A/D converter with an 8-channel multiplexer, and a sender/receiver module with antenna). The sampling rate is 1000 S/s for torque, 8 S/s for temperature, and 1 S/s 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 brings it back on-line.

The control unit in the vehicle consists of a 32-bit processor and digital, serial, CAN, and analog 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. Several 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 synchronization delay between the control unit and each transceiver head is less than 1 ms.

The control unit accepts commands through a CAN-bus or RS-232 interface. A digital input that serves as trigger for measurement sequences to begin is also available. Measured data—converted into physical units—are output through the CAN or RS-232 interface. An analog output (one per transceiver head) is also available as an option.

The RWT is calibrated on a 3-component calibration stand at Kistler. The calibration stand is extremely rigid, allowing forces to be applied exactly in the desired direction. Consequentially, it is possible to determine with a high degree of accuracy not only sensitivity, linearity, and hysteresis, but also the true crosstalk values. The calibration stand has a large operating space, making calibration possible not only along the coordinate axes through the center of the RWT, but also at the center of the tire footprint. This is achieved by using appropriate rigid lever arms for applying the calibration forces eccentrically.

Jean L. Broge