INSTRUMENTATION • ELECTRONICS
In the rotational viscometer, the
TorqSense, the wireless torque sensor, provides a robust and reliable way to measure viscosity
data from earlier mixes to give detailed information that would not be available from fi nal target viscosity readings alone. However, torque can be diffi cult to
measure. In fact, traditional measuring technologies introduce as many problems as they solve. Because the spindle is rotating, wires attached to a torque sensor on the shaft would wind up and quickly snap. Approaches using slip rings are available but far from ideal because of costs, set-up time and the inevitable wear and tear. A wireless technique has been developed
using TorqSense rotary torque transducers from Sensor Technology. T ese do not need a physical connection to the rotating shaft, but instead use a radio frequency (RF) link to both send power to the
sensing element on the spindle and to receive torque reading signals back from it. TorqSense torque sensors use two tiny
surface acoustic wave (SAW) devices, made of ceramic piezoelectric material containing frequency resonating combs. T ese are glued onto the drive shaft at 90° to one another. As the torque increases one comb expands and the other contracts proportionally to the torque being experienced. In eff ect, the combs act as strain gauges but measure changes in resonant frequency. An RF transmitter/receiver mounted close to the spindle emits radio waves towards the SAWs and collects them when they are refl ected back. T e change in frequency of the refl ected waves identifi es the current torque.
TorqSense transducer is mounted between the motor and the paddle. A double bearing eliminates any side loads, while a torque limiting coupling provides protection in the event of the paddle mechanism seizing. With the motor operating at constant speed, the transducer provides an output of torque that changes proportionally to the viscosity during mixing, thus enabling the operator to accurately measure the relative viscosity of the mix. Where it is important to determine absolute viscosity rather than relative viscosity, the system can be pre-calibrated using specifi c paddles immersed in fl uids with a known viscosity. Water, for example, has a viscosity of 0.001 Pascal seconds at room temperature. By calibrating the viscometer to a known sample, the absolute torque fi gure can then be derived in the application from the relative torque fi gure, and the required viscosity then accurately measured. Using TorqSense rotary torque transducers has simplifi ed the design of these new rotational viscometers while also increasing accuracy, providing a robust and reliable solution
Mark Ingham is with Sensor Technology.
www.sensors.co.uk
NEW TORQUE SENSORS S
ensor Technology has launched its digital ORT 230/240 series. These new optical rotary torque
sensors are ideal for applications when the demand is for low torque and/or high bandwidth, providing precise, dynamic measurement of rotary and static torque of less than 100Nm and for bandwidths of up to 50kHz. The new ORT 230/240 devices replace the fi rm’s E200 ORT series,
benefi ting from
new electronics that deliver signifi cant gains in resolution, frequency response, reduced sensor
current consumption and faster digital data throughput.
The high-speed capability comes
from an inherently low inertia, since the electronics are not fi xed to the shaft, while non-contact operation ensures a long and reliable life
(backed up by Sensor Technology’s lifetime warranty) with high accuracy. The optical operating principle also ensures excellent noise immunity ●
www.engineerlive.com 57
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