The File - Nov 16, 2008 - (Page 7)

In Focus | Data acquisition Digital telemetry advances torque measurement By Chris Novak Product Manager Honeywell Sensing and Control Monitoring and controlling torque can be critical in optimising overall system efficiency, as torque and RPM determine horsepower, and horsepower determines system efficiencies. Torque measurement is vital in engine and transmission testing, turbine testing, pump testing, and testing of gear trains and power measurement within propulsion systems. Really, torque is an important factor in anything that rotates or spins on a shaft, spindle or axle. So, what’s new and how did we get there? A brief history of what has been used and how torque measuring has changed may help illustrate the benefits of the latest technologies. Digital telemetry is the latest technology and, so far, seems to have a number of benefits over older systems. But before digital telemetry was viable, there were several other methods, starting in recent history with the slip ring signal transfer method. As testing demanded more and more, so evolved the total non-contact solution of using analogue telemetry. Analogue FM wireless telemetry systems use a sensor with a built-in radio transmitter module, a power supply and a receiver. Low voltage signals from the strain gauges are amplified and modulated to a radio frequency signal by the transmitter. This radio signal is picked up by a hoop antenna and decoded into analogue voltage by the receiver. This voltage is amplified and scaled to show the torque value on an LED display. This signal is routed to an external connector for interfacing with a data acquisition system. While this form of telemetry is an improvement over other mechanical methods, it is bulky and requires additional receivers for multiple channels. The frequency response is limited to Figure: Components of new-generation digital telemetry systems. Discuss Torque measurement: Digital vs. analogue Are the benefits of a digital telemetry system worth the cost difference in your application? Discuss. Mechanical, analogue The slip ring is a simple means of getting a signal transferred from the rotating element to the stationary element. It has low speed limits. Ring wear and dust generated by the brushes can quickly impede signal transfer, so you must routinely maintain the rings and the brushes to ensure clean signal transfer. As testing requirements became more demanding, so evolved the rotary transformer. Compared to the slip ring, the rotary transformer method tolerates higher speed, is noncontact and typically more accurate. However, it is less tolerant to extraneous loading conditions like bending moments and thrust loads. It also requires more sophisticated signal conditioning instrumentation using an AC carrier excitation. about 1,000Hz with an analogue signal output. The antenna, straps and battery-powered systems can be difficult to install and tune. As technology and electronics advanced, digital telemetry became practical. Digital telemetry The first generation of digital telemetry systems frequently used a hoop antenna, a bulky receiver and had limited data processing capability. A coil acted as the rotary transformer. The coil is excited by the radio frequency that is transmitted via the antenna. These early systems did allow two-way communication flow, but because they had no microprocessors, data processing was very limited. All the discrete components added to the cost and design limitations. As microprocessor and surface mount technologies progressed, multiple chips were added to the next generations of systems to vastly improve data management and control. A rotor electronics circuit board module is embedded in the sensor and is potted and sealed. Signal conditioning and digitising is done on the rotating sensor using this module. Antenna and caliper-style coupling modules eliminate the need for hoop antennas and are more immune from vibration problems. Antennas are limited to a few select sizes and are matched to the caliper modules at the factory. This eliminates the need to tune the system in the field. Outer protective layers on the antenna and caliper module give outstanding protection against moisture and oil. A signal-processing module acts as the receiver unit, handling the communications with the rotor electronics. Resolution, stability and accuracy are all improved over the early digital telemetry systems. The new systems transfer digital data at very high speeds, providing a frequency response up to 3,000Hz. More than standalone sensors, these are complete torque measurement systems, with standard analogue, frequency and digital outputs. Fully software-driven, these digital telemetry systems can be changed “on-the-fly” without affecting calibration. They can be designed with clear upgrade paths for custom software, developments and fu- ture requirements. It is also where the “fit and forget” concept can be used to lower maintenance costs of existing solutions. If you are measuring torque “the old fashioned way,” you may be unaware of some inherent inefficiencies. A common setup when one measures torque (or electrical current for that matter) the “old” way is to use a torque sensor or transducer that converts torque into an electrical signal. This conversion requires some method to transfer the signal, which requires more equipment and more work. Common signal transfer methods include a slip ring or transformer. To operate properly, both methods need stand-alone signal conditioning equipment. This means more setup and configuration, involving potentiometers and switches, which can be problematic. This sensor is interconnected with cabling to the strain gauge instrumentation or amplifier. Let’s say you are measuring torque on an axle and you need a range of 1,000 footpounds. However, you also want to measure at lower range of 500 foot-pounds and would like a voltage output at full scale for both. Both the “old” and “new” ways require you to calibrate the sensor and connect it to your instrument with cabling. Making these adjustments using analogue potentiometer pots or switches can be time consuming and labour intensive. continued on page 11 7 EE Times-India | November 16-30, 2008 | www.eetindia.com http://www.powerdesignindia.co.in/SEARCH/SUMMARY/technical-articles/power supply.HTM?ClickFromNewsletter_081116 http://forum.embeddeddesignindia.co.in/FORUM_POST_1000039251_1200094001_0.HTM?ClickFromNewsletter_20081116 http://forum.embeddeddesignindia.co.in/FORUM_POST_1000039251_1200094001_0.HTM?ClickFromNewsletter_20081116 http://www.embeddeddesignindia.co.in/SEARCH/SUMMARY/technical-articles/microprocessor.HTM?ClickFromNewsletter_081116 http://www.embeddeddesignindia.co.in/SEARCH/SUMMARY/technical-articles/microprocessor.HTM?ClickFromNewsletter_081116 http://www.eetindia.co.in/SEARCH/SUMMARY/technical-articles/signal conditioning.HTM?ClickFromNewsletter_081116 http://www.eetindia.co.in/SEARCH/SUMMARY/technical-articles/signal conditioning.HTM?ClickFromNewsletter_081116 http://www.eetindia.com/STATIC/REDIRECT/Newsletter_081116_EETI02.htm?ClickFromNewsletter_081116

Table of Contents for the Digital Edition of The File - Nov 16, 2008

EETimes India - November 16, 2008
Contents
Farnell
Embedded System Eases Rail Maintenance
National Semiconductor
Working With IT for Networked DAQ
Digital Telemetry Advances Torque Measurement
Events

The File - Nov 16, 2008