Wireless Torque Sensor
The transport and mixing of angular momentum of fluid parcels by turbulent flow is an area of substantial interest in geophysical and astrophysical fluid dynamics. The torque on a boundary in a rotating or swirling shear flow is a measurement of the average angular momentum flow.
Strain gauge torque sensor and XBee wireless digitizer.
It's useful to have very high dynamic range for torque measurements in this kind of experiment, so that small fluctuations can be measured over a very wide range of mean torque values. It can sometimes be scientifically useful to measure relative fluctuations even when they may be smaller than the stated hysteresis and and repeatability limits of commercially available strain-gauge torque sensors. I wasn't happy with the commercial offerings at the time of this project, so I coupled a Futek TFF600 torque sensor with a custom 22-bit digitizer.
The Futek strain gauge bridge is excited with about 12 volts DC and the millivolt-level output is amplified by a Linear LT1167 instrumentation amplifier. The amplified signal is digitized using a Microchip MCP3553 22-bit analog to digital converter running at 30 samples per second, controlled by an Arduino microcontroller board. The Arduino then sends torque samples to the lab frame using an XBee wireless serial link. The firmware for the Arduino is here: xbeetorque.pde on GitHub.
Schematic: Click image to enlarge or click here for a PDF version.
In addition to the electronic design, fabrication, testing, and calibration, I designed, welded and machined the black anodized shaft adapter in the photo at the top. To save material, the part is built up of two parts; a tube with a cap welded in before final machining. I also modified half of the flexible shaft coupler to mate with the Futek transducer.