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Transcript
ME343 Chapter 12 Force Measurement, Load Cells and Dynamometers Force Measurement Load Cells • Transducers that generate a voltage signal as a result of an applied force. • These transducers usually consist of an elastic member and a deflection sensor. Force Measurement Force Measurement Force Measurement Force Measurement Force Measurement Force Measurement Force Measurement Force Measurement Force Measurement Torque Measurement Torque Measurement Torque Measurement Torque Measurement Mechanical Power Measurement (Dynamometer) A dynamometer is an instrument for measuring the power exerted by a source or the amount of power consumed by a load. • The following two types of dynamometers are considered: 1- Absorption type This type of dynamometer measures torque (and power) by dissipating mechanical energy and are suitable for power measurement of engines (such as internal combustion and gas turbine engines) and electrical motors (ac and dc motors). A Prony brake, water brake, and cradled electric motor are of this type. 2- Driving type This type of dynamometer measures torque (and power) and supply energy to operate the device being tested. This is convenient for testing such devices as pumps and compressors, which require a driving source. A rotating electric machine can be used as a driving dynamometer. Mechanical Power Measurement (Dynamometer) Dynamometer Dynamometer Prony Friction Brake (Absorption Type) • Historically, a device called a Prony brake was used to measure shaft power (e.g. IC engine). • The Prony brake serves to provide a well-defined load for the engine, with the output power of the engine dissipated as thermal energy in the braking material. • By adjusting the load, the output power over a range of speeds and throttle settings can be realized. • The power P is measured by recording the torque FL acting on the torque arm and the angular speed ω of the engine which can be measured by using a tachometer. • Hence, P=ω FL. Dynamometer Water-brake dynamometer is similar to a Prony brake but employs fluid friction (rather than dry friction) to dissipate energy. • When testing engines using a water-brake dynamometer, the braking action (or load) is developed by the principle of direct momentum exchange between the rotor(s) and stator(s) of the dynamometer. • The dynamometer rotor(s) directs the water against the waterbrake housing or stator. The stator in turn, redirects the water back against the rotor so that it opposes the movement of the rotor(s). • It is this turbulence and back pressure which causes the braking action or load. The greater the flow of water through the dynamometer, the greater the braking action or load. • The stator is held in position by a strain gauge (torque link) which will measure the force acting on the stator. With proper calibration, this force is a measure for the torque produced by the engine when it moves the rotor in the water. Dynamometer Cradled Dynamometers A cradled dynamometer is supported in bearings with a moment arm connected to a force measurement device. ▫ 1. Absorption Type When a source (motor, engine, etc.) is connected, this dynamomete acts as a generator and dissipates the power in the form of thermal energy in a resistive load connected to it. The mechanical power generated by the source can be found by measuring the rotational speed of the shaft using an RPM sensor and the steady-state force required to prevent rotation of the dynamometer. Dynamometer Cradled Dynamometers 2. Driving Type The torque and power of a dc motor can be measured by itself, by supplying the electrical power to the motor, in which case a separate load is required to dissipate the power and adjust the rotational speed. Dynamometer Dynamometer