Download ME343 Chapter 12

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