Force, Torque, and Shaft Power Measurement By Aj. Somchai Triratanajaru M.Eng. KMUTT, B.Eng KMITL Term 1/2554

LECTURE OUTLINE 1. Standards and Calibration 2. Basic Methods of Force Measurement 3. Characteristics of Elastic Force Transducers 4. Resolution of Vector Forces 5. Torque Measurement on Rotating Shafts 6. Shaft Power Measurement (Dynamometers) 7. Gyroscopic Force and Torque Measurement 8. Vibrating-Wire Force Transducers *Major part of this presentation are from Measurement Systems, Ernest O. Doebelin

1.Standards and calibration Force, (F=ma); thus a standards for force depends on standards of mass and acceleration. Mass standards-> cylinder of platinum-iridium called International Kilogram. Acceleration -> drive from standards of length and time. g -> sea level and 45o latitude 980.665 cm/s2 If not , must be compensated.

Force standards

2. Basic methods of force measurement There are seven methods introduce in the reference text book. Some methods are only use in lab. We will focus on weighting and torque measuring instrument But keep in mind that it is still have so many methods out there. Use this information as the guide line for further study.

2.1 Balancing it against the known gravitational force on a standard mass, either directly or through a system of levers.

2.2 Measuring the acceleration of a body of known mass to which unknown force is applied.

2.3 Balancing it against a magnetic force developed by interaction of a current-carrying coil and a magnet.

2.4 Transducing the force to a fluid pressure and then measuring the pressure.

2.5 Applying the force to some elastic member and measuring the resulting deflection.

Strain-Gauge Load Cell

Strain-Gauge Beam Transducers

Young's modulus Young's modulus, E, can be calculated by dividing the tensile stress by the tensile strain: where E is the Young's modulus (modulus of elasticity) F is the force applied to the object; A0 is the original cross-sectional area through which the force is applied; ΔL is the amount by which the length of the object changes; L0 is the original length of the object.

Young's modulus.. Young's modulus is the ratio of stress, which has units of pressure, to strain, which is dimensionless; therefore Young's modulus itself has units of pressure.

Young's modulus… Material Modulus (GPa) Aluminum 69 Copper 117 Steel 207

LVDT Force Transducer

Piezoelectric Load Cells

Torque Measurement of Rotating Machines: Dynamometers

Schematic of Dynamometers

Chassis Dynamometer

Servo-controlled Dynamometer

Further reading and more exercises Text. Process control Instrumentation technology Curtis D. Johson