Mass, Force, And Torque Measurement

Mass, Force, and Torque Measurement

Mass, Force, and Torque MeasurementMass (Weight) MeasurementThe mass of a body is always quantified in terms of a measurement of the weight of the body, this being the downward force exerted by the body when it is subject to gravity. Three methods are used to measure this force:The first method of measuring the downward force exerted by a mass subject to gravity involves the use of a load cell. The load cell measures the downward force F, and then the mass M is calculated from the equation:

M = F/g

where g is acceleration due to gravityThe second method of measuring mass is to use a spring balance. This also measures the downward force when the measured mass is subject to gravity. Hence, as in the case of load cells, the mass value can only be calculated exactly if the value of g is known exactly. Like a load cell, the spring balance is also a deflection-type instrument and so is easy to use.The final method of measuring mass is to use some form of mass balance instrument. These provide an absolute measurement, as they compare the gravitational force on the mass being measured with the gravitational force on a standard mass. Because the same gravitational force is applied to both masses, the exact value of g is immaterial. However, being a null-type instrument, any form of balance is tedious to use.Mass-measuring instruments1. Load CellIt is a passive transducer or sensor which converts applied force into electrical signals. They are also referred to as load transducers.

Load cells use different operating principles:Based on fluid pressure

Based on elasticity

Based on magnetostriction effect or piezoelectric effectCommon types of load cells: Electronic Load Cell (Electronic Balance)- It is now the preferred type of load cell in most applications. Within an electronic load cell, the gravitational force on the body being measured is applied to an elastic element. Pneumatic and Hydraulic Load Cells- It translate mass measurement into a pressure measurement task, although they are now less common than the electronic load cell. Intelligent Load Cells- It is formed by adding a microprocessor to a standard cell. This brings no improvement in accuracy because the load cell is already a very accurate device.The term load cell means strain gage-based load cells. The reason behind the wide adoption of strain gage-based load cells is their characteristics:Highly precise and linear measurementsLittle influence due to temperature changesSmall size compared with other types of load cellsLong operating life due to lack of moving parts or any parts that generate frictionEase in production due to small number of componentsExcellent fatigue characteristics

Load cells are used in several types of measuring instruments such asuniversal testing machines.2. Mass Balance (Weighing) InstrumentsIt is based on comparing the gravitational force on the measured mass with the gravitational force on another body of known mass. This principle of mass measurement is known commonly as weighing and is used in instruments such as the beam balance, weigh beam, pendulum scale, and electromagnetic balance.Beam balance (equal arm balance)It is a balance consisting of a lever with two equal arms and a pan suspended from each arm. It is mostly useful in qualitative chemical analysis.

Weigh beamIt is a kind of large steelyard for weighing merchandise. It is also calledweighmaster's beam.

Pendulum scalePendulum type scales do not use springs. This design uses pendulums and operates as a balance and is unaffected by differences in gravity.

3. Spring balanceIt is anapparatus that simply a spring fixed at one end with a hook to attach an object at the other. It works by Hookes Law, which states that the force needed to extend a spring is proportional to the distance that spring is extended from its rest position.

Mass CalibrationThe primary requirement in mass calibration is maintenance of a set of standard massesapplied to the mass sensor being calibrated. Provided that this set of standard masses isprotected from damage, there is little reason for the value of the masses to change. Despite this, values of the masses must be checked at prescribed intervals, typically annually, in order to maintain the traceability of the calibration to reference standards.Force MeasurementThis is concerned with the measurement of horizontal forces that either stretch or compress the body that they are applied to according to the direction of the force with respect to the body. If a force of magnitude, F, is applied to a body of mass, M, the body will accelerate at a rate, A, according to the equation:F = MAThe standard unit of force is the NewtonOne way of measuring an unknown force is therefore to measure acceleration when it isapplied to a body of known mass. An alternative technique is to measure the variation in the resonant frequency of a vibrating wire as it is tensioned by an applied force. Finally, forms of load cells that deform in the horizontal direction when horizontal forces are applied can alsobe used as force sensors.1. Use of AccelerometersIt is a device that measures proper acceleration. The proper acceleration measured by an accelerometer is not necessarily the coordinate acceleration (rate of change of velocity).

2. Vibrating Wire SensorIt measures force (a mechanical load) using a wire that vibrates at a high frequency.

It consists of a wire that is kept vibrating at its resonant frequency by a variable-frequency oscillator. The resonant frequency of a wire under tension is given by:F = (0.5/L){(M/T)}where M is the mass per unit length of the wire, L is the length of the wire, and T is the tension due to the applied force, F. Thus, measurement of the output frequency of the oscillator allows the force applied to the wire to be calculated.Force Sensor CalibrationForce sensors are calibrated using special machines that apply a set of known force values to the sensor. The machines involved are very large and expensive. For this reason, force sensor calibration is normally devolved to either specialist calibration companies or manufacturers of the measurement devices being calibrated, who will give advice about the frequency of calibration necessary to maintain the trace ability of measurements to national reference standards.Torque MeasurementTorque,momentormoment of forceis the tendency of aforceto rotate an object about an axis,fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist to an object. Mathematically, torque is defined as the cross productof the lever-arm distance and force, which tends to produce rotation.1. Measurement of Induced StrainThe method involves bonding four strain gauges onto a shaft as shown in Figure, where the strain gauges are arranged in a d.c. bridge circuit.

The output from the bridge circuit is a function of the strain in the shaft and hence of the torque applied. It is very important that positioning of the strain gauges on the shaft is precise, and the difficulty in achieving this makes the instrument relatively expensive. This technique is ideal for measuring the stalled torque in a shaft before rotation commences.2. Optical Torque MeasurementOptical techniques for torque measurement have become available recently with the development of laser diodes and fiber-optic light transmission systems. One such system is shown in Figure.

Two black-and-white striped wheels are mounted at either end of the rotating shaft and are in alignment when no torque is applied to the shaft. Light from a laser diode light source is directed by a pair of fiber-optic cables onto the wheels. The rotation of the wheels causes pulses of reflected light, which are transmitted back to a receiver by a second pair of fiber-optic cables.3. Reaction Forces in Shaft BearingsThe magnitude of the transmitted torque can be measured by cradling either the power source or the power absorber end of the shaft in bearings, and then measuring the reaction force, F, and the arm length, L, as shown in Figure

The torque is then calculated as the simple product, FL. Pendulum scales are used verycommonly for measuring the reaction force. Inherent errors in the method are bearing friction and windage torques. This technique is no longer in common use.4. Prony BrakeThede Prony Brakeis a simple device invented byGaspard de Pronyto measure thetorque produced by an engine. The term "brake horsepower" is one measurement ofpower derived from this method of measuring torque. (Power is calculated by multiplying torque byrotational speed.)Essentially the measurement is made by wrapping a cord or belt around the output shaft of the engine and measuring the force transferred to the belt throughfriction.

Calibration of Torque Measuring SystemsSpecial machines are required for torque measurement system calibration that can apply accurately known torque values to the system being calibrated. Such machines are very expensive. It is therefore normal to use the services of specialist calibration companies or to use similar services provided by the manufacturer of the torquemeasurement system.