Force Sensors The basic principle behind the measurement

Force Sensors

The basic principle behind the measurement of force is – when a force is applied on an object, the object gets displaced. The amount of displacement occurred can be calculated using the various displacement transducers, and thus force measurement can be done. Some of the methods for measuring force are given below: • Strain Gauge Transducer • Piezo-Electric Transducer • Magnetostrictive Transducer • Force Measurement using Pressure • Other Force Measuring Systems

Strain Gauge Transducer • Force transducers based on strain gauges have a so-called spring element or loaded member where the forces to be measured are applied. The spring element deforms and strain is produced on the surface. The task of the spring element is therefore to convert the forces to be measured into strain as reproducibly and linearly as possible. Many force transducer properties are defined by the selection of the spring element material and design. In general, four strain gauges are used, installed so that two are stretched and two are compressed when force is applied. • These four SG are connected in a Wheatstone bridge circuit. The complete Wheatstone Bridge is excited with a stabilised DC supply and with additional conditioning electronics, can be zeroed at the null point of measurement. As stress is applied to the bonded strain gauge, a resistive changes takes place and unbalances the Wheatstone Bridge.

This results in a signal output, related to the stress value. As the signal value is small, (typically a few millivolts) the signal conditioning electronics provides amplification to increase the signal level to 5 to 10 volts

• four gauges (full bridge), two gauges (half bridge), a single gauge (quarter bridge)

Piezoelectric Transducer The main principle of a piezoelectric transducer is that a force, when applied on the quartz crystal, produces electric charges on the crystal surface. The rate of charge produced will be proportional to the rate of change of force applied as input. As the charge produced is very small, a charge amplifier is needed so as to produce an output voltage big enough to be measured. If a force is applied on the pressure port, the same force will fall on the force summing member. Thus a potential difference will be generated on the crystal due to its property. The voltage produced will be proportional to the magnitude of the applied force.

Applications • Due to its excellent frequency response, it is normally used as an accelerometer, where the output is in the order of (1 -30) m. V per gravity of acceleration. • The device is usually designed for use as a pre-tensional bolt so that both tensional and compression force measurements can be made. • Can be used for measuring force, pressure and displacement in terms of voltage. Advantages • Very high frequency response. • Self generating, so no need of external source. • Simple to use as they have small dimensions and large measuring range. • Barium titanate and quartz can be made in any desired shape and form. It also has a large dielectric constant. The crystal axis is selectable by orienting the direction of orientation. Disadvantages • It is not suitable for measurement in static condition. • Since the device operates with the small electric charge, they need high impedance cable for electrical interface. • The output may vary according to the temperature variation of the crystal. • The relative humidity rises above 85% or falls below 35%, its output will be affected. If so, it has to be coated with wax or polymer material.

Magnetostrictive Transducer Magnetostriction can be explained as the corresponding change in length per unit length produced as a result of magnetization. The material should be magnetostrictive in nature. This phenomenon is known as Magnetostrictive Effect. The same effect can be reversed in the sense that, if an external force is applied on a magnetostrictive material, there will be a proportional change in the magnetic state of the material. This reverse process is also called Villari Effect. A magnetostriction transducer is a device that is used to convert mechanical energy into magnetic energy and vice versa. Such a device can be used as a sensor and also for actuation as the transducer characteristics is very high due to the bi-directional coupling between mechanical and magnetic states of the material.

This device can also be called as an electro-magneto mechanical device as the electrical conversion to its appropriate mechanical energy is done by the device itself. In other devices, this operation is carried out by passing a current into a wire conductor so as to produce a magnetic field or measuring current induced by a magnetic field to sense the magnetic field strength.

Force Measurement Using Pressure 1. Hydraulic Load Cell As shown in the figure given below, the inside chamber of the device is filled with oil which has a pre-load pressure. The force is applied on the upper portion and this increases the pressure of the fluid inside the chamber. This pressure change is measured using a pressure transducer or is displayed on a pressure gauge dial using a Bourdon Tube.

• When a pressure transducer is used for measuring the value, the load cell is known to be very stiff. Even at a fully forced condition, it will only deflect up to 0. 05 mm. Thus, this device is usually used for calculating forces whose value lies between 500 N and 200 Kilo. N. The force monitoring device can be placed at a distance far away from the device with the help of a fluid-filled hose. Sometimes there will be need of multiple load cells. If so, a totalizer unit has to be designed for the purpose. • The biggest advantage of such a device is that it is completely mechanical. There is no need of any electrical assistance for the device. They can also be used for calculating both tensile and compressive forces. The error percentage does not exceed more than 0. 25% if the device is designed correctly. • The device will have to be calibrated according to the temperature in which it is used as it is temperature sensitive.

2. Pneumatic Load Cell The working of a pneumatic load cell is almost same to that of a hydraulic load cell. The force, whose value is to be measured, is applied on one side of a piston and this is balanced by pneumatic pressure on the other side. The pressure thus obtained will be equal to the input force applied. The value is measured using a bourdon tube. The pneumatic load cell has an inside chamber which is closed with a cap. An air pressure is built up inside the chamber until its value equals the force on the cap. If the pressure is increased further, the air inside the chamber will forcefully open the cap and the process will continue until both the pressures are equal. At this point, the reading of the pressure in the chamber is taken using a pressure transducer and it will be equal to the input force.

Other Force Measuring Systems 1. Elastic Devices • The strain gauge can be replaced with a Linear Voltage Differential Transformer (LVDT) inside a load cell to know the displacement of an elastic element. The device is best suitable for dynamic measurements as it has good features like high resolution and hysteresis. • Another device most suitable for the measurement of force in an elastic element is the capacitive load cell. With the device, the displacement can be calculated by measuring the capacitance. The sensor has two parallel plates with a small gap in between. According to the force applied on the device, there will be a change in length of the spring member, which in turn changes the gap distance between the plates and produces a proportional capacitance. This measured capacitance value will be proportional to the force applied. • Optical fibers can also be used to design an optical strain gauge to measure force. When a force is applied on the force summing member, it causes a change in length of the optical fibers that are bonded to the strain gauge. If the level of strain is different for two optical fiber strain gauges, then the phase difference between the monochromatic beams that strike the optical gauges will be proportional to the value of force applied. • For obtaining a displacement value of high resolution, a device called interference-optical load cell can be used. A Michelson Interferometer is used to measure the amount of force that has caused the change in shape of the fork-shaped spring. The highest amount of elastic deformation and along with it, the strain of the material need not be as large as in the case of the strain gauge load cell. The spring is made of quartz with very little temperature dependence.

2. Vibrating Elements The principle of resonance is used in the force transducer of vibrating elements. If a tuning fork load cell is used, the transducer will have two parallel band plates connected at both ends and will be made to vibrate in opposite directions. The change in resonance thus caused will be proportional to the force applied. The transmission and reception of the signals are carried by two piezo-electric elements kept very close to the fork.