Strain Gauge Load Cell - Sensors and Transducers Pedagogy Zone

A load cell is an electromechanical transducer that converts load acting on it into an analog electrical signal. Strain Gauge Load cell provide accurate measurement of compressive and tensile loads. Load cells commonly function by utilizing an internal strain gauge that measure deflection. Because the modulus of elasticity of a load cell is constant the amount of strain can be calibrated to determine the force upon the load cell. Typically the force creates the train in the load cell which is measured by strain gauge transducer.

Strain gauge is attached to the object or the strained element where the force is being applied. As the object is stressed due to the applied force, the resulting strain deforms the strain gauge attached with it. This causes an increase in resistivity of the gauge which produces electrical signal proportional to the deformation. The measurement of resistivity is the measure of strain which in turn gives the measurement of force or load applied on the object. The change of resistance is generally very small and is usually measured using a Wheatstone bridge circuit where the strain gauges are connected into the circuit. The strain gauges are serving as resistors in the circuit.

The Wheatstone bridge circuit produces analog electrical output signal. In a typical strain gauge load cell for measuring force, four strain gauges are attached to the surface of the counterforce and are electrically connected in a full Wheatstone bridge circuit as shown in image. Load cells have different shapes (cylindrical tubes, rectangular or square beams, and shaft) for different applications and load requirements to ensure that the desired component of force is measures, thus strain gauges having different shapes are positioned in various orientations upon the load cell body. The different configurations of strain gauges are already discussed under strain gauges displacement sensors.

Fluid Pressure Sensors

Pressure is an expression of the force required to stop a fluid from expanding, and is usually expressed in terms of force per unit area. A pressure sensor measures pressure of gases or liquids. These sensors generate a signal as a function of the pressure applied by the fluid. Pressure sensors are used in many applications such as automotive vehicles, hydraulic systems, engine testing etc. Pressure sensors may required to measure different types of pressures: 1. Absolute pressure where the pressure is measured relative to the perfect vacuum or zero-pressure, 2. Gauge pressure where the pressure is measured relative to the atmospheric pressure, and 3. Differential pressure where a pressure difference is measured. The devices which are used to measure fluid pressure in industrial processes are:

- Diaphragm pressure sensor
- Capsule pressure sensor
- Bellows pressure sensor
- Bourdon tube pressure sensor
- Piezoelectric sensor
- Tactile Sensor

The construction and working principle of these sensors are explained here.

Diaphragm pressure sensor

The diaphragm pressure sensor uses the elastic deformation of a diaphragm (membrane) to measure the difference between an unknown pressure and a reference pressure. Diaphragm is a thin circular elastic membrane made of generally silicon as show in image. As pressure changed, the diaphragm moves, and this motion is the measure of differential pressure. Diaphragms are popular because they require less space and the motion they produce is sufficient for operating electronic transducers. They also are available in a wide range of materials for corrosive service applications.

A typical diaphragm pressure gauge contains a chamber divided by a diaphragm. as shown in the image. One side of the diaphragm is open to the external targeted pressure. P ext, and the other side is connected to a known pressure, P ref,. The pressure difference, P ext – P ref mechanically deflects the diaphragm.

The diaphragm deflection can be measured in any number of ways. For example, it can be detected via a mechanically-coupled indicating needle, an attached strain gauge [refer image (a)], a linear variable differential transformer (LVDT) [refer other displacement/velocity sensors. Once known, the image (b)], or with many deflection can be converted to a pressure loading using plate theory.

Strain gauge arrangement consists of four strain gauges with, two measuring the strain in a circumferential direction while the remaining two measure strains in a radial direction. The four strain gauges are connected to form the arms of Wheatstone bridge. The sensitivity of pressure gauges using LVDTs is good and. therefore, stiff primary sensors with very little movement can be used to reduce environmental effects. Frequency response is also good.

Advantages – Strain Gauge Load Cell

Much faster frequency response than U tubes.
Accuracy up to ±0.5% of full scale.
Good linearity when the deflection is no larger than the order of the diaphragm thickness.

Disadvantages : More expensive than other pressure sensors.

Capsule pressure sensor

In order to improve the sensitivity, two corrugated diaphragms are combined by arranging these in back-to-back and sealed together at the periphery to obtain shell like shape as shown in image. These are called as capsules. One of the diaphragms is provided with a central reinforced port to allow the pressure to be measured, and the other is linked to a mechanical element. The difference in pressure between inner and outer surface of the capsule produces displacement. These capsules can also be attached with the LVDT as described in the diaphragm pressure gauge.