Strain Gauge - how does it work?

Strain Gauge, also known as Foil Gauge is a device whose electrical resistance varies in proportion to the amount of strain in the device. The most commonly used strain gauge is the bonded metallic strain gage. It consists of a fine wire or metallic foil arranged in a grid pattern as shown below.

The white part in the above image is a high quality metallic wire of uniform cross-section which is bonded to a thin backing, called the carrier, which is attached directly to the test specimen or the transducer element. The strain experienced by the specimen/element is transferred to the strain gage, which responds with a linear change in electrical resistance.

Strain gauges are used for Stress Analysis and Manufacture of Transducers. Two distinct classes of strain gauges are available for Stress Analysis and Transducers. Further, within the transducer class, strain gauges for steel and aluminium are available. Strain gages are available commercially with nominal resistance values of 120Ω, 350Ω and 1000Ω.

A fundamental parameter of the strain gage which expresses it's sensitivity to strain is the gauge factor. Gage factor is defined as the ratio of change in resistance to the change in length (strain). The gage factor for metallic strain gages is typically around 2.

Strain gauges must be properly mounted onto the element/test specimen so that the strain is accurately transferred from the test specimen > adhesive > strain gage > foil.

Below are types of transducer class strain gauges-

• Linear strain gages
• Double linear strain gages
• Single shear strain gages
• Double shear strain gages
• Columnar strain gages
• T rosettes
• Half bridge strain gages
• Full bridge strain gages
• Membrane rosettes

HBM, Omega and Micro Measurements are few well known strain gauge manufacturers.

The article on Sensorland explains the working of a strain gauge.

Measuring Strain with Strain Gages - This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series will teach you a specific topic of common measurement applications by explaining theoretical concepts and providing practical examples.This tutorial introduces and explains the concepts and techniques of measuring strain with strain gages.

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Types of load cells

Types based on working principle-

• Cantilever/Bending beam
• Compression
• Tensile or In line
• Universal (tensile/compression)
• Shear
• Torque

Types based on design

• Bending beam
• Binocular Beam
• Column (Single column/Multi-column)
• Shear beam (Single ended/Double ended)
  
• S-type
  
• Load button
• Load pin
• Pancake

Types based on output signal

• Analog
  
• Digital

Types based on electrical properties

• Resistive (Analog)
• Piezoelectric (Analog)
• Capacitance (Digital)

Types based on connectivity

• Wired
• Wireless

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Load Cell manufacturers - few international brands

Vishay BLH

Transducer Techniques


Strainsert


Sensortronics


Precision Transducers


Omega


Mettler Toledo


Loadstar Sensors


Interface


Honeywell


HBM


Haris Sensor


Futek


Eilersen


ADI ARTECH


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How does a load cell work?

Load Cell is a device that converts force into electrical signal.

A strain gauge based load cell is made of a specially engineered mechanical component called as an element. Four strain gauges are positioned precisely and bonded on to the element using a special adhesive. (strain gauge is a device that changes resistance when it is deformed or stressed).


The force-electrical signal conversion happens in two stages-

• force applied deforms the element which in turn deforms the strain gauges
• strain gauges converts the deformation to electrical signals

The strain gauges are wired in Wheatstone bridge configuration. The bridge is connected to a power supply unit and a signal conditioner through a 4-core cable. When an input voltage (10VDC) is applied to the bridge, the output is a voltage in the range of few millivolts.


The load cell output is proportional to the load applied to the element.


Basic specifications of a load cell-

• Capacity (kg)
• Full scale Output (mV)
• Sensitivity (mV/V)
• Excitation Voltage (VDC)
  
• Input resistance (ohms)
• Output resistance (ohms)

Below are the commonly used standard values-

• Excitation voltage: 10V or 12V DC
• Full scale output: 10mV, 20mV and 30mV
• Load cell sensitivity: 1mV/V, 2mV/V and 3mV/V
• Input resistance: 350Ω and 700Ω
• Output resistance: 350Ω and 700Ω

Sensitivity = full scale output/excitation voltage
Example: 20mV/10V = 2mV/V

Two articles to you can read-
Introduction to Load Cells
How Does a Load Cell Work?

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How does a Digital Weigh Scale work?

Imagine you pick an apple and try to guess it's weight. You would place it in your palm and toss it lightly to feel the weight. After 3 or 4 tosses you would say "200 grams". Your hand sensed the weight and sent a signal to your brain which after some calculations estimated the weight. Right? Similarly, when you place the same apple on a digital weigh scale, the load cell senses the weight and sends the signal to the indicator. The signal conditioner processes the load cell signal and displays weight.

Basically all weigh scales consist of-

• a structure
• one or more load cells
• signal conditioner

Structure is the load bearing part. It transfers the weight to the load cell(s). In low capacity scales (table-top/bench/platform) the structure houses the load cell and electronic components. In higher capacity scales (floor/dormant/weigh bridge) the structure rests on load cells. The weight indicator

Load Cell is basically a transducer which converts force into electrical signal. Every digital weigh scale has one or more load cells. Table-top/Bench/Platform scales are single load cell weighing machines. Floor/Dormant scales have four load cells. Truck scales or Weigh bridges have 4 to 8 load cells depending on the rated capacity and length of the bridge. For all scales load cell is the part which determines the scales rated capacity. Of course the mechanical structure is designed to withstand loads well beyond the rated capacity. Load cells capacities range from 500g to 100T.

Signal Conditioner is the electronics part, it powers the load cell, receives signal from the load cell(s) and converts that signal to readable numbers. The numbers- digital output- can be made to display in desired unit of weight. Signal conditioners have extended features like RS232 or 4-20MA output which can be used for data acquisition or control other electrical equipment. An electronic weigh scale hooked up to a computer can automate many aspects of business operations like materials management or process control or billing automation.

Compare a mechanical scale to an electronic scale. Mechanical scale just shows the weight but an electronic scale does much more than that- it helps automate.

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