FLX Force Sensor Study Guide

Force SensorsWhat is a force sensor?In physics, the definition of force is any agent that causes a mass to move. When you push an object, say a toy wagon, youre applying a force to make thewagon roll. Whether the wagon actually does roll depends upon the applied force overcoming other forces that oppose the motion, such as the force from friction. A force sensor, then, is a device that mea- sures the amount of force applied.There are many ways to measure force, and major differences among force mea- surement devices. Factors that engineers must consider when making a force mea- surement decision include determining the proper output range, accuracy, price, and the ease of project integration provided by the sensors signal conditioning electronics.Force, mass, and weightThe most commonly known force is that of gravity, which continuously tries to pull objects to the earth. Holding an object stationar y in your hand, say a 2-kg mass, means your hand is applying an upward force that exactly opposes the downward force of gravity. The measure of force is called a Newton (N). Gravityexerts a 9.8 N force per kilogram of mass, so a 2-kg mass exhibits a force of 19.6

N. Your hand must be exerting a 19.6 N force upward to hold the mass stationar y against gravity s downward tug.Note that the previous discussion used the term mass rather than weight. In ever yday use, the mass of an object is often referred to as its weight. However, this is incorrect. The physical sciences rigidly define mass and weight as sepa- rate measures. The weight of an objectactually depends on several factors, most notably the force of gravity. Surprisingly, the force of gravity changes with latitude, altitude, and subsurface densities. Thus the same object can possess different weights at different points on the earth.

The mass of an object, however, does not change and represents the total amount of matter in the object. For best results,the idea of weight in force sensing should be avoided.Other confusions arise with the use of the term pressure. While pressure does exert force, the amount of force is con- trolled by the size of the area to which the pressure is applied:Force = Pressure X AreaFor example, lets start with three weights, each with a mass of 2 kg. The bottom of the first weight has a surface area of 10 cm2, the second weight 1 cm2, and the third weight 0.1 cm2. Holding each weight stationar y in your hand means that you are applying an upward force of 19.6 N for each weight. But how the weights feel in your hand will be quite different. The first weight is easy

to hold, while the second creates some discomfort. Holding the third weight becomes outright painful. In each case, the force to hold the weight stationar y remained the same: 19.6 N. But the pres- sure changed from 1.96 N/cm2 to 19.6 N/ cm2 for the second, and 196 N/cm2 forthe third!Stress vs. strainAn object will change its size or shape at the application of any force. A prime example of this is a diving board. As a diver walks to the end of the board, it bends downward due to the force applied to the board by the divers weight. Once the diver leaps from the board, it snaps back to its original shape. The diving board is said to have elasticity.Material can shift many different ways in reaction to an applied force depend- ing upon how the force is applied. Such forces typically fall into one of three clas-sifications: tension, compression, or shear.

Presented bySponsored bywww.tekscan.com/flexiforce.html

1 June 2012F FA L A F A LL LO LOF F FTension Compression Shear

a force of 196 N has been applied to the wire. If the wire had a cross-sectional area of 0.04 cm2, the amount of stress applied to the wire becomes 196 N/0.04 cm2, , or 4,900 N/cm2 of stress.Stress = Force / Cross-sectional AreaLO Confusion can arise between the values of stress and pressure because this equation for stress looks similarto the equation for pressure. However, pressure is applied to the surface of an object, while stress occurs within the body of the object.For real materials, stress is pro- portional to strain only when strain is sufficiently small. It is possible toTension occurs when the force pulls on an object, increasing its length. Compression does just the opposite, pushing against an object shortening its length. In shear, the elastic object is subjected to equal but opposite forces across its opposing faces.

The degree to which the object changes shape is a function of the stress and strain on the element.Strain is the relative change in the shape or size of an elastic object due to an applied force. For example, a 10-kg mass attached to a wire applies a ten- sion force that makes the wire stretch0.01 mm over a 20-mm length. The strain on the wire is 0.01/20 or 0.0005. The strain value thus tells us how much a particular length of wire will stretch with the same amount of force. Note that strain does not have a unitof measure.Strain = Change in Length / OriginalLength (for the same applied force)

Because strain is typically such a small number, the value is usually measured in microstrain (strain). Microstrain equals the strain value times 106. For example, an elastic ele- ment has a strain value of 0.0000032. To convert this reading to microstrain, multiply the strain value by 106:0.0000032 106 = 3.2 strain.Stress is the measure of the internal forces acting within an object. In the wire example, the wire grew longer when attached to a 10-kg mass. We say

exceed the elastic limit of the mate- rial. The elastic limit is defined as the maximum force that can be applied toa material without permanently chang- ing its shape. Forces kept below the elastic limit let the material snap back to its original shape when the force is removed. However, if the elastic limitis exceeded, the materials shape is per- manently changed, destroying its cali- bration to measure the applied force.As even more weight is added, the wire eventually breaks. This is the breaking stress of the wire. Every ma- terial has its own elastic modulus, elas- tic limit, and breaking stress.Hookes Law states that stress is directly proportional to strain as long as the load does not exceed the elastic limit of the material being stretched. That means if the weight attachedto the wire should double, the wire should stretch to 0.02 mm, twice the amount. By measuring the amount the wire stretches, it should be possible to calculate the amount of force applied to the wire, and thus the amount of mass attached to the wire. If the wirestretched 0.005 mm, then the mass is 5 kg. However, if the wire stretched 0.015 mm, the mass equals 15 kg.

Measuring strainNow that it has been demonstrated that the elastic element changes its shape when a force is applied, a way to measure that change is needed. The most common method uses an electri- cal resistance strain gauge. (Note thatwww.tekscan.com/flexiforce.html

2 June 2012Elastic limit

Breaking point

and pressure sen- sors whose sensing element may be micro-machined out of a single pieceElastic region

Plastic regionChange in length, L

of silicon.Wire strain gauges were the original resistance- type strain gauge. Even though they are more expensive to produce than semiconductor orsome texts refer to these devices as strain gages. This is an accepted alter- nate spelling.)

Electrical resistance strain gauges work under a simple principle: All conductors exhibit some degree of re- sistance that is directly proportional to the conductors length, and inversely proportional to its cross-sectional area. Make the conductor longer, and its re- sistance goes up. Conductors with large diameters have lower resistance than those with small diameters.If a predetermined length of wire with a specific resistance is bonded to an elastic element, its size and shapewill change with changes in the size and shape of the element. By measuringthis change in resistance, the change in size of the elastic element can be deter- mined, and the force applied to the elas- tic element calculated.The two most common strain gauges use either a metallic foil or wire, or a semiconductor material. Each has a specific gauge factor, the measure of the output for a given strain. Semiconductor gauges typically have a 100 to 150

gauge factor while metallic wire and foil gauges typically only have a 2 to 4 gauge factor. The output of semiconductor gauges is non-linear with strain, and so they usually need special linearization circuitry. They are sensitive to tempera-

thin-film gauges, they are still the gauge of choice for high temperatures and stress analy- sis. A 20-to-30-m diameter wire is

bonded to a substrate material that is in turn bonded to the elastic element. To improve sensitivity, the wire makes sev- eral back-and-forth paths to extend its length along the force axis.

A relative newcomer to the force sensing arena is made of piezoresistive material sand- wiched between two conduc- tive plates. Piezoresistive material differs from other strain gage material in that its resistance depends upon the amount of force applied to the material rather than changein overall length or volume.With no force applied, piezoresistive material offers an electrical resistance of several megohms (M) almost an open circuit. However, as force is applied its resistance drops to the low kilo-ohm (k) range. The large swing in resistance with changes in force helps simplify the sens-ing electronics as well.

Load cellsThe most common means for measuring force is the load cell.

Silicon

Contactture changes, especially high tempera- tures, thus need careful matching of the gauges within any given load cell. Even so, they may still need a high degree of temperature compensation. The high gauge factor of semiconductors leads them to be the element of choice for small transducers. Typical uses areas force transducers, accelerometers,

The geometric shapeand modulus of elas- ticity of the elastic ele- ment within the load cell determines the range of force that can be measured, the di- mensional limits of the cell, its final perfor-

Contactwww.tekscan.com/flexiforce.html

3 June 2012+ExcR1 R2

+ExcR1

Strain gage

Strain gage

+Exc

Strain gageSig +Sig

Sig +Sig

Sig +SigStrain gageR3

Strain gageR3

Strain gage

Strain gageExc

Exc

ExcQuarter-bridge Half-bridge Full-bridgemance, and its production costs.Each load cell contains an elastic element to which the force is ap- plied. It is the change in shape of this elastic element that measures the overall force applied to the load cell. The load cell housing merely protects the elastic element and the sensing gauges attached to it.The elastic element can take on many different shapes. Some shapesthe elastic element may assume include that of a simple solid cylinder, a hol-low cylinder, a bending beam, a shear beam, an S-beam, a double-ended shear beam, a ring, or a toroidal ring.The material used for the elastic element is usually tool steel, stain- less steel, aluminum, or ber yllium copper. The best materials exhibita large linear relationship between stress and strain with no noticeable change over time.There must also be a high level ofLoad Spherical load buttonDiaphragm-1Diaphragm-2Housing (enclosed inert gas)Elastic bodyStrain gage

repeatability between applications of force to ensure that the load cell is a reliable measuring device. To achieve these characteristics it is usual to subject the material to a special heat treatment. This may include a sub-zero heat treatment cycle to get maximum stability.Circuits to Measure ChangeBecause of the extremely small resistance changes that occur with both semiconductor and metallic wire and film load cells, the most common measuring circuits for those devices use a Wheatstone Bridge. The load cell makes up one or more legs of the bridge. A sensi- tive voltmeter or other electronic circuit monitors the amount of im- balance in the bridge, and thus the level of applied force.Bridge circuits are classified as quarter, half, and full bridge depend- ing upon how many load sensing ele- ments are used and how they wire into the bridge. Less than full bridges need completion resistors to complete the other legs of the bridge circuit.An excitation voltage is applied to the bridge (+Exc, -Exc) to create voltage drops across the four resis- tive elements. The output signal (+Sig, -Sig) measures the difference in voltage drops from one side of the bridge to the other. When +Sig

and Sig are equal, the bridge is said to be balanced. Any force applied to a strain gauge changes the gauges resistance, producing a change inthe signal voltage. For half and full- bridge circuits, the strain gauges are arranged in such a way that as one gauge rises in resistance, the other drops. This enhances the measuredwww.tekscan.com/flexiforce.html

4 June 2012100lb Sensor120010008006004002000

Force (lbs)

Conductance:1/RResistance

0.0200.0180.0160.0140.0120.0100.0080.0060.0040.0020.000

over a larger surface area. Measuring the different forces applied over a large area can be daunting,in that it needs an individual force sen-sor for each measurement point. This can easily reach into the hun- dreds, if not thousands, of force sensors distributedover the sur-signal over the smaller bridge types.A piezoresistive force sensor has a much larger range of resistance output which lends itself to a simpler electronics implementation. In addi- tion, the drop in resistance is inversely proportional to the force applied tothe material. The inverse resistance ef- fect means that the conductance of the sensor becomes directly proportional to the force applied.There are a variety of circuit op- tions available to measure this rela- tionship. A simple voltage divider configuration is easily integrated into a small portable device where overall packaging size is of critical importance.However, as shown earlier, it is conductance that responds in a linear fashion with force. As current flow maintains a linear relationship with

face of an object.However, thin-film piezoresis- tive force sensors simplif y that task . The piezoresistive material of the sensor is crossed with two sets of parallel lines set in a crosshatch pat- tern. A simple scanning multiplexer checks the resistance at each point where the lines cross. If there are10 horizontal and 10 vertical lines, sensing for 100 points is possible. A 20 by 20 line matrix produces 400 sensing points. Dynamic pressure distribution systems currently available can contain as many as 1,600 sensing points per square inch.By analyzing the reading at each point, an overall distribu- tion of the forces applied to the surface area of the sensor can be displayed.

INFLEXFORCE

= 5V R1

OUTconductance, a standard I-V op-ampcircuit is recommended for applica- tions that need optimal linearity.This simpler sensing arrangement is easily adapted to microprocessor- based operation such as the type

SummaryForce sensors can measureany push from a feather land-

VOUT = VIN * RFLEXIFORCE / (R1 + RFLEXFORCE)C1RFEEDBACK

used for embedded control systems.Surface force distributionAll of the prior force measurement systems have one common limitation: They can only measure the force ap- plied to one point. However, thereare times when its desired to look at the distribution of force applied

ing on a brick tothe thrust of the space shuttles rocket engines. Its adaptablefor many other types of mea- surements, such as pressure,

Piezoresistive elementVT= 5V

VEE = Ground

MCP6002VCC= +5V

VOUTwww.tekscan.com/flexiforce.html

5 June2012mass, weight, and torque. When used with proper temperature compensation, its capable of operating overa wide tempera- ture range fromnumbing Antarctic cold to blistering desert heat.While load cells offer the great- est sensitivities to force measure- ments, their bulk and operational needs place definite limitations on their use in areas where weight and size are at a premium. Thin-film piezoresistive sensors built on flex-

ible circuit materials typically less than 0.01-in. thick overcome many of these size limitations. In addi- tion, their simpler interface and low-power operation makes theman ideal candidate for portable, low- cost force measuring systems.Though force sensors can only detect the force applied to a single point, surface force distribution measurement designs using thin-film piezoresistive materials can incor- porate thousands of test points per- mitting display of the distribution of forces across the entire surface.www.tekscan.com/flexiforce.html 6

June 2012

Force, F

Ressitance (K-Ohms)

0

10

20

30

40

50

60

70

80

90

100

110

120

V

R

V