If you have seen large concrete bridges being constructed, you will be aware that pre-cast concrete deck elements are cast with ducts passing through in carefully designed paths in the casting yard.  Once the concrete has cured to a pre-determined strength, the elements are placed into position and then the elements have steel hawser tendons passing right through them and post-tensioned using large hydraulic jacks with tension force sensors incorporated.

Instead of using such a cumbersome force sensor arrangement, there is now a much neater way of monitoring the tension force using a battery-operated handheld non-contact tension force sensor available on the market called the Digiforce.  Not only does this now simplify work on site during construction, but also it means that bridges needn’t be closed or weight restricted whilst testing post-tensioned elements and suspension cables.  All that is needed is the length of cable being measured and it’s weight per metre.

 How it works is brilliantly simple and very accurate according to the Digiforce meter manufacturer.  It works on the principle that when a tensioned cable is struck, it oscillates to a unique base frequency based on its length between the two clamps and the mass per unit length.  The tension is thus derived using digital sensors and converted to a tension value electronically.

The magnetic sensors need a ferrous wire.  However, if other non-ferrous wires are to be measured a ferrous clip to fix on to the wire, is supplied along a bush hammer to create the vibration.  This dinky little device, as you can see comes with a keypad for entering data as well as a USB or standard RS232 nine-pin cable to output data onto a PC, ruggedised palmtop or laptop using the supplied software..

 You can easily see that that such a device has a wide application.  Indeed it is now typically being used in precast concrete yards where long planks and columns are being cast.  Before the concrete is poured, each wire can be checked for the specified tension in the mould.

In essence, this is a fine grid that is interleaved into a thin sheet carrier material and is connected to digital decoders and thence analysed by software to show pressure changes over the whole of the wafer.  This thin sheet can be cut into any shape required to suit the application and when pressure is imposed upon it, it’s output is shown in real time as a colour-coded pressure map on the computer screen.   For the first time, a dynamic picture in real time of pressure maps of a surface can be monitored over a long period and thus this is a godsend to testing and development over an extremely wide area of applications!

Some applications of these new tactile force sensor arrays follow to demonstrate their value to research and development as well as active everyday applications.

Tyre manufacturers can now use this novel application of technology to pressure map the tyre ‘footprint’ to accurately assess the effectiveness of their car designs without hours of live testing on the road and then check for wear.  It can be seen immediately by inserting a pad under a tyre and loading the wheel.

Dentists can now measure occlusions or bite patterns and record the results directly which makes for much more accuracy of corrective action without resorting to the traditional trial and error iterative methods.

Other applications in the automotive research area include, for example, head gasket/head bolting arrangements designs by inserting this thin film force sensor array and bolting down the head onto the engine.  Yet another example are brake discs and pads.  This latter application demonstrates how robust these arrays are.

We will be returning to explain how they work in more detail in future posts.

For measurement of small changes in length (or strains) where the material is not disrupted as the brick wall was, the change as measured by a strain gauge is not visible to the eye, this is the basis for all force sensors.  To briefly explain the physics, for a homogeneous material such as steel alloy, aluminium, other metals and ceramics, there is a fixed and proportional relationship between stress to which the material is subjected and the strain that occurs.  This is known as Young’s Theorem and the constant for the relationship between stress and strain is known as Young’s Modulus.  Each material has a known Young’s Modulus provided that the material has not been stressed beyond its ‘elastic limit’ which is usually close to but not coincident with its ‘yield point’.

Typical metal foil strain gauge element

Most commercially available force sensors consist of 4 piezo-resistive strain gauges bonded to a specially machined steel billet and arranged in a diamond along the axis of the force to be measured.  These strain gauges are wafer thin and comprise a specially printed circuit that changes it’s electrical resistance and the four gauges are wired as a Wheatstone Bridge to measure the changes in resistance.

Thus the force sensor can be calibrated in force or torque units depending on the design of the steel body.  It can be compressive as in pancake or load cells,  tensile loads as in the ‘Z’ load Cell such as can be found in craneage, or torsion force as can be found in dynamometers; the applications are legion.

Motorsport is a testing ground

Motorsport is a very competitive field in which a great deal of research and development takes centre stage.  The competitive edge is usually obtained by paring the last few milligrams of each component and like military aircraft, this process is taken to the maximum to keep ahead of the competition and not to lose that edge through over-design of any component.  To achieve this, the research and development uses force sensors for on-car testing as well as testing components to destruction on materials testing in the workshop and manufacturers’ testing lab.

On-car testing of structural components takes many forms and all use piezo-resistive force sensors (or transducers).  Let us take a quick canter through the applications of force sensors commonly used in motorsport and how these motorsport sensors are used in this area of automobile development.

Fatigue testing in the laboratory of suspension, structural members and other components is a very necessary starting point when testing a new car design.  Individual components are tested to destruction for both absolute strength and fatigue life on materials test rigs containing force sensors and torque sensors.  This is followed by testing of the whole car on a rolling road followed by the test track to amass dynamic loading data using components that are especially constructed with load cells and static torque sensors incorporated in the linkage.  The measurements include include suspension linkage force, gear change force, steering force and wheel bearing force.

Steering Force Load Cell

When designing a new body shape for a car, especially within motorsport, it is essential to establish how the car performs aerodynamically both from a fuel consumption and more particularly, from a safety point of view.  Initially, car mock ups are wind tunnel tested to test for lift, drag and yaw using load cell sensors upon the mock up’s platform.  This way, spoilers and other drag reduction measures can be modelled and improved as well as dynamic suspension tweaks.

Other measurements used on instrumented cars on the test tracks include integral wheel drag, foot brake force, engine & braking dynamometer readings, hand brake force, instrumented spherical rod end bearings and pitch link force for motor bikes.

Find out about position sensors from Applied Measurements on engineeringface.

Typical metal foil strain gauge element

First, a word or two on the piezo-resistive effect as nearly all modern load sensors rely on this effect.  The strain gauge is usually a very thin silicone/metal foil bonded sandwich with the metal foil arranged as shown in the illustration.  They are bonded to the steel body with either cyanoacrylic glue or more usually a form of epoxy cement for long lasting applications.  The elongation in the long direction causes a change in the electrical resistance of the array.  The low temperature coefficient of expansion of the alloy avoids much of the problems of resistance changes with temperature. Nearly all such stain gauges are arranged in a diamond with 2 points arranged through the axis of the force to be measured.  The gauges are wired into in a resistance (Wheatstone) bridge to ensure that the arrangement has self-temperature compensation (STC) across a wide temperature range usually -20°C to +80°C.  This simple arrangement is repeated depending on how many axes  are to be sensed in the tension/compression body.

The button load cell is a particular variant that is used for compressive loads and usually it can be found at the base of weighing vessels, foundations, compressive testing machines for construction materials and weigh bridges at the larger end and in weigh scales and other lab test machines at the lower end.  They are capable of weighing compressive loads between 0-250kN to a massive 0 – 30 tonnes and they in common with most force sensors of this type, can follow dynamic loading variations to a maximum of some 50kHz.

Other variants of this type is the cannister load cell for high accuracy applications and the pancake load cell.  Yet another form is the donut or low profile load cell that is used typically as a washer replacement in structural elements including post-tensioning anchors and mast guy anchor blocks.

S Type Force Sensor

The body of the S-Beam or Z-Beam force sensor is, as it’s name suggests, thus shaped and is used for both tension and compression loading with threaded holes for in line fixing as shown.  These are often used to measure tension forces in suspended hoppers or as a load link in a tensioned cable assembly for lifting operations.  They are also found in materials testing machines for both compressive and tension forces with ranges of less than 50kg or greater than 5000kg, our  covers load ranges from 0-1kg (10N) up to 0-30,000kg (300kN) with typical accuracies of 0.03% FS .

We have already covered torsion load cells on this blog with the torque sensors page and these are used particularly for monitoring rotating shafts for the dynamic type and static torque on various testing and industrial applications.

Another type that is quite distinct is the shear and loading beam load cell type in which a cantilever body is deformed under load.   They are specifically designed for the measurement of tensile and comprehensive forces where they form the bridge between the system framework and the live load as can be found in foundations for measuring vessel charge loads.

Finally, there are the multi axis force sensor products that measure 2 or 3-axes that are usually custom built to measure both torque and tension in the case of the 2-axes and torque, compression and side force for such applications as continuous low-stir friction welding and load and torque monitoring for wind turbines, in soil mechanics and medical industries applications.

Depth and level measurement in liquids is a vital requirement in a number of process and utility sectors where industry-standard fieldbus outputs are required  for integration of depth measurement into process control.   This can include wells, boreholes, waste water, lakes, reservoirs, rivers, water and sewage treatment plant as well as other liquids in bulk such as chemicals, fuel and diesel oil.

These days, it is possible to get a very wide range of depth sensor products that are very rugged and are capable of achieving ±0.1% accuracy and can be fully integrated into process plant or into hazardous areas by virtue of their ATEX certification.  What is more, they normally are designed and rated to resist the electomagnetic surges caused by nearby lightning strikes.  At first this appears to be a strange requirement given that they are normally at earth potential in say the ocean.  However, the transducer’s leads go into the areas that are not so protected.

Applied Measurement's ATEX Depth Sensor pi99ex

Depth transducer equipment that are commercially available, nearly all use ceramic (Aluminium Oxide) piezoresistive sensors as the barometric pressure sensing element for depths exceeding 10mWG and these have sealed (or closed) bodies.  For depths less than 10m, a vented stainless steel sensor is normally employed for greater accuracy at low barometric pressures.  The vent passes to the open atmosphere via the combined cable tube.

The casings are very rugged and are normally stainless steel turnings although I have seen titanium (Tn) bodies where mechanical strength in extreme conditions is specified.

Some models, instead of having a nose cone with an open end to the liquid being monitored, have a modified nose with a tapping to ½” BSPP male for insertion into a submerged pipeline.  Neat!

Force sensors have become very sophisticated these days and they are now routinely fitted in all types of industrial machinery or weighing devices to measure weight, compressive or tensile forces and, of course, dynamic and static torque.  In this article, we deal with the equipment needed that is needed to convert the raw output electrical signal from each sensor into an industry-standard digital signal and the equipment to read that digital output signal as a display.

Now force sensors need to be hooked up into a circuit that is excited by a DC voltage.  The analogue process signal output is almost always one of the industry-standards 0-5Vdc, 0-10Vdc or 4-20mA.  The force sensors are pre-assembled with usually, 4 strain gauges bonded within a sensor body in a diamond or cruciform pattern set at 45 degrees to the axis of force to be sensed (or 45 degrees to the shaft in the case of torque sensors).   The analogue output signal is delivered via a 350Ω Wheatstone bridge circuit.

Signal conditioners  are used to take analogue output from up to four 350Ω bridges connected in parallel and turn it into highly stable industry-standard analogue outputs.  Conditioners are capable of amplifying very low-level input signals right down to 0.06mV/V into a high-level industry standard process signal such as 0-10Vdc or 4-20mA.

The digitiser then converts the analogue dc voltage signal value into one or more of the digital signal formats in common use such as RS232, RS485 and USB and with protocols  such as ASCII and MODBUS RTU.  For most remote control and monitoring purposes a digital amplifier is required to ensure that a reliable digitised signal is delivered remotely.

Applied Measurement's 2.4GHz Telemetry Module

Specific on-board display units, remote units for use with control panels and handheld wireless remote units are available to complete each sensor manufacturer’s force sensor line up.  These units can take either one channel of digital signals or more commonly, a number of channels that are switchable.

Many force sensors that are sold for specific purposes are integrated pieces of equipment that contain both the sensors, the conditioners, amplifiers  and digitiser so that they can be used “plug and play” into most fieldbus applications.

Find out about the range of load cell digitiser and load cell signal conditioner products from Applied measurements on their website.

Measurement of torque is undertaken either as Static where little or no movement is observed in the manner of a torque spanner (less than one revolution) or Dynamic where the shaft is rotating such as in a vehicular drive shaft.  Excessive dynamic torque induced shear stress is very harmful to rotating drive shafts in that the stress is cyclic and fatigue failure will occur over time, even if the shear stress in the material is below the elastic limit for the material.

It is normal to measure torque in the centre of the shaft as far as possible from the spurious stresses induced at each end and four strain gauge resistors are mounted on the shaft surface at 45 degrees from the shaft axis either in a cruciform or box configuration. The resistors are then connected to form a wheatstone bridge circuit powered by with a 5 to 18V DC power source.

DTDR-F Wireless Telemetry Torque Sensor for Rotating & Static Applications

Traditionally, the dynamic torque  sensor needed slip rings to connect the circuitry to the strain gauge resistors. These have been developed to give low-loss connections but there are limitations.  However, with the advent of high charge density of lithium- polymer battery technology with a 1-year battery life possible, a flanged integrated version has been developed by Applied Measurements with the signal data being transmitted on a 2.4 GHz radio carrier wave.  This simplifies the whole system with remote telemetry being quite practicable.

We will be visiting the important subject of signal output considerations for the critical dynamic case in more detail in subsequent posts – so keep visiting

Mode of operation

The electrical resistance measurement in four strain gauges arranged in a wheatstone bridge circuit is at the heart of the modern load cell or force sensor.   The four strain gauge resistors are firmly set in a vertical plane as four sides of a square.  The wheatstone bridge circuit is powered by a constant 5V,  10V or 18V DC power source and the instrumentation is set to give a digital output signal that is calibrated in weight units.

Additional bonded strain gauge resistors are included to provide a comparative no-load reading at any particular temperature within the unit’s range thus  providing a control for the wheatstone bridge circuitry for null point readings.

The analogue or digital output signals are compatible with a range of bespoke strain gauge instrumentation digital transmitters in common use in the process and other industries.  Digital amplifiers typically provide an RS232, RS422 or RS485 output using either the common ASCII protocol or one of the many of more specialised, industry-specific protocols that have been developed such as CANbus or Modbus.

Canister Load Cell

The canister load cell is the common single point application with a rugged construction to IP67 enclosure standard and with a compressive load capacity typically from 0.5 Te to 3,000 Te.  These are usually of stainless steel construction and can be submerged. Canister load cells are available to measure both tensile and compressive forces.

S-Beam or Z-Beam Load Cells

S Type Force Sensor

The S-beam load cell and Z-beam tension load cells are usually relatively simple designs in which the structure is shaped as an ‘S’ or ‘Z’, with strain gauges bonded to the central sensing area in the form of a Wheatstone bridge.   They can measure both compressive and tension loads along with good dynamic side load rejection.  These force sensors can be found in tank level, hoppers and truck scales.

Link Load Cells

The link load cell can be found in lifting applications on cranes and winches.  The cells can be self-indicating links, remote display by wire links or remote display by telemetry links. They are also useful for measuring forces in vertical or horizontal tension frames and marine mooring applications. There are other types of force cell in this area and these include Bending Beam and shear beam (or pancake).  We shall cover these in future articles so keep visiting.

The definition of a force sensor (according to Wikipedia) is a device that converts a phyical applied force into another form of energy.   Other words for this are transducer,  strain cell, strain gauge touch sensor or load cell.  The application for force sensors are very wide ranging from compression load cells in structural buildings that measure  dynamic forces of 100′s or even 1,000′s of tonnes, to the force sensitive resistor (FSR) that can detect microscopic strains and is used typically for concealed alarms.

We will be exploring this wide-ranging subject in some detail in future articles on this website but today, we will concentrate on the most common type, the piezoelectric force sensor.   The piezo-electric effect is when certain cystalline and ceramic materials, when subjected to a force, develop an electrical potential difference across them.  This EMF (or voltage) is directly proportional to the pressure applied and this is used and measured by simple electrical circuitry for various purposes.   The effect is reversible and repeatable over 1,000′s of cycles.  Therefore it’s uses in  highly accurate strain and force measurement are legion.

piezoelectric guitar pickup

Some of the most well-known everyday uses are the cigarette lighter,  the push-start butane barbeque lighter and the modern accurate weighing machines found in many domestic kitchens .    Other uses using this principle are the production and detection of sound, generation of high voltages, electronic frequency generation, micro-balances, and ultrafine focusing of optical assemblies.

Just as when a force is applied to a piezoelectric crystal a voltage is induced, the application of a voltage will change the shape of the crystalline substance such as can be found in modern camera focusing applications where a crysalline lens itself can be manipulated.

The piezoelectric effect was first discovered in the 1880′s and it’s first practical use was as ASDIC (or SONAR) in first World War but  it wasn’t until the 1950′s  piezoelectricity came into everyday use in such things as load cells and sound recording.