Tuesday, September 28, 2010

Things to consider before buying a load cell

When your application needs to measure static or quasi-static loads, forces or weights, you would need to use a load cell. A load cell is a device that converts the applied force into an electronic signal that can be used to compute the magnitude of the force applied. Although relatively simple in concept, you’ll find that there are many complexities that might affect measurements when you actually implement your load cell system. One needs to consider a number of factors before you decide which load cell to use. These include:

  1. Magnitude of force/load/weight being measured
    • This is important to know because typical load cells offer overload protections of about 1.5 times the rated load capacity. If you apply loads – even temporarily – beyond this maximum load – you can seriously damage your load cell and might need to recalibrate it to ensure accurate readings
    • If you suspect that there could be an occasional spike in loading conditions, be conservative in picking a load cell with adequate capacity to handle the highest load condition with at least 50% margin to avoid exceeding the elastic limit of the load cell
  2. Accuracy needed for application
    • Accuracies of load cells are generally rated as a percentage of full scale. So a load cell with 0.1% accuracy rating for a 100 lb load cell would offer +/- 0.1 lb accuracy, but the for a 1000 lb load cell offer a +/- 1 lb accuracy. So you need to pick the right capacity load cell for your application
    • Certain test and measurement applications may need the accuracy as a percentage of reading so you may need to pick a load cell with adequate accuracy to address those applications
    • You also need to consider the accuracy of the complete system – not just the load cell. Depending on how you plan to use the load cell, the accuracy of the system could be a function of the load cell, the signal conditioning equipment, the resolution of the digitizer, the variation of the power being supplied and other environmental factors
  3. Operating Temperature Conditions
    • Although most load cells offer certain degree of temperature compensation, you need to consider the operating conditions before you pick a load cell.
    • Is your application in a well controlled, room temperature or outdoor, harsh environments with changing humidity and temperature? If it is outdoor – you should look for load cells with IP68 rating
    • Even with temperature compensated load cells, there could be transient temperature conditions that affect the load cell readings. Generally speaking the compensation works best when used under constant temperature conditions
  4. Duration of measurement – Short Term vs. Long Term
    • An important consideration in picking the right load cells is to understand the stability of a load cell under short term vs. long term test conditions
    • Most scales work under the scenario where someone “
    • ” the load cell and then takes a reading. Accuracies are generally very high for these applications especially under room temperature conditions
    • But when temperatures are changing or when a constant load is placed on a load cell for a long time, and one cannot “Tare” the reading periodically there could be drift or Creep in the baseline reading. If your application cannot tolerate this kind of drift then you need to purchase a load cell with good long term stability and low creep
    • Process control applications that cannot be periodically tares and still require very high resolutions and accuracy with temperature over long periods of time – are some of the most demanding applications to outfit with a load cell
  5. Data Update Rate
    • How fast do you need the reading to be? 1 Hz, 10 Hz, 100 Hz or faster?
    • This is important to know because load cells are generally speaking "quasi static" devices. That is they are not meant to measure the dynamic forces acting on the sensor - rather just the static loads
    • When you need to measure forces at a very high frequency to study the dynamics of a system - you need to purchase a force sensor with those characteristics
    • The response time of a load cell is generally of the order of 3-5 milliseconds - BUT this does not take in to consideration the mass of the body attached to the load cell. Depending on what is attached to the load cell - the response time could be much slower than what you expect
    • Furthermore, the response time is generally indicated as that of the load cells' analog output - and not that of the complete digitized system. You need to consider the data rate of the data acquisition system, the PC or PLC being used etc. to get very high data rates
  6. Direction of loading
    • Load cells generally are designed for measurement in one direction - either in Tension or in Compression
    • The reason for this is that -calibration systems and calibration processes are designed for one direction.
    • If your application needs forces to be measured in both directions i.e. Universal loads - then the accuracy of the system may be lower than it is in just one direction.
    • Some load cells such as the S-Beam load cells are better for such universal applications and if your application permits the special mechanical configuration of the S-Beam load cell - then you should consider using that kind
    • Manufacturers of load cells may charge you an extra fee for calibrating the load cells in both directions
  7. Mounting Options
    • This may be one of the most often overlooked aspect of load cells! Designers, engineers or users often pick a load cell and then discover they have to spend hundreds of dollars more to design fixtures or other accessories to actually use it in practice!
    • Pay special attention to the mounting options offered by load cells on the top as well as bottom of the load cell. Threaded holes are easier to work with and are generally lower in profile and less expensive to use than if you have to drill a hole and thread it on your end!
    • Also pay special attention to off-center loading conditions when you design your application. Some load cells such as pancake, button style and other circular load cells are more accurate when used axially along the center of the load cells. When off center loading conditions exist - then certain load cells such as cantilevered single point load cells offer better performance than others.
  8. Output Required (Analog or Digital)
    • This is one of the most important factors to consider when picking a load cell since the type of output you want for your application is one of the biggest determining factors in the final cost of your solution
    • The most common output available from conventional load cells is mV/V. When you apply 10V DC power - you get about 20 milliVolt for full scale outputs. This is a low level analog signal which cannot be input into general purpose DAQs or PLCs and requires amplification before it can be used in that mode. Look for load cells with amplified outputs to reduce the headache associated with this step and the additional cost incurred
    • If the distance between the load cell and the DAQ/PLC is large - you may need to use the 4-20 mA type of analog output.
    • Better still - if your application works with a PC or a DAQ/PLC with digital inputs then look for load cells with Serial/RS-232/RS-485/USB or Wireless outputs
  9. Total cost of Ownership
    • The cost of the solution depends on many factors:
      • Cost of load cell
      • Cost of signal conditioning equipment needed
      • Cost of digitizers/control systems
      • Cost of Programming or applying calibration to get readings in lb/Kg/N
      • Cost of periodic recalibration
    • You need to consider the total cost of solution before making a decision about which load cell to purchase
    • In addition, there could be significant unit to unit variation in performance and accuracy - so you need to consider the cost of calibrating the system - if you are an OEM designing a system to be used by an average consumer
  10. Certifications Needed
    • If your application is going to be used commercially to transact business on the basis of weight, there are rules and regulations that you'll need to follow
    • For example if you are selling coffee beans packaged in 1lb bags, then you need to use NTEP certified load cells for measuring the coffee bags during packaging
    • In Europe and other areas this certification is called "OIML".
    • In general R&D, Education and Test and measurement applications do not require such NTEP/OIML certifications, but there might be industry guidelines and recommendations you need follow

Source: http://www.loadstarsensors.com/
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Tuesday, September 21, 2010

Digital Weigh Scale types

Weighing scales can be broadly classified into-
  1. fixed
  2. portable

Fixed scales can be classified as below-

Fixed Type

Description

Weight Range

Dormant scale

This scale is fixed in a pit such that it's top surface is in level with the floor. Heavy objects like cylinders/barrels can be rolled on to the scale for easy weighing. Dormant scales are commonly used in gas-filling areas to monitor cylinder weight. Filling takes place with the cylinder placed on the scale. On reaching the limit filling is cut-off or an alarm is sounded to alert the operator.

100kg to 10000kg

Tank/Hopper/Silo weighing system

Weighing system is built into the structure of the tank support. Load cells are positioned to take the tank/hopper/silo's weights. This system helps monitor/control material stock/movement.

500kg to 20000kg

Conveyor weighing system

A conveyor weighing system measures rate of flow of material- tonnes per hour (TPH). It is commonly used in industries such as mining, cement, fertilizer, etc. Load cells are fixed into under the conveyor belt which senses the weight at that point. The system includes other components like proximity switch to monitor speed. A special digitizer calculates TPH using input from load cells and proximity switch. Conveyor weighing system helps monitor/control rate of flow of material which is a critical parameter in process industries.

Crane weighing system

A weighing system is built into the structure of the crane. Load cells are positioned to measure the load lifted by the crane. This system helps avoid overloading. It is also used to monitor/control material stock/movement.

500kg to 20000kg

Filling/Packing system

A Packing/Filling system is commonly used in industries liquid or solid material are packed in containers like plastic bags, cardboard cartons, cans, bottles, etc. The system measures and fills the container with the specified quantity. Load cells installed in the system are connected to a special purpose digitizer cum controller which monitors & controls material feeders. Packing/Filling systems are common in food, sugar, cement, fertilizer, etc.

200g to 50kg

Check weighing system

A check weighing system is commonly used in industries where material (liquid or solid) is packed in containers like plastic bags, cardboard cartons, cans, bottles, etc. Check weighers are used to ensure that material quantity is right- not less or not more. The system alerts the operator whenever a package falls outside the specified limits. Some systems automatically send non-qualifying packages to reject bin. Check weighers are normally built into the conveyor system on which packed material move. Load cells installed in the system are connected to a special purpose digitizer which monitors weight data of every package moving over the conveyor.

50g to 200kg

Weigh bridge

Also called truck scales, these scales are used to weigh commercial vehicles like trucks and trailers. Weigh bridges are a must for medium and large manufacturing industries.

20T to 100T

On-board truck scale

Load cells are mounted on the truck chassis and connected to digitizer in the driver's console. This system measures the weight of the material in the truck/tipper. Onboard weighing is used as a safety feature or to track material movement continuously.

10T to 200T

Rail weighing system

Rails are modified to accommodate strain gauges and hence work as load cells. Such rails, installed in the tracks and connected to a digitizer, help measure railcar weight. The train need not stop for the measurement- weighing is done in-motion.

100T to 500T

Off-Road Truck Scales

These scales are heavier versions on truck scales. Heavy tippers used coal/iron ore mines use such scales to track material movement.

50T to 500T

Weigh-in-motion system

These systems are used to check overloading of commercial vehicles on highways. A weigh bridge positioned under the surface of the road measures the weight of truck/trailers while the vehicles are in motion.

100T



Portable scales can be classified as below-

Portable Type

Description

Weight Range

Laboratory scale

Laboratory scales are made for high accuracy-high precision measurement of light objects weighing few grams. These scales are delicate & sensitive and need to be handled with extreme care. Most cases these scales are placed in a special enclosure to protect the scale from dust, moisture and even blowing air. Test weights or objects to be weighed are handled with forceps, not with bare fingers.

10g to 1kg

Human scales

Human scales designed for home/clincal use are generally low accuracy scales. Compact floor models are the most commonly ones used to weigh human beings other than infants. There are other special types for the disabled- wheel chair scales or scale mounted stretchers. To weigh infants and small kids, baby scales are employed by hospitals.

200kg

Table top scale

Table top scales are used for weighing light weight objects. Retail stores is the biggest market for these scales. Dealers offer various models- low-cost to high precision/high accuracy models. These scales are easy to use and easy to maintain but delicate and need to be handled with care.

10kg to 30kg

Bench scale

Bench scales are low capacity scales mainly used at a retail stores. Most scales are simple low-cost, easy to use, easy to maintain devices which can be purchased of the shelf.

30kg to 50kg

Platform scale

Platform scales are available in low to high capacities having a wide range of applications right from a retail store to a warehouse to a manufacturing setup. While lower capacity scales have one load cells, higher capacity scales come with four load cells. Scale manufacturers/dealers offer simple low-cost models to expensive custom-built models with extended features like connectivity to PC or ticket-printing facility.

100kg to 500kg

Floor scale

Floor scales are designed for heavy-duty usage in industrial, manufacturing or warehouse setting. These scales usually have a two steel frames- lower and upper- between which four load cells positioned. Objects to be weighed are placed on the top deck plate which is fastened to the top frame. Load cells are connected to a digitizer via a junction box. Digitizer can be a simple weight indicator or possess advance features to transfer weight data to a PC where data is captured & stored for further use like generating reports on material movement.

200kg to 1000kg

Pallet weighing scale

Pallet Weighing Systems (PWS) helps weigh load on the spot or as it is being handled. PWS are commonly used in manufacturing setups to track material movement. Load cells and necessary accessories are integrated into the pallet truck frame so that load cells sense the weight of the material handled. A battery powered digitizer powers the load cells and displays weight on a ergonomically positioned display.

500kg to 2000kg

Hook Scale

Hook scales is a compact device with a tensile load cell and digitizer components housed in special enclosure. The load cell has hooks fixed on both ends- upper hook for hanging the scale and lower for hanging the object to be weighed. Hook scales are commonly used in cranes in loading area so that weight of the material can be checked. They are also used in production area to ascertain weights of various objects. They can also be used as safety equipment in cranes while hoisting delicate or sensitive items.

500kg to 2000kg

Livestock scale

Livestock scales or cattle scales are commonly used on farms to weigh animals. These scales are designed to accommodate the animals and coated with special paints or coatings to resist corrosive properties of animals wastes. Livestock scales are easy to carry, setup and use.

100kg to 500kg

Ramp-End Scale

Weigh pads or Portable Ramp-end Scales help weighing vehicles on any level surface. Weigh pads are used to check weight distribution on wheels/axles. These light weight and low profile scales can be carried around in a car or a van. Setting up the scales is simple- just position scales to match axle width & wheel base and connect to digitizer. The battery powered digitizer powers the weigh pads and displays weight- individual wheel weight, weight on each axle or total weight. Portable Ramp-end Scales are useful to vehicle manufacturers, car racing teams, fleet owners and highway authorities (to check overloading of vehicles).

5T to 10T

Aircraft weighing system

Aircraft weighing systems are similar to Portable Ramp-end Scales. AWS is used to check weight distribution on the wheels and also to measure total weight of the aircraft. These light weight and low profile scales can be carried around and set-up easily. Normally the aircraft to be weighed is jacked up, weigh pads positioned under the aircraft and then lowered down so that it rests on the weigh pads. A battery powered digitizer powers the weigh pads and displays weight- weight on each of the pads and also the total weight. AWS are useful to aircraft manufacturers and aircraft maintenance companies.

2T to 10T

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Tuesday, September 14, 2010

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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Tuesday, September 7, 2010

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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