The History and Early Development of Load Cells

Before strain gage-based load cells became the method of choice for industrial weighing applications, mechanical lever scales were widely used. Mechanical scales can weigh everything from pills to railroad cars and can do so accurately and reliably if they are properly calibrated and maintained. The method of operation can involve either the use of a weight balancing mechanism or the detection of the force developed by mechanical levers. The earliest, pre-strain gage force sensors included hydraulic and pneumatic designs.
 

In 1843, English physicist Sir Charles Wheatstone devised a bridge circuit that could measure electrical resistances.  The Wheatstone bridge circuit is ideal for measuring the resistance changes that occur in strain gages. Although the first bonded resistance wire strain gage was developed in the 1940s, it was not until modern electronics caught up that the new technology became technically and economically feasible. Since that time, however, strain gages have proliferated both as mechanical scale components and in stand-alone load cells.
  

load Cells come of age 

Today, except for certain laboratories where precision mechanical balances are still used, strain gage load cells dominate the weighing industry. Pneumatic load cells are sometimes used where intrinsic safety and hygiene are desired, and hydraulic load cells are considered in remote locations, as they do not require a power supply. Strain gage load cells offer accuracies from within 0.03% to 0.25% full scale and are suitable for almost all industrial applications.

In applications not requiring great accuracy--such as in bulk material handling and truck weighing--mechanical platform scales are still widely used. However, even in these applications, the forces transmitted by mechanical levers often are detected by load cells because of their inherent compatibility with digital, computer-based instrumentation. The features and capabilities of the various load cell designs are summarized in Figure 7-1.

 

Figure 1: Load Cell Performance Comparison
TYPE OF LOAD CELL	WEIGHT RANGE	ACCURACY (FS)	APPLICATIONS	ADVANTAGES	DISADVANTAGES
Mechanical Cells
Hydraulic	Up to 10,000,000 lb	0.25%	Tanks, bins and hoppers. Hazardous areas.	Takes high impacts, insensitive to temperature.	Expensive, complex.
Pneumatic	Wide	High	Food industry, hazardous areas	Intrinsically safe. Contains no fluids.	Slow response. Requires clean, dry air
Strain Gage Cells
Bending Beam	10-5,000 lb	0.03%	Tanks, platform scales,	Low cost, simple construction	Strain gages are exposed, require protection
Shear Beam	10-5,000 lb	0.03%	Tanks, platform scales, off- center loads	High side load rejection, better sealing and protection
Canister	to 500,000 lb	0.05%	Truck, tank, track, and hopper scales	Handles load movements	No horizontal load protection
Ring and Pancake	5- 500,000 lb		Tanks, bins, scales	All stainless steel	No load movement allowed
Button and washer	0-50,000 lb 0-200 lb typ.	1%	Small scales	Small, inexpensive	Loads must be centered, no load movement permitted
Other Types
Helical	0-40,000 lb	0.2%	Platform, forklift, wheel load, automotive seat weight	Handles off-axis loads, overloads, shocks
Fiber optic		0.1%	Electrical transmission cables, stud or bolt mounts	Immune to RFI/EMI and high temps, intrinsically safe
Piezoresistive		0.03%		Extremely sensitive, high signal output level	High cost, nonlinear output

Common Questions About Load cells

Q1:  How can load cell designs be distinguished?
A:  They can be sorted according to the type of output signal generated (pneumatic, hydraulic, electric) or according to the way they detect weight (bending, shear, compression, tension, etc.).

Q2:  How do hydraulic load cells work?
A:  They are force-balance devices, measuring weight as a change in pressure of the internal filling fluid.  A load or force acting on a loading head is transferred to a piston that in turn compresses a filling fluid confined within an elastomeric diaphragm chamber. As force increases, the pressure of the hydraulic fluid rises. This pressure can be locally indicated or transmitted for remote indication or control.   

Q3:  How is hydraulic load cell pressure indicated?
A:  Pressure can be locally indicated or transmitted for remote indication or control.

Q4:  Is hydraulic load cell output stable?
A:  Output is linear and relatively unaffected by the amount of the filling fluid or by its temperature. If the load cells have been properly installed and calibrated, accuracy can be within 0.25% full scale or better, acceptable for most process weighing applications.

Q5:  When are hydraulic load cells ideal for hazardous area? 
A:  Because this sensor has no electric components.

Q6:  Are there any marked disadvantages to using hydraulic load cells? 
A:  One drawback is that the elastomeric diaphragm limits the maximum force that can be exerted on the piston to about 1,000 psig. All-metal load cells also are available and can accommodate much higher pressures. Special metal diaphragm load cells have been constructed to detect weights up to 10,000,000 pounds.

Q6:  What are some of the typical applications of hydraulic load cells? 
A:  Hydraulic load cell applications include tank, bin, and hopper weighing.        

Q7:  How can maximum accuracy of hydraulic load cells be attained?
A:  The weight of the tank should be obtained by locating one load cell at each point of support and summing their outputs. As three points define a plane, the ideal number of support points is three.

Q8:  What is a hydraulic totalizer?
A:  It is the area where cell outputs are sent.  The totalizer then sums the load cell signals and generates an output representing their sum. Electronic totalizers can also be used to attain the same result.

Q9:  How do pneumatic load cells work?
A:  Pneumatic load cells operate on a force-balance principle utilizing multiple dampener chambers to provide higher accuracy than can a hydraulic device.  These type of load cells are often used to measure relatively small weights in industries where cleanliness and safety are of prime concern.

Q10:  What are a few advantages of pneumatic load cells?
A:  The advantages of this type of load cell include their being inherently explosion proof and insensitive to temperature variations. Additionally, they contain no fluids that might contaminate the process if the diaphragm ruptures.

Q11:  Are there any disadvantages to using pneumatic load cells?
A:  Yes, they have a relatively slow speed of response and they require clean, dry,  regulated air or nitrogen.

 Q12:  What are strain gage load cells?
A:  This type of load cell converts the load acting on them into electrical signals.  The gauges are bonded onto a beam or structural member that deforms when weight is applied.

Q13:  How many strain gages are usually used in testing?
A:  Four strain gages are used to obtain maximum sensitivity and temperature compensation. Two of the gauges are usually in tension, and two in compression, and are wired with compensation adjustments.

Q14:  What happens when weight is applied to the strain gage?
A:  The strain changes the electrical resistance of the gauges in proportion to the load.

Q15:  How popular have strain gage load cells become in recent years?
A:  Most other load cells are becoming obscure.  The popularity of strain gage load cells is in great part due to their ever-increasing accuracy and lower their unit costs.

Q16:  What are Piezoresistive Sensors?
A:  Similar in operation to strain gages, Piezoresistive sensors generate a high level output signal, making them ideal for simple weighing systems because they can be connected directly to a readout meter. The availability of low cost linear amplifiers has diminished this advantage, however. An added drawback of Piezoresistive devices is their nonlinear output.

Q17:  What are Inductive and Reluctance Sensors?
A:  These devices respond to the weight-proportional displacement of a ferromagnetic core. One changes the inductance of a solenoid coil due to the movement of its iron core; the other changes the reluctance of a very small air gap.

Q18:  How does a Magnetostrictive sensor work?
A:  The operation of this sensor is based on the change in permeability of ferromagnetic materials under applied stress.  It is built from a stack of laminations forming a load-bearing column around a set of primary and secondary transformer windings. When a load is applied, the stresses cause distortions in the flux pattern, generating an output signal proportional to the applied load. This is a rugged sensor and continues to be used for force and weight measurement in rolling mills and strip mills.

Q19:  Tell me about new sensor developments?
A:  Fiber optic load cells are center stage because of their immunity to electromagnetic and radio interference (EMI/RFI), suitability for use at elevated temperatures, and intrinsically safe nature.  Two techniques are showing promise: measuring the micro-bending loss effect of single-mode optical fiber and measuring forces using the Fiber Bragg Grating (FBG) effect. Optical sensors based on both technologies are undergoing field trials in Hokkaido, Japan, where they are being used to measure snow loads on electrical transmission lines.