Position: In this page, you will learn about Load Cells Design.
Before strain gauge 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 gauge force sensors included hydraulic and pneumatic load cell designs.
In 1843, English physicist Sir Charles Wheatstone devised a bridge circuit that could measure electrical resistances. As was discussed in detail in Chapter 2, the Wheatstone bridge circuit is ideal for measuring the resistance changes that occur in strain gauges. Although the first bonded resistance wire strain gauge 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 gauges have proliferated both as mechanical scale components and in stand-alone load cells.
Today, except for certain laboratories where precision mechanical balances are still used, strain gauge 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 gauge 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 weighing 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 7-1: 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 gauge Cells | |||||
| Bending Beam | 10-5,000 lb | 0.03% | Tanks, platform scales, | Low cost, simple construction | strain gauges 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 | ||
New Sensor Developments
In the area of new sensor developments, fiber optic load cells are gaining attention because of their immunity to electromagnetic and radio frequency interference (EMI/RFI), suitability for use at elevated temperatures, and intrinsically safe nature. Work continues on the development of optical load sensors. 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.
A few fiber optic load sensors are commercially available. One fiber optic strain gage can be installed by drilling a 0.5 mm diameter hole into a stud or bolt, and then inserting the strain gage into it. Such a weight sensor is completely insensitive to off-axis and torsion loads.
Micromachined silicon load cells have not yet arrived, but their development is underway. At the Universiteit Twente in the Netherlands, work is progressing on a packaged monolithic load cell using micromachining techniques, and it is possible that silicon load cells might dominate the industry in the future.
article source: http://www.omega.com/
