The Swan Song of Mechanical Weighing?


(source: Electronics World, Nov. 1971)

By John Frye

Sophisticated electronic weighing equipment is rapidly replacing mechanical scales in industry.

WINTER'S first snowflakes were lazily drifting down as Mac entered his service shop. "I surely hate to think about having to wear that overcoat for the next several months," he said to Barney, already at work at the service bench as he hung up the garment. "It seems to weigh a little more every winter." "Ha! You said the magic word!" Barney exclaimed, laying down his solder gun.

"What magic word?" "Weigh." Last evening an uncle, who is a sales-engineer for electronic-weighing equipment, was at our house for supper. I don't know how good an engineer he is, but he certainly comes on strong as a salesman. He bent my ear for three solid hours about how electronic-weighing equipment works, how good it is, and what it can do for industry.

But you know something? I enjoyed it. Ordinarily, I'm bored stiff when a layman tries to talk electronics to me.

You know how that goes. He has so many wrong and half baked ideas it's hopeless to try and straighten him out. But this uncle of mine really spoke our language, and he knew what he was talking about." "Okay, you may as well regurgitate what you learned. You are not going to put out any work until you do. How big is this new market? Who manufactures the electronic equipment? What are the advantages over mechanical weighing? How does it work? How--" "Hold it!" Barney interrupted,"let's take things one at a time. For several years the mechanical industrial-weighing market has coasted along on annual sales of about $160 million. Even as late as ten years ago, the electronic portion of this market was insignificant. But this year electronic weighing equipment grossed at least $80 million. One manufacturer estimates the electronics side of the scale market at only 35 %, but it is rapidly overtaking the mechanical side. Weighing manufacturers are buying more and more semiconductors and other electronic components. And you must remember that electronic weighing can do many things, which I'll discuss later, that are impossible with mechanical weighing. The potential of this market is tremendous.

"Right now there are three major producers of the new equipment: Fairbanks Morse Weighing Systems Division of Colt Industries, St. Johnsbury, Vermont; the Howe Richardson Scale Company, Clifton, N.J.; and Toledo Scale and Systems Company, Toledo, Ohio. Two or three other companies I'll mention later are getting into the act. But for right now, I want to talk a little about the development of electronic weighing and how it works.

"Electronic-weighing equipment really shines in automatic weighing and proportioning. Back in the thirties the Richardson Scale Company (now Howe Richardson) supplied automatic proportioning systems for mass-production process companies such as distilleries or those making glass, prepared mixes, tires, and abrasives. In these systems, each ingredient in the formula was weighed out by its own individual scale, usually to a collector conveyor, and the loading and discharge of each scale was controlled through relays and timers. Changing formulas called for time-consuming readjustment of each scale. While this worked quite well in high-volume operations where formulas were changed infrequently, what was needed was a system that could handle frequent formula changing quickly and use one scale for several ingredients.

"The development of the servomechanism during World War II, for radar, made such a system possible. It provided a basic electronic positioning device that could be adapted to remote weight setting in formulation, and Richardson electronic engineers realized these possibilities and came out with their first system about 1951.

"This system used a control panel with weight-selector dials (one for each ingredient in the formula) having the same graduations as the scale dial at the weighing site. A potentiometer was mounted on the spindle of each weight selector dial, and a similar potentiometer was mounted on the spindle of the dial-scale pointer. Each control potentiometer was switched into a bridge circuit with the scale potentiometer for weighing a particular ingredient. The wiper contacts were wired into an electronic circuit which amplified any voltage differential between them. This output operated a small relay which caused the ingredient feeder to start or stop. A null position occurred when the weight of material in the hopper equaled the preset weight on the selector dial. Later, a plug-in formula capsule was substituted for the adjustable selector dial panel, and still later a punched card contained the formula information.

"Along with greater sophistication in formula input devices came greater variety in formula readout devices: remote indicators and recorders, counters and totalizers, digital printers and calculators, typewriter readout, and others.

Automatic proportioning systems are now used in most plants proportioning ingredients for rubber tires, plastics, animal feeds, baked goods, coffee, ice cream, glass manufacturing, ceramic ware, alcoholic beverages, and cement and asphalt highway paving.

"Space-age developments applied to electronic weighing added new dimensions in reliability, standardization, faster delivery, lower cost, and quicker service. In a process built around an automatic weighing system, downtime is dangerously costly. Mechanical-weighing systems required talented service personnel. Modern electronic systems require only the plugging in of a new module to correct a difficulty, and users are encouraged to keep such modules on hand." "I see what you mean by saying electronic weighing can perform many operations impossible with mechanical systems." "That's right. Fairbanks Morse considers the application of electronics to weighing as falling into two categories: Category One covers the use of electronics instruments and techniques with existing scale designs that previously used a mechanical-type of indicator (pendulum, spring, beam, etc.). Category Two includes completely new products designed to take full advantage of electronic techniques.

"In the first category, FM markets both analog and digital instruments for use with every type of platform scale, motor truck, and railroad scales for static weighing purposes. Ancillary devices such as printers, remote displays, tare entry facilities, etc. are available with these. Their 'Digitruck-Levetronic' weighing system is a good example of a digital readout system for use with new scales or with existing weigh-beam or dial-scale systems.

"The second category employs small load cells or transducer strain gages in place of weigh-beams, springs, etc. The weight on the platform causes an electrical change in the load cell that can be processed to produce an analog or digital readout. FM's 'Slim Jim' low-profile floor scale is an example. The platform stands only 1 3/4" above the floor and is simply free-standing on a suitably solid floor. It was originally developed for the U.S. Post Office and comes in two models with maximum limits of 7000 and 14,000 pounds, respectively. The digital indicator can be up to 200 feet from the platform. If the platform must be flush with the floor, a pit only 2 1/2" deep is required. A printer is available if desired.

"In-motion weighing requires speed that is duck-soup for electronics. Here we need to weigh discrete objects anything from a whole train, a tank car, or a package--while in motion. An example is the weighing of meat carcasses transported on hooks suspended from a monorail through a processing plant. Lessons learned from in-motion weighing techniques (how to overcome vibration, for example) led to the FM 'Livestock Weighing Scale' in which a number of live cattle are weighed on a large platform scale, as at a public auction, and the average weight per beast is calculated and indicated.

"A still tougher problem overcome by electronics is continuous weighing of ore, coal, quarried materials, etc., being transmitted in bulk on a conveyor belt. In one system, load cells under idler rollers are fed pulsed signals from a tachometer driven by the belt.

The output of the load cells thus consists of pulses whose height is a function of the belt speed. The signals are fed to a weight /speed integrator and totalizer that keeps a continuous accumulating record of the weight of the material passing along the conveyor." "Are electronic weighing devices only useful in large-scale industry? Aren't small electronic scales practical ?" "Toledo Scale is checking that out.

They have been field-testing a retail point-of-sale digital computing scale in food chains. Separate digital displays face the clerk and the customer, each of which indicates unit price, weight, and total price. Toledo, which got into electronics about ten years ago, uses a strain-gage type load cell as the weight sensor; and two years ago they changed their readout system from a November, 1971 servo-type analog system to a digital-IC system and their control system from photoelectric to a remote-control straight digital-IC system. As a result, they claim their scale is accurate to better than 0.1 %. A Toledo spokesman says there are definitely fewer service problems with electronic equipment than with mechanical equipment and that he thinks eventually electronic equipment will be less expensive than mechanical equipment.

"A couple of smaller companies edging into the market are Data Controls Systems, Inc., Danbury, Connecticut and Metrodyne Corp., Riverside, Connecticut. DCS is specializing in hatching by weight-that is mixing up to 16 feeds per formula for corporate farmers--but they are interested in tailoring equipment to customer needs instead of pushing standard products.

"Metrodyne manufactures 'Dyne–A-Mat,' a portable electronic weighing system light enough to be hand-carried and with a capacity of 10,000 pounds.

It consists of a 3/8-inch thick elastomeric mat which can be spread on the floor anywhere and attached to the digital readout and print-out equipment. The mat is actually a capacitor with black layers of conductive neoprene alternating with non-conductive layers that act as a dielectric. Applied weight corn presses the dielectric allowing the conductive layers to come closer together.

The resulting change in capacitance is proportional to the deforming weight and can be used to produce a signal actuating the readout equipment.

"Finally, a barely tapped market lies in 'on the fly' weighing of tractor-trailer trucks approaching toll stations at up to 20 miles per hour. Experiments have been carried on in Pennsylvania with transducerized load cells in an attempt to weigh trucks traveling 4 to 6 miles per hour, but truck oscillations especially in partially loaded tank trucks-and-driver braking create recalibration and maintenance problems.

The Japanese are quite interested in truck weighing in connection with a series of new asphalt roads they are building, and they have been looking into the 'Dyne-A-Mat' system." "Well," Mac said, as he switched on his bench light, "all this is a switch, anyway. Physicists used to be quite interested in weighing the electron, and now the electron is doing the weighing."

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