In most commercial weighing installations the design of the control system must be engineered for the job. Rarely is a so-called "off-theshelf" system suitable. Since the design of the control system only indirectly affects the weighing accuracy, and only then in the event of a circuit malfunction, the important design considerations are those related to cost, reliability, and sequencing speeds. In instances where the maximum number of weighings per unit time is important, then the very fast switching times of solid-state components may be the only answer, whereas in other cases relays, stepping switches, and other similar control devices may be most advantageous. In all design formulations, the economics of the system components must be balanced against the engineering excellence desired. Data Processing. Data processing is strictly related to the requirements of the overall system and not to the weighing portion alone. It is a matter of, "now that you have the weight figures, what do you want to do with the information?" No one weighs an object just for the fun of doing it, but rather to use the information for some specific purpose. In industry this information may be necessary to expedite or control some manufacturing process or material-handling requirement. In the past, a scale manufacturer's responsibility was to provide an indication of the object weight and at most, to give a printed record of the weight, but with the advance of science on all fronts, such simple weighing systems are no longer adequate. Most data processing systems today depend upon some form of digital coding. Digital coding offers several advantages. One of the most important being the ability to transmit and mathematically manipulate the information without error. Weighing instruments are, therefore, of the direct digital type, or a means is available to convert the analog information to digital. A pendulum-dial scale, for example, may have an analog-to-digital conversion mechanism mounted on the dial indicator shaft to provide the desired digital output. Translation matrices are available to convert any coded system into any other coded system so the design problem of interfacing the weighing instrumentation with any commercially available data processing equipment presents no particular problem. Assuming that the information taken from the weighing instrument is correct, then the transmittal and manipulation of this data to and through data processing equipment can be handled virtually error-free. Measuring Instrumentation. Mechanical lever systems, beams, and pendulum-dial scales were the work-horses of industrial and commercial weighing for many, many years. The demands for faster weighing procedures, automatic controls, and data processing soon made evident that these mechanical devices were inadequate for the requirements. The mechanical scale required too much hime to come to a balance condition so the direction of development turned toward electronics. Weighing times of three seconds or more permitted the use of Servomotor-driven null-balancing bridge networks in association with load cells. Such systems, being electro-mechanical in nature, were more than adequate to meet this time requirement. However, as it became evident that weighing times of one second, one-tenth second, and even milliseconds would be needed to integrate weighing into the new industrial processes, new concepts in weighing instrumentation were developed. As a result, several new designs have made their appearance in the past few years. These new instruments are almost entirely electronic in order to take advantage of the higher switching speeds offered by electronic circuitry. Electronic weighing instruments are essentially digital voltmeters. The signal fed into the instrument is a voltage whose magnitude is proportional to the weight of the object on the platform. The instrument measures this voltage to produce a digital indication and output. There are several different principles employed to convert the voltage to a digital output, among which are the following: (a) Ramp Type.-An unknown voltage is compared to a linearly-increasing voltage called a "ramp." The time required for the ramp to rise from a fixed reference voltage to a value equal to the unknown voltage is a measure of the unknown voltage. (b) Voltage to Frequency.-The unknown voltage is used to control an oscillator whose output is a series of pulses in which the frequency of pulses is proportional to the value of the unknown voltage. By counting the generated pulses over a precisely controlled interval of time, the value of the unknown voltage is determined. (c) Step-Voltage Type.-A series of independent reference voltage steps is provided in digital, digital-decade, binary, or binary-decade sequence to which the unknown voltage is compared. By comparing the unknown voltage to the reference voltage steps in sequential order, the value of the unknown voltage is established. (d) Voltage-Divider Type.-In this method, the unknown voltage is compared to a voltage derived from a series array of precision resistors across a very stable reference voltage source. Many configurations of the precision resistors values may be employed, such as binary, binary-decaded, or digital-decaded. Each type has advantages particular unto itself. No one instrument is universal in its application and each has a place in the general scheme of motion-weighing. Measuring times as low as one millisecond and up to ten seconds can be chosen from among these various types. Instrument accuracies of 0.01 percent or better are possible with the types mentioned. A wide choice of instrumentation is, therefore, available from which the one most suitable in regard to measuring time and acuracy can be chosen to meet the particular requirements of a particular motion-weighing installation. Transducers. It is the function of the transducers to change the gravity-weight of the object being weighed into a proportional voltage output which can be measured by the instrument. There are several different types of transducers being employed in weighing systems today, such as, pneumatic, hydraulic, and electrical. Among these, one of the most popular types is the bonded strain-gage load cell. Strain-gage load cells have been manufactured in sizes varying from ounces-full-load rating to twelve-million-pounds full load rating now in use at NASA. Strain-gage load cells have several advantages in motion-weighing installations since they have very fast response times, are compact, are not subjected to wear, and, since the output is electrical, the coordination with other control equipment is quite straight forward. The acuracy of the overall motion-weighing system is dependent, in large measure, on the characteristics of the load transducers. If the output of the transducer is in error with respect to the applied load, the most perfect measuring instrument will indicate a system result that is in error. Linearity of voltage output with respect to applied load on strain-gage load cells is normally 0.1 percent for most weighing applications, with some special applications using load cells of 0.05 percent linearity. Variations in ambient temperatures in the vicinity of the load cells can cause errors in the output voltage. Most load cells are temperature compensated, but usually only in the of range +15°F. to +115°F. Many weighing applications exceed these temperatures in both extremes. Changes in barometric pressures also can cause an error in the output voltage of load cells and some manufacturers have included in their designs means to compensate for such pressure changes. Since strain-gage load cells are usually of the wheatstone-bridge resistive configuration requiring an input voltage to produce the output voltage, it is most essential that well regulated power supplies be employed. The extra effort and cost required to provide excellent load-cell power supplies is more than paid for in the stability and accuracy achieved. It is evident that if a motion-weighing system is to approach a 0.1 percent accuracy, and if load cells of even 0.05 percent linearity are used, and if the accuracy is to be maintained over an extended period of time, then many design complexities must be introduced into the system. Unfortunately, as design complexities increase, the cost of the system increases, and reliability decreases. Reliability can be restored through deliberate redundancy, but at a further increase in cost. the Aside from the influence that the choice and application of the load cells have on the overall accuracy of a motion-weighing system, most important problems center about the design of the weigh-platform. In fact, as a design problem, the load cells and the weigh-platform must be considered as a unit since the problems of one are directly related to the other. Many motion-weighing scales are designed so that a complete belt-conveyor system is mounted on load cells; the conveyor then becomes the weigh-platform. The objects to be weighed are brought to the scale by conveyors, are transported across the scale platform by the weigh-conveyor, and are then carried away by other conveyors. In such systems, it is common to include all necessary components of the weigh-conveyor, such as drive motors and reduction gears, as an integral part of the conveyor, so that the entire dead weight of the conveyor is carried on the transducing system. Because most motion-weighing scales are part of a more general system, and because many of these have special design features to meet a customer's particular requirement, it is not always possible to thoroughly check out the completed design in a laboratory before installation in a customer's plant. No amount of laboratory testing can duplicate the environmental and dynamic conditions under which the scale will be operating in day-in, day-out service. In order to illus LIDRAREs trate how certain design modifications to a weigh-platform improved the overall system accuracy, the results of a series of field tests are herein included and discussed. Illustration I is of a high-speed motion-weighing scale installed in the field early in 1963. Instrument: Single-Sample Ramp System-Electronic. Indication: Digital, 99.9 pounds maximum reading capacity, 0.1 pound minimum graduation. Measuring Speed: One-tenth second maximum. Readout: Parallel entry into totalizing adding machine. Transducers: Four strain-gage load cells. Installation and Operation: The scale is located between the end of a production line and the shipping dock. Packaged production items are randomly placed on a continuous conveyor belt traveling at 75 feet per minute. They are discharged onto a short length of speedup conveyor traveling at 120 feet per minute. This exchange performs the desired separation between items. From the speed-up conveyor the items are transferred to the weigh conveyor. When the item is fully scale-borne, and at a point near the discharge end of the weigh conveyor, a photocell light beam is interrupted, triggering the weigh cycle. Upon completion of the weigh cycle, the item weight is displayed on the digital indicators and is also automatically entered into the parallel-entry adding machine. The result is a printed record of each individual item. Available to the shipping dock foreman is the facility for pulling out a subtotal or total for each production run as desired. In order to appreciate some of the salient points brought out by the mathematical treatment of the test data herein presented, the conditions under which the weighing system operated are worthy of con sideration. 1. The instrument is digitally indicating with a minimum grad uation of 0.1 pound. This is equivalent to 1.6 ounces, and if this 1.6 ounces is placed anywhere on the weigh-conveyor (having a total surface area of 8 square feet), either concentrated or evenly distributed, the scale will indicate this change in load. It is not difficult to visualize the many ways in which the force acting upon a moving conveyor can change by the very small amount of 1.6 ounces, particularly on a system operating under dynamic condi tions associated with full production. 2. This particular system is fully-automatic with no weighman in attendance. It is a "command" device, its weigh-cycle being initiated by the object being weighed. Consider a static weighing scale under the operation of a weighman when a fork lift truck |