Weight may be defined as the gravitational attraction or earthpull on a body (mass). Thus, from a practical viewpoint, a scale or other form of weighing device is a means for determining mass. Weighing devices fall into two principal categories: (1) gravimetric; and (2) non-gravimetric or nuclear devices. At present, after many centuries of use, the effect of gravitational force as a treasure of mass in industry and commerce remains by far the most common method for mass (weight) determination. Non-gravimetric methods are just becoming established in the industry.
The basic components of a belt scale system are:
- Belt conveyor to convey the load over the scale.
- Scale suspension to transmit and direct the load force.
- Sensing device to measure the load force and belt speed.
- Integrating, totalizing and recording the scale output.
The scale suspension must function to transmit the force on the weigh idlers to the load balancing device. This is basically a problem of transmitting only the force normal to the conveyor belt without resolving any lateral forces into the vertical direction.
The following criteria must be met:
- Rigidity, minimal deflection.
- Torsional stability.
- Elimination of lateral forces.
- Minimize effects of excentric belt loading.
- Alignment provisions.
- Force reduction through leverage.
- Minimize tare weight portion on sensor.
- Maximize belt load portion on sensor.
- Minimize horizontal surface area for dirt collection.
- Unit construction for easy installation.
- Frictionless pivot point or fulcrums.
Load Sensing Devices
To measure the gravimetric pull on a body being weighed (the load), the gravimetric force must be countered by an opposing force. Measurement of this exposing or counterbalancing force thus becomes the measurement of weight. The conventional even-arm balance shows very clearly the concept of balancing an unknown mass with an equal known counterbalancing mass. Because the loads encountered in industry and cameras generally range up to many thousands of pounds, it is obvious that the even-arm approach is not practical.
Thus, with the exception of direct load-bearing load cells, most of the weighing equipment employs same form of lever system (usually a combination of first- and second-class levers to gain a mechanical advantage). These levers permit the load force to be counterbalanced and hence measured within a practical force range. Lever systems not only perform this force-reducing action; they also serve to focus the load force at a single point for convenient measurement. They align the force vertically in one direction so that measurement accuracy will not be affected by unrelated side forces.
In its simplest form, a conventional scale beam carries an adjustable poise that will counterbalance a given load (providing it is within the range of the scale) at only one position along the beam. Adjustment can be made manually or automatically through the use of a servo system. For automatic adjustment, some means most be provided to move the poise along the beam until exact balance is achieved. Several kinds of beam-balance detectors are in commercial use – notably, magnetic switches, proximity switches, pneumatic pickups, and photoelectric pickups. Movement of the poise generally is accomplished through a reversible electric servo motor that turns a screw threaded through the poise.
Strain-gage load cells may be applied in numberous configurations in either tension or compression. Most commonly, they are used in compression – either with a load-reducing, load-focusing lever system or where the load is applied directly to one or more cells. The first is the more common.
Semi-conductor load cells are becoming more popular because they have higher electrical outputs for the same loading than regular strain-gage load cells. They are, however, more temperature sensitive.
Integrators & Totalizers
The final link in the weighing system is the integrator and totalizer. Integrators may be grouped as follows:
- Mechanical, such as the ball-disc, cone-disc or belt-disc integrators.
- Electrical, integrating either an analog signal or digital pulses.
- Hybrid, a combination of mechanical and electrical systems.
The basic design criteria for the integrator and totalizer are:
- Negative integration feature.
- High resolution for calibration.
- Non-interacting zero and span adjustments.
- Optional low-load cut-out.
- Compatibility with necessary controls and instrumentation.
- Belt speed condensation.
- High stability and repeatability.
The hybrid type of integrators still utilize the mechanical approach such as ball-disc, but instead of a direct mechanical link between the position of the ball carrier, the belt load and belt speed, electrical or pneumatic signals are used. This again offers both advantages and disadvantages. The advantages lies in the fact that electronic load sensors can be used with small deflections thereby minimizing vertical misalignment and belt tension problems.