The system configuration of such scales is shown in Fig.1.4. In addition,most of the scales have the following feature:
3)The scale has a function of mass control.
The name and function of representative scales for industrial use are tabulated in Table 1.1
Table 1.1 Industrial scales and their functions
Figure 1.4 System configuration of the scale for industrial use
1.3.2 Control Purpose
Paying attention to the difference of the mass flow of the object being fed onto the load receiving element and the mass flow of the object after the measurement, we could say the purpose of the mass control written in table 1.1 is mass flow control of the object. From this point of view, the control purpose of the hopper scale or the weigh packer is to attain an intermittent flow each amount of which is pre-determined. The associative(selective combination) weigher is also
regarded as this type of control. The control purpose of the checkweigher is to attain the diverging flows according to the pre-determined grades in mass. As for the weigh feeder, the purpose is to attain a flow of pre-determined flow rate in mass or to attain a flow the total amount of which coincides with the pre-determined value. 1.3.3 System Configurations
Generally, a control system is composed of a controlled object, detecting means, controlling
means or controller and control element.In the industrial weighing systems, the controlled objects include the feeding devices, distributing devices, and discharging devices, and mass is the
controlled variable. The system shown in Fig.1.4 corresponds to the detecting means, and various kinds of actuators are used as the control element.
Figure 1.5 shows the classification, from the view point of the system configuration, of the industrial weighing systems. The system configuration of a hopper scale or a weigh packer is shown in Fig.1.5(a). The controlled object is a feeding device,whose typical example is a screw feeder. The control element is a variable speed motor driving the feeder in the case. The hopper corresponds to the load receiving element The desired value is denoted by the symbol R and the manipulated variable the symbol C. The symbols m and m＇represent respectively the states of mass flow and the differences in symbol mean the differences between the two states.
Figure 1.5(b) shows the system configuration of a weigh feeder, the typical example of which is a variable speed belt-feeder. The load receiving element is composed of a portion of the belt and
the weigh roller(s). The controlled object is the belt-feeder and the control element is the variable speed motor. The total amount of the detected mass is denoted by the symbol Q .Either Q or its derivative Q＇is chosen as the controlled variable and the measurement of the belt speed V is needed for obtaining the value Q.
Figure1.5(c) shows the system configuration of a checkweigher. The controlled object is the distributing device and the belt conveyer is normally adopted as the load receiving element.
Figure1.5(d) shows the system configuration of an associative(selective combination) weigher. Normally, small hoppers are used for the load receiving elements, each of which is equipped with a gate controlled by an actuator. The gates are controlled objects and the actuators are the control elements.
For the scales shown in Figs. 1.5(a) and 1.5(b), feedback control is adopted since the mass flow control has to be carried out while measuring the mass. On the other hand, for the scales shown in Figs.1.5(c) and 1.5(d), the mass flow control is fundamentally sequential control since the control is carried out after the measurement of the mass.
(a) Hopper scale or weigh packer (b) Weigh feeder
(c) Checkweigher
(d) Associative weigher
Figure 1.5 Industrial scales with mass control
CHAPTER 2 STATICE OF SCALES 2.1 STATICE OF LEVERS 2.11 Classification of Levers
A straight lever normally has a fulcrum pivot, load pivot, and power pivot, which is referred to as
the fundamental lever. Each position of the pivots is referred respectively to as fulcrum point, the load point, and the power point. The fulcrum point is a point at which a lever is supported and about which it is vibrationable. The load and power points are points at which a load and counterbalancing force are applied,respectively.
Fundamental levers are classified into three types according to the arrangement of the above three points; the first-order lever, the second-order-lever, and the third-order-lever. The detail is shown in Fig.2.1 in which point F is the fulcrum point, point A the load point, and point Bthe power point, being in a straight line.
(a) First-order lever (b) Second-order lever (c) Third-order lever
Figure 2.1 Classification of levers
The lever (system) is called the single lever or the compound lever ( system) according to the number of connected levers. The single lever is a lever used independently, such as a weigh beam of balance, and the compound lever is a lever system composed of connected levers.
Each number of the fulcrum point, load point, and power point is not limited to one point for one lever. For example, The lever illustrated in Fig.2.2, which may be regarded as a two-united lever, has two fulcrum point and two load points. A compound lever system including such levers is shown in Fig.2.3.
Figure 2.2 A two-united lever Figure 2.3 Compound lever system 2.12 Single levers
In practical application, there are two cases as to the position of a lever in static equilibrium under loading;the case that the position is always identical with the position under a zero load(Case 1), and the case that the position varies as the load(Case 2). We examine the static equilibrium conditions for the above two cases,assuming the lever to be a rigid body. (1) Static Equilibrium Condition
The necessary and sufficient conditions for the static equilibrium of a single lever are as follows: Σ(forces)=0 and Σ(moments)=0 (2.1)
To examine the static equilibrium conditions for a single lever in Case 1 and Case 2,we apply the above conditions to the lever whose fulcrum, power, and load points are not in a straight line.
We assume that, when the load is zero, the lever is in static equilibrium under the initial forces, W0 at point A,P0 at point B, R0 at point F, and G at point C (the center of gravity), as shown in Fig.2.4. We also assume that, when applying the load W and the counterbalancing force
P , the lever remains at the same position. The equilibrium conditions before and after the loading are
W0+P0+G+R0=0
W0a+P0b+Gc=0 (2.2) And
(W0+W)+(P0+P)+G+(R0+R)=0
(W0+W)a+(P0+P)b+Gc=0 (2.3) Where R is the increment of the force at point E.
In Fig.2.4 we must take the sign into account for the forces and their application points. The downward forces are considered positive and the upward forces negative. The force application points are considered positive when they locate on the load point side in the origin taken at the fulcrum point, and are considered negative when they locate on the power point side.Hence, the counterclockwist moment is positive and the clockwise moment is negative.
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