英文原文名 ADVANCED WEIGHING
TECHNOLOGY
中文译名 现代称重技术
现代称重技术
第一章 秤的功能与结构 1.1 基本结构和称重原理
两种不同类型的机械秤示于图1.1。那么，秤的基本结构和称重原理方面的共同特征是什么呢？
对于图1.1(a)所示的天平或杠杆秤，放在载荷盘上的被测物体的质量，与放在砝码盘上的砝码的质量是利用它们的自重对支点的力矩，通过计量杠杆进行比较的。这也可以看作是对物体载荷产生的作用力与砝码自重产生的反作用力进行比较，而且两者同时作用在计量杠杆上。对于图1.1(b)所示弹簧秤，由弹簧伸长而产生的恢复力，应被视为反作用力或抗力。
综上所述，我们认识到通常可以把秤分解成三个功能部分，即载荷接受部分 或受载器，力比较部分、反力部分。载荷接受部分（例如载荷盘等），它作为秤的一部分用于接受载荷，并将载荷产生的力施加到力比较部分上。反力部分（例如，带砝码的砝码盘或弹簧等），它作为秤的一部分产生反作用力，并将其施加到力比较部分上。力比较部分（例如计量杠杆等），它作为秤的一部分接受以上两种力。
(a)天平或者杠杆秤 (b)弹簧秤
图1.1 机械秤的两种类型
当我们检查任何一种机械秤时，会注意到它们通常都具有以上结构。所以，我们可以认为这种结构式秤的基本结构。此外，测量是以物体质量产生的作用力与反力部分产生的反作用力之间的平衡为基础的。所以，我们可以认为秤的称重原理是利用了力的平衡。现代科技的发展，使我们在质量测量方面不仅能够利用力的静平衡，而且还可以利用力的动平衡。
载荷传递杠杆应该包括在载荷接受部分之中。对于料斗秤中称重传感器直接支撑料斗的情形，可以认为它属于力比较部分被省略的一种特例。
对于天平或杠杆秤，其测得值可以从反力部分上的砝码变化中获得。对于弹簧秤，其测得值可以从反力部分的弹簧伸长变化中获得。一般来说，机械秤的测得值可以从反力部分产生的某些量值变化中获得。 1.2 电秤和电子称系统的构成
机械秤是指包括显示功能在内的所有功能都能通过机械手段实现的一种秤，而电秤和电子称具有一个能将反力部分产生的变化转换成电量的传感器，还具有一个能处理电量信号以获得测量值的信号处理装置。所以，电秤和电子称的特征在于有传感器和信号处理装置。
图1.2说明了电秤和电子称的基本系统构成。传感器将转化了的电信号，输送给由3种基本电路组成的信号处理装置，它们是输入电路、数据处理电路、输出电路。输入电路上有例如滤波器、放大器、A/D转换电路等，它们将传感器的输出信号变换成更适用于数据处理的信号。数据处理电路通过处理其输入信号，来获取测得值以及与测量有关的必须值。输出电路则是传输处理好的测量结果的电路。
图1.2 电秤和电子称基本系统框图
按照反力部分是否承受了反作用力，可以将传感器如图1.3所示划分为两类，即非反力型传感器和反力型传感器。
图1.3 秤用传感器按是否承受反力所作的分类
1.3 工业秤的功能及其系统构成 1.3.1 功能特征和分类
工业秤主要用于工业称重，它们具有以下特征： (1) 对载荷接受部分的加载或卸载时自动进行的。
(2) 用物体自重W确定物体质量值M的过程是自动进行的。
这种秤的系统框图示于图1.4。此外大多数工业秤还有以下一个特征，即 (3) 具有质量值的控制功能。
典型工业秤的名称和功能列于表1.1。
表1.1 工业秤的名称及其功能
图1.4 工业自动秤的系统框图
1.3.2 控制的目的
若注意观察一下加到载荷接受部分上的物体的质量流动状态，及其与经过测
量后的物体的质量流动状态之间的差异，我们可以得知表1.1中所说的质量值控制的目的就是为了控制质量流动的状态。从这个观点出发，料斗秤或包装秤的控制目的，就是为了获得一种断续流动状态，而每次断续流动的量都是预定的。联合（分选组合）秤的控制目的也属于这种类型。检重秤的目的，是为了按照预定质量等级获得离散的流量动状态。至于喂料的控制，则是为了获得一个预定质量的流动状态，或者获得一个总量与预定值相同的流量东状态。 1.3.3 系统的结构
一个控制系统通常包括被控对象、检测部分、调节部分或控制器，以及操作部分。在工业称重系统中，被控对象包括供料装置、分配装置、排放装置、而被控变量就是被测质量。图1.4所示的系统相当于一个检测部分，而各种执行器则用于操作部分。
图1.5显示了从系统构成观点对工业称重系统的分类情况。图1.5（a）所示为料斗秤或包装秤的系统构成图。被控对象是供料装置，其典型实例为螺旋喂料器。此时操作部分是驱动喂料器的一个变速电机。称量斗相当于载荷接受部分。目标值用符号R表示，操作变量用符号C表示。符号m和m＇分别代表质量流动的状态；用不同的符号意指两种状态有所不同。
图1.5（b）所示为一台喂料秤的系统构成图，它的典型实例是一台变速的皮带喂料秤，载荷接受部分皮带称重段和称重托轮组成。被控对象时皮带喂料器或供料装置，而操作部分是变速点击，被检测质量的总量用符号Q表示。选择Q或其对时间的导数Q＇为被控变量，为了得到Q值，就需要测量皮带的运行速度v。
图1.5（c）所示为一台检重秤的系统构成图。被控对象是分配装置，而皮带输送机通常被用作载荷接受部分。
图1.5（d）所示为一台联合（分选组合）秤的系统构成图，通常以一些小的称量斗作为载荷接受部分，并且每个斗都装有一个用执行器控制的阀门。这些阀门就是被控对象，而执行器即为控制元件操作部分。
对于图1.5（a）和1.5（b）中所示的秤，由于在测量质量的同时必须控制质量的流动状态，所以应采用反馈控制。另一方面，对于图1.5（c）和（d）中所示的秤，由于对质量流动状态的控制是在测量质量之后进行的，所以基本上是进行顺序控制。
（a）料斗秤的系统框图 （b）喂料秤的系统框图
（c）检重秤的系统框图 （d）联合秤（分选组合秤）的系统框图
图1.5 工业称重系统的构成框图
第二章 秤的静力学 2.1 杠杆的静力学 2.1.1 杠杆的分类
通常把具有交点轴，载荷轴和力轴的直杠杆称为基本杠杆。每个轴的位置分别被称为支点、重点和力点。支点就是杠杆的支承点，杠杆可以围绕它转动。重点和力点分别是载荷和力的作用点。
按照以上3个点的分布，可以把基本的杠杆分为3种类型，即第一类杠杆，第二类杠杆和第三类杠杆。在图2.1所示的分类图中，F是支点，A是重点，B是力点，它们作用在同一直线上。
（a）第一类杠杆 （b）第二类杠杆 （c）第三类杠杆
图2.1 杠杆的分类
按照联接杠杆的数量可以将杠杆（系）称为单一杠杆或复合杠杆（系）。单一杠杆是独立的，例如天平的横梁，而复合杠杆则是由相关联的杠杆组合而成的一个杠杆系。
支点、重点和力点的数量，在一个杠杆上并不限于一个。例如对于图2.2所示的杠杆，我们可以看作是一个双联杠杆，它有两个支点和两个重点。包含这种双联杠杆的一个符合杠杆系，见图2.3所示。
图2.2 双联杠杆
图2.3 复合杠杆系
2.1.2 单一杠杆
在实际应用中，杠杆在载荷作用下保持其静平衡位置的情况有两种：第一，总是与空载下杠杆的平衡位置相一致（第一种情况）；第二，平衡位置随载荷而变化（第二种情况）。当我们研究以上两种情况下的静平衡条件时，我们将杠杆设想为一个刚体。
（1）静平衡条件，单一杠杆保持平衡的必要和充分条件是
Σ（诸力）=0 和 Σ（诸力矩）=0 （2.1）
为了研究单一杠杆在第一种情况和第二种情况下的静平衡条件，我们将上述充分必要条件应用于那些支点、力点和重点不在同一直线上的杠杆。
假设当载荷为零时，杠杆在初始力的作用下保持静平衡，如图2.4所示。W0作用于A点，P0作用于B点，R0作用于F点，G作用于C点（重心）。再假设当施加载荷W和反力P时，杠杆仍保持在相同的位置上。那么，静平衡条件在加载前后即为
W0+P0+G+R0=0
W0a+P0b+Gc=0 （2.2） 并且
（W0+W)+(P0+P)+G+(R0+R)=0
(W0+W)a+(P0+P)b+Gc=0 （2.3）
式中，R是作用于F点的力的增量。
在图2.4中，我们必须考虑力的符号和作用点。向下的力为正，而向上的力为负，以支点为原点，当力的作用点位于重点一方时为正，而位于力点一方时为负。所以，逆时针方向的力矩为正，顺时针的力矩为负。
图2.4 单一杠杆的静平衡条件
ADVANCED WEIGHING TECHNOLOGY
CHAPTER 1 FUNCTIONS AND STRUCTURES OF SCALES 1.1 BASIC STRUCTURE AND WEIGHING PRINCIPLE
Two different types of the mechanical scale are illustrated in Fig.1.1.What are the common features to the scales in basic structure and weighing principle?
(a) Balance (b)Spring scale
Figure 1.1 Mechanical scale
In the balance or lever scale illustrated in Fig.1.1(a),the mass of the object to be measured and located at the load plate is compared with the mass of the weights to be locates at the weight plate as the moments due to their gravity around the fulcrum by means of the weighbeam. This can be considered a comparison of the force due to the load of object with counterforce due to the weights,both acting on the weighbeam.As for the spring scale illustrated in Fig.1.1(b),the restoring force due to the elongation of spring is considered the counterforce or resistant.
The above consideration leads us to the recognition that those scales can be divided into three functional elements in common,which are the load receiving element or load receptor,the force comparing element and the counterfore element.The load receiving element,such ad a load plate,is a portion of the scale which receives an object to be measured and applies the force caused by the mass of the object to the force comparing element.The counterforce element,such as a weight plate with weights or a spring,is a portion of the scale which develops a counterforce,applying it to the force comparing element. The force comparing element, such as a weighbeam, is a portion of the scale to which the above two forces are applied.
When examining any types of the mechanical scale, we notice they have the above structure in common. Then, the structure can be regarded as the basic structure of scales. Furthermore, the measurement is based on the equilibrium in the force due to the mass of an object and the
counterforce developed in a counterforce element. Therefore, the application of the equilibrium in forces can be regarded as the weighing principle of scales. The modern technological development enables us to apply not only the static equilibrium but also the dynamic equilibrium in forces for mass measurement.
The load transmitting levers should be included in the load receiving element. For the hopper scale whose hopper is directly supported by loadcclls, we should regard it as a case that the force comparing element is omitted.
In the balance or lever scale, the measured value can be obtained from the weight change in the counterforce element. In the spring scale the measured value can be obtained from the
elongation change of the spring as a counterforce element. Generally, the measured value in the mechanical scale can be obtained by using some quantity changes developed in the counterforce element.
1.2 SYSTEM CONFIGURATION OF ELECTRICAL AND ELECTRONIC SCALES
A mechanical scale is a scale in which all functions including display function are realized by mechanical means. On the hand, an electrical and electrical scale is a scale with a transducer
which inverts the change developed in the counterfore element to an electrical quantity and with a signal processing device which processes the signal of that electrical quantity to obtain the
measured value. Therefore, the electrical and electronic scale are characterized by the transducers and signal processing devices.
Figure 1.2 shows the basic system configuration of the electrical and electronic scale. The electrical signal converted with the transducer is sent to the signal processing device composing of three functional circuits, which are input circuit, data processing circuit, and output circuit. The input circuit functional circuits, which are input circuit, data processing circuit, and output circuit. The input circuit is associated with the circuits, such as filtering, amplifying and A/D converting circuits, which manipulate the output signal from the transducer into a more usable signal for data processing. The data processing circuit is a circuit which processes the input signal to obtain the measured value and the necessary values related to the measurement. The output circuit is a circuit which send out the processed results.
Figure 1.2 basic system configuration of the electrical and electronic scale
According to whether or not they undertake the counterforces as counterforce elements, the transducers are classified into two types which are the noncounterforce-type and the counterforce-type transducer,as shown in Fig.1.3.
Figure 1.3 Classification of transducers
1.3 FUNCTIONS AND SYSTEM CONFIGURATIONS OF THE SCALES FOR INDUSTRIAL UES
1.3.1 Functional Characteristics and Classification
The scales mainly used for industrial weighing hace the following features:
1)The loading on and unloading from the load receiving element are automatic.
2)the determination process of the mass value M of the object by using its weight W is automatic.
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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