Some Development Trends of Measurement And Weighing

First, dynamic measurement

It is often hoped that force weighing is completed in the normal operation at the same time, that is, as far as possible do not occupy or occupy less time.For example, in the rolling mill, from billet heating to rolling is a continuous operation, in order to master the heating process of burning loss rate and product yield, it is required to do quick or short time weighing of steel without interfering with normal production.

In the metering of cargo handled at port, it is often required that the cargo be weighed at the same time in the process of transportation or hoisting, so as not to affect the loading and unloading speed.At this time, whether it is the bulk material flowing through the belt scale, or the goods hanging under the movement of the hanging scale, are in a state of motion.Sometimes a car is weighed on a highway or a train is weighed on a railway.Sometimes live animals, such as cattle or medical animals, are weighed.Sometimes objects are weighed in a moving or vibrating environment, such as on an ocean-going ship, aircraft, or truck.

In the machining industry, the flexible manufacturing system (FMS) combined with the machining center greatly improves the machining adaptability of machine tools

Response and processing efficiency.But the tool damage has become the main fault of the system normal operation, often need to use dynamic dynamometer to continuously monitor the running status of the rotary tool.The "joint" multi-component sensors, mounted between the robot's arm and paw, provide three-dimensional forces and torques at the manipulator's elbow, which are also measured under motion conditions.

All the above conditions form the dynamic force measurement weighing measurement, their characteristics can be summarized as:

(1) the measured object is in a non-static state, that is, the measured force or weighing object is in motion;

(2) the measuring environment is in a non-static state, that is, the measuring bed, table or bracket which the force measuring weighing instrument is placed in is in motion;

(3) rapid measurement in a short time, that is, the measurement time is shorter than the setting time of force measuring weighing instrument.

In order to fast, continuous and accurate measurement, obtained by measuring the steady-state value, is required to load weighing sensors, signal processing system, data acquisition system and dynamic load weighing system which composed of correct description and analysis of demand also weaken the dynamic load weighing measuring error and dynamic compensation.For example, in order to weaken the high frequency interference caused by longitudinal vibration, the bridge dynamic compensation and the threshold dynamic compensation can be carried out.In order to weaken the low frequency interference caused by transverse swing, three-point compensation can be carried out.

In the study of dynamic weighing, the usual method is to simultaneously or separately measure the acceleration, displacement and velocity of the heavy object, and then solve the differential equation of the weighing process by numerical integration method or direct method to get the weight.The current method is to treat dynamic weighing as a problem of parameter estimation and prediction. Firstly, a model with unknown parameters is derived based on the prior knowledge of the weighing system, and then the model is used to fit the signal of the weighing transition process, so as to obtain the parameter estimation in the sense of the least square error.Because the measured weight can be regarded as the final value of the weighing process, it can be estimated or predicted by the model parameters.

In the aspect of dynamic force measurement and weighing, although the latest hardware technology of the sensor plays an important role, the software technology based on the dynamic system model is more essential and more important for the design of dynamic measurement algorithm.That is to say, the mathematical model of the measurement system should be used to focus on the software to solve the problem. Admittedly, the hardware and software approaches are necessary for the study of dynamic force measurement and weighing technology.

Two, automatic online measurement

Before the advent of automatic on-line continuous weighing devices in 1958, the weighing or weight control of products on the production line was done manually, i.e., taking them out at regular intervals and weighing them separately.This "sampling inspection" method is obviously not suitable for the current high-speed production, quality management and practical protection of consumer rights and interests requirements.In order not to violate regulations such as NIST44 and 133 in the United States, manufacturers have begun to adopt the 100% weight inspection principle for products sold directly.If online automatic weighing is not possible, manufacturers prefer to use the principle of slightly overloading, in case the weight is insufficient and credibility is lost.

In practice, statistics need to be collected and verified online quickly and easily during the production process. If a series of defective products occur, the producer should shut down the production line and immediately check the cause.Therefore, the traditional spot check method is no longer applicable, there is a trend of gradually being replaced by online testing.At the end of 1970s, microcomputer introduced weighing instrument, which injected vigor into the development of weighing technology.At present, the automatic scale used for online measurement is not only for eliminating the products with insufficient weight, but also for weight control, statistical analysis and treatment.

When the weighing meter is connected to the computer, it can produce written reports that the producer wants, such as real-time counting histograms, standard deviations of weights, running averages, related parameter reports, statistical reports, and interval reports by time, quantity, or weight.Instead of walking up and down the production line, a producer can change parameters using a remote control panel in a quality control room away from the measuring station. He can even watch several production lines at once.For food, cosmetics, medicine and chemical industries, this online measurement of automatic scale will undoubtedly make the production manager's work become simple, accurate, safe and efficient.

There are mainly four kinds of automatic scales for online weight measurement:

(1) gravity loading automatic scale.It is through the automatic weighing program, the weight of the predetermined, in fact is the weight of the constant bulk goods into the container of the automatic scale, referred to as quantitative scale.It is mainly composed of an automatic feeder or a device related to one or more weighing units, as well as the appropriate control device and unloading device.

(2) automatic material grasping scale.It is an automatic weighing scale for unpredetermined and separated items, which can be used to classify items by weight, such as inspection scale and sorting scale, and can also be used to price individual items by weight and label them, such as price label scale.

(3) continuous cumulative automatic scale.It is installed in the appropriate position of the belt conveyor, the bulk material for continuous, cumulative weighing automatic scale, referred to as the belt scale.

(4) Non-continuous cumulative automatic scale.It is a batch of bulk material is divided into several parts of the separation, discontinuous load, according to the predetermined procedures in turn weighing each load after respectively cumulative, in order to obtain the batch of material total automatic scale, referred to as cumulative hopper scale.

Inspection scale is the different weight of the articles, according to its weight and the nominal set value of the difference, subdivided into two or more groups of automatic balance.Sorting balance is according to the given weight range, subdivided into several groups of automatic balance for grabbing material.The belt scale does not need to subdivide the material by weight, and the belt of the conveyor can be operated at a single speed or a variety of speeds.Hopper scale in successive weighing of the weight of the material is usually not equal, in determining the actual weight of each bucket that is cumulative.A quantitative scale with a packing mechanism is a packing scale or a bag scale.The above kinds of automatic scales have their own characteristics in the automatic online measurement of weight and will play different roles.

Three, model measurement

As a kind of commonly used measuring and measuring instrument, force weighing instrument can be considered to have experienced four stages in the last hundred years.First, there was the era of mechanical analog instruments for more than half a century, followed by the era of electromechanical electrical instruments and electronic instruments, and then in the late 1970s, it entered the era of digital instruments and microcomputer instruments, followed by the era of model measurement (MBM) instruments in the 1990s.

This is because people require not only the measurement results, but also the comprehensive evaluation of the measurement results, that is, state estimation, diagnosis or trend analysis of the weight and force values being measured.What people actually face is often a multi-variable dynamic process or system that needs real-time measurement.Therefore, it is not enough to use only the traditional measurement methods and numerical processing means, but need the help of model building and parameter estimation to achieve intelligent measurement.

The significant difference between intelligent force measuring and weighing instrument and microcomputer force measuring and weighing instrument lies in the knowledge layer composed of various mathematical models whether it is learning, reasoning, judgment or adaptive function in intelligent instrument:

In fact, the method of weighing force measurement using mathematical model or model measurement is very promising.It regards measurement as a process and measuring instruments as a system.According to the prior information, i.e., prior knowledge, and the experimental data, i.e

Posterior knowledge, the use of system identification to establish the mathematical model of the measuring instrument, and through the corresponding algorithm to process data and describe the instrument comprehensively, so as to its performance state estimation, or through software to improve the hardware environment of the measuring instrument.

The model measurement has opened up a new way to solve the increasingly complex dynamic measurement problems.For example, the weighing system adopts the autoregressive moving average model of the second-order system, and with the help of this model and the recursive least square method, namely RLS, the model parameters and the weighed weight can be estimated from the extremely short weighing step response.The simulation results show that the weight can be estimated by RLS when there is white noise at the input end.The measurement time required by this method is very short, usually no more than one oscillation period can get good results.

In the microcomputerized force weighing instrument, the knowledge model is also introduced to form the expert system, that is, the thinking process of excellent force weighing experts is solidified into the software of the measuring program, and combined with the computer correction program, so as to improve the testing ability and fault detection ability of the measuring instrument.From these we can see that the significance of measuring software for the future development of force measuring and weighing technology cannot be underestimated.

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