What are Weight Sensors?

Catalog

I What are Weight Sensors?

A weight sensor is a device that converts the quality signal into a measurable electrical signal output. When using the sensor, we must first consider the actual working environment, which is very important for the correct selection of the weight sensor. Besides, it's also related to whether the sensor can work normally, its safety and service life, and even the reliability and safety of the entire weighing instrument. In the following, we're going to talk about the classification, structure, and installation of weight sensors.

II Classification of Weight Sensors

According to the conversion method, weight sensors are divided into 8 categories: photoelectric type, hydraulic type, electromagnetic force type, capacitive type, magnetic pole variation form, vibration type, gyro type, and resistance strain type. The most widely used is the resistance strain type.

1. Photoelectric Weight Sensor

It includes the optical grating type and code disc type.

Optical grating sensors use moire fringes formed by gratings to convert angular displacement into photoelectric signals. There are two gratings, one is a fixed grating, and the other is a moving grating mounted on the dial axis. The test object put on the load-bearing platform rotates the dial shaft through the transmission lever system and drives the moving grating to rotate so that the moire fringe also moves accordingly. Using photocells, conversion circuits, and display instruments, we can calculate the number of moire fringes to measure the rotation angle of the grating, and finally read the quality of the measured object.

Figure 2. Moire Fringe Effect

The code wheel(symbol board) of the code wheel sensor is a piece of transparent glass mounted on the dial shaft with black and white codes compiled according to a certain coding method. When the test object on the load-bearing platform rotates the dial shaft through the force transmission lever, the code wheel also rotates a certain angle. The photocell will receive the optical signal through the code wheel and convert it into an electrical signal, then be digitally processed by the circuit, and finally, the number of the measured quality will be on the display. Photoelectric weight sensors were mainly used in electromechanical combined scales.

2. Hydraulic Weight Sensor

When the gravitational force P of the measured object acts, the pressure of the hydraulic oil increases and the degree of the increase is proportional to P. By measuring the increase in pressure, we can obtain the quality of the object. The hydraulic weight sensor has a simple and firm structure and a large measuring range, but its accuracy generally does not exceed 1/100.

3. Capacitive Weight Sensor

It works by using the proportional relationship between the oscillation frequency f of the capacitor oscillation circuit and the electrode plate distance d. There are two electrode plates, one fixed and the other movable. When the test object is placed on the load-bearing platform, the leaf spring deflects, the distance between the two electrode plates changes, and the oscillation frequency of the circuit also changes accordingly. By measuring the change in frequency, we can determine the object of the object. Capacitive weight sensors consume less power and cost less, with an accuracy of 1/200 to 1/500.

4. Electromagnetic Force Weight Sensor

It uses the principle that the load on the load-bearing platform is balanced with the electromagnetic force. When the object to be tested is placed on the load-bearing platform, one end of the lever is tilted upward; the photoelectric component detects the tilt signal, which is amplified and flows into the coil to generate an electromagnetic force to restore the lever to a balanced state. By digitally converting the current that generates the electromagnetic balance force, the quality of the measured object can be determined. The electromagnetic force weight sensor has high accuracy, which can reach 1/2000 to 1/60000, but the weighing range is only between tens of mg and 10 kg.

5. Magnetic Pole Variation Weight Sensor

When the ferromagnetic element is mechanically deformed under the influence of the gravity of the measured object, internal stress is generated and the permeability is changed so that the induced voltage of the secondary coil wound on both sides of the ferromagnetic element (magnetic pole) changes accordingly. We can obtain the force applied to the magnetic pole by measuring the amount of voltage change, and then determine the mass of the object. The accuracy of the magnetic pole variation weight sensor is not high, generally 1/100. Its weighing range is tens to tens of thousands of kilograms, which is suitable for large tonnage weighing work

6. Vibrating Weight Sensor

After there is a force on the elastic element, its natural vibration frequency is proportional to the square root of the applied force. By measuring the change in natural frequency, we can obtain the force of the measured object acting on the elastic element, and then its mass can be obtained. There are two types of vibrating sensors: vibrating wire and tuning fork type.

The elastic element of a vibrating wire sensor is a string wire. When the object is on the load-bearing platform, the intersection point of the V-shaped string is pulled down, and the pulling force of the left string increases, and the pulling force of the right string decreases. The inherent frequencies of the two strings change differently. If we calculate the difference between the two frequencies, we can obtain the mass of the object.

Figure 3. Structure of vibrating wire sensors

The accuracy of the vibrating wire sensor is high, up to 1/1000-1/10000, and the weighing range is 100 grams to several hundred kilograms, but this sensor has a complex structure, the processing is difficult, and the cost is high.

The elastic element of the tuning fork sensor is a tuning fork. A piezoelectric element is fixed at the end of the tuning fork, which oscillates at the inherent frequency of the tuning fork, and the oscillation frequency can be measured. When an object is placed on the load-bearing platform, the inherent frequency of the tuning fork is under stress in the tensile direction, and the degree of increase is proportional to the square root of the applied force. After we measure the change of the natural frequency, the force exerted by the heavy object on the tuning fork can be obtained, and then the mass of it can also be obtained. The tuning fork sensor has small power consumption, the measurement accuracy is as high as 1/10000-1/200000, and the weighing range is 500g-10kg.

Figure 4. Tuning Fork Sensor

7. Gyro Weight Sensor

The rotor is installed in the inner frame and rotates steadily around the X axis at an angular velocity ω. The inner frame is connected to the outer frame via a bearing and can tilt and rotate about the horizontal axis Y. The outer frame is connected to the machine base via a universal coupling and can rotate around the vertical axis Z.

The rotor shaft (X axis) remains horizontal when it is not acted by an external force. When an external force (P/2) acts on one end of the rotor shaft, it tilts and rotates about the vertical axis Z. The precession angular velocity ω is proportional to the external force P/2. And the value of the external force and the mass of the object can be obtained by detecting the frequency.

The gyro weight sensor has a fast response time (5 seconds), no hysteresis, good temperature characteristics (3ppm), small vibration effects, and high accuracy in frequency measurement, so high resolution (1/100000) and high measurement accuracy can be obtained (1/30000-1/60000).

8. Resistance Strain Weight Sensor

It uses the principle that the resistance of strain gauges changes when the strain gauge is deformed. It is mainly composed of 4 parts: elastic element, resistance strain gauge, measuring circuit, and transmission cable.

Figure 5. Working of Strain Gauges

9. Annular Plate Weight Sensor

The structure of the annular plate weight sensor has clear stress streamline distribution, high output sensitivity, an elastomeras a whole, a simple structure, a stable stress state, and an easy processing process. At present, it still occupies a relatively large proportion in the production of sensors, but the structure design formula for this sensor is not yet perfect. Because the strain calculation of this kind of elastomeris relatively complicated, it is usually regarded as an annular elastomerfor estimation in design. In particular, the design and calculation error of the annular plate weight sensor with a range of 1t and below is greater, and at the same time, a larger nonlinear error often occurs.

Annular plate weight sensors have a compact structure, good protection performance, high precision, and good long-term stability. It's suitable for measuring crance scale, electromechanical scale, and other force values

Figure 6. Annular Plate Type

10. Digital Weight Sensor

The Digital weight sensor is a force-electricity conversion device that can convert gravity into electrical signals. It mainly refers to a new sensor with a resistance strain sensor, electronic amplifier, and analog-to-digital conversion technology(ADC), and a microprocessor.

The development of digital weight sensors and digital measuring instrument technology has gradually become the new favorite in the field of weighing technology, with the advantages of simple and efficient debugging and strong adaptive ability.

As shown in the figure, the S-type weight sensor is the most common type of sensor. It is mainly used to measure the tensile force and pressure between solids. Because its shape is like an S shape, it's also commonly known as the S-type weight sensor. This sensor is made of alloy steel and is sealed and protected by rubber. It is easy to install and easy to use, which is suitable for electronic force measurement systems such as steelyards and batching scales.

Figure 7. S-type Weight Sensor

III Weight Sensor Composition

1. Sensitive Element

It directly senses the quality and output of other quantities related to the measured quality. For example, the elastomer of the resistance strain weight sensor transforms the mass of the measured object into deformation; the elastomer of the capacitive weight sensor transforms the measured mass into displacement.

2. Conversion Element

It converts the output of the sensitive element into a signal that is convenient for measurement. For example, the resistance strain type weight sensor converts the deformation of the elastomer into a change in resistance; the capacitor of the capacitive weight sensor converts the displacement of the elastomer into a change in capacitance. Sometimes some components function as both sensitive components and conversion components, such as the piezoelectric material of a voltage weight sensor, which deforms under the action of an external load, and outputs electricity.

3. Measuring Element

The output of the conversion element is converted into an electrical signal, which is convenient for further transmission, processing, display, recording or control. Such as the bridge circuit in the resistance strain weight sensor, and the charge pre-amplifier of the piezoelectric weight sensor.

4. Auxiliary Power Supply

It provides energy for the electrical signal output of the sensor. Generally, weight sensors need an external power supply to work. Therefore, the requirements for power supply must be marked on the products, but not as part of the weight sensor. Some sensors, such as magnetoelectric speed sensors, can work normally without auxiliary power due to their large output energy. So not all sensors need an auxiliary power supply.

IV Weight Sensor Materials

The performance of weight sensors depends largely on the choice of manufacturing materials. The weight sensor material includes the following parts: strain gauge material, elastomer material, patch adhesive material, sealant material, lead sealing material, and lead material.

1. Strain Gauge Material

The strain gauge is the sensing part of the weight sensor. It converts the external force into an electrical output, which is the most important part of the sensor. The commonly used strain gauge base material is polymer film material, and the strained material is usually high-purity constantan. The performance of the strain gauge is not only related to the purity of the base material and constantan but also related to the manufacturing process. Thus, improving processing technology is also an important aspect of sensor performance.

Figure 8. Strain Gauge

2. Elastomer Material

The function of the elastomer of the weight sensor is to transmit external force. It must have the same deformation when the same force is applied. Because the strain gauge is attached to the elastic body, and the deformation of the elastomer is the deformation of the strain gauge. At the same time, the elastomer should be resettable and can reset automatically when the external force disappears. Elastomer materials are mainly aluminum alloy, stainless steel, and alloy steel.

3. Adhesive Material

The patch adhesive is used to firmly fix the strain gauge and the elastomer together so that their deformation will always be consistent, which is also an important component. At the beginning of the 21st century, the most commonly-used patch adhesive was a two-component polymer epoxy adhesive. Its performance is closely related to its purity, mixing method, storage time, solidification method, solidification time, etc., so the detailed specification should be read carefully before we use it.

Figure 9. Polymer Adhesive

4. Sealant Material

Early weight sensors are sealed with sealants. Later, due to the development of manufacturing technology, welding technology can extremely improve the stability and service life of the weight sensor. Although many welding techniques were used in the early 21st century, some important parts still need to be coated with the sealant. The silica gel is often used as the sealant because of its good stability, moisture resistance, corrosion resistance, and excellent insulation performance.

5. Lead Seal

If the output lead of the sensor is not fixed, it will be damaged or loose, resulting in an unstable signal or no output. At the beginning of the 21st century, all the weight sensor outputs with connectors, so the material and tightening strength of the connector will affect the output. Therefore, it is better to use both connectors and sealants.

Internal leads also need to be fixed to avoid moving around. The quality of the lead is also very important, and the order of the material performance from high to low is:

silver-plated wire > copper wire > aluminum wire

If there are serious high-frequency signal and radio wave interference, shielded cables must be used; in corrosive environments and inflammable and explosive occasions, anti-corrosion, flame-retardant and explosion-proof cables with sleeves need to be installed for protection.

Figure 10. Shielded Cables

V Installation Precautions

1. The weight sensor should be handled with care, especially for small-capacity sensors that use alloy aluminum as the elastomer. Any shock or drop caused by vibration is likely to cause a large output error.

2. In the design of the loading device and the installation, we should ensure that the loading axis of the weight sensor overlaps with the loading force so that the impact of the inclined load and the eccentric load is minimized.

3. In terms of horizontal adjustment, the mounting surface of the weigh sensor base should be leveled with a level gauge. if multiple sensors are measuring at the same time, the installation surface of their base should be kept on the same level as much as possible to ensure that the force that each sensor bears is basically the same.

Figure 11.  Level Gauge

4. Determine the rated load of the weight sensor according to the measuring range sensor in its specification. Although the weight sensor itself has a certain overload capacity, this should be avoided as much as possible during installation and use. Sometimes overloading for a short time may also cause permanent damage to the sensor.

5. To prevent chemical corrosion. Vaseline should be used to coat the outer surface of the weight sensor during installation. Avoid using the weight sensor in direct sunlight or the ambient temperature with a sudden change

6. Add a bypass made of copper braided wires to both ends of the loading device of the weight sensor.

7. The cable should not be lengthened casually. When there is a need to lengthen, the joint should be soldered and moisture-proof sealant should be added.

8. It is best to use some baffles to cover the weight sensor. The purpose is to prevent debris from falling into the moving part of the weight sensor and affecting its measurement accuracy.

9. The cable of the weight sensor should be far away from the strong power line or the place with pulse waves. When these situations cannot be avoided, the cable of the weight sensor should be inserted into the iron pipe separately, and the connection distance should be shortened as much as possible.

10. In high-precision applications, the weight sensor and instrument should be used after 30 minutes of warm-up.

VI Error Analysis

1. Operational Error

It is caused by the operator, which also means that there are many causes, such as the errors that occur at different temperatures or during the purification of air or other gases, including faulty probe placement or incorrect insulation between the probe and the measurement address. It can also be caused by the faulty placement of the transmitter, so positive or negative pressure will affect the reading.

2. Property Error

It's inherent in the equipment itself. It is the difference between the well-recognized handling function characteristics and actual characteristics of the equipment. Such errors include DC drift, incorrect or non-linear slopes.

3. Dynamic Error

The characteristics and calibration of many weight sensors are suitable for static conditions, which means that the input parameters used are static or similar to static. Many sensors have strong damping, so they will not respond quickly to the changes in input parameters. For example, it takes a few seconds for the thermistor to respond to a step-change in temperature.

The thermistor will not immediately jump to a new impedance or change suddenly. On the contrary, the impedance is slowly changed to a new value. Therefore, if the weight sensor with a delay characteristic responds to the rapid change in temperature, the output waveform will be distorted due to dynamic errors. The factors that cause dynamic errors include response time, amplitude distortion, and phase distortion.

4. Insertion Error

Insertion errors occur when a sensor is inserted into the system due to changes in measurement parameters. This is usually the problem during the electronic measurements, but similar problems can also occur in other measurements, such as using a voltmeter to measure the voltage in a circuit. The voltmeter must have an inherent impedance, which is much larger than the circuit impedance. As a result, the circuit load appears, which leads to a large error in reading. The reasons for this type of error may be:

●the transmitter is too large for the system (eg, pressure system);

●the dynamic characteristics of the system are too slow;

●too much thermal energy is loaded by self-heating in the system.

5. Environmental Error

Environmental errors come from the environment where the weight sensor is used, which includes the temperature, swing, sensation, altitude, chemical evaporation, or other factors. These often affect the characteristics of the sensor, so in practice, these elements are always classified and gathered together.