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Classification and Disadvantages of Weighing Sensors
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Relevant sensor technology papers
Publish Time:
2020-07-08
A weighing sensor is a device that converts a mass signal into a measurable electrical signal output. There are qualitative differences between the old and new national standards in the basic concepts and evaluation methods of the main technical indicators of weighing sensors. There are several styles, mainly S-type, cantilever type, spoke type, plate ring type, bellows type, bridge type, and column type.
When it comes to weighing sensors, many people think that this is just a sensor for measuring the weight of an object, but this is not entirely correct; it has also been applied in many fields. Weighing sensors are similar in principle to the pressure sensors described earlier, but their shapes are different. Weighing sensors are used everywhere in modern scientific and technological industries. Below, we will introduce the classification and disadvantages of weighing sensors.
Classification of Weighing Sensors
Weighing sensors can be divided into eight categories according to their conversion methods:
1. Resistive Strain Gauge Type
Resistive strain gauge type is a commonly used weighing sensor. It works on the principle that the resistance of the strain gauge changes as it deforms. It mainly consists of four parts: an elastic element, a strain gauge, a measuring circuit, and a transmission cable.
2. Hydraulic Type
Hydraulic sensors have a simple and robust structure and a large measurement range, but the accuracy generally does not exceed 1/100.
3. Capacitive Type
It works using the proportional relationship between the oscillation frequency f of the capacitor oscillation circuit and the distance d between the plates. Capacitive sensors have low power consumption, low cost, and an accuracy of 1/200～1/500.
4. Digital Type
A digital weighing sensor is a force-to-electricity conversion device that can convert gravity into an electrical signal. It mainly refers to a new type of sensor that integrates a resistive strain gauge weighing sensor, an electronic amplifier, analog-to-digital conversion technology, and a microprocessor.
The development of digital weighing sensors and digital instrumentation technology has gradually become a new favorite in the field of weighing technology. Its advantages, such as simple and efficient debugging and strong adaptability to the site, are showing promise in this field.
5. Plate Ring Type
The structure of the plate ring type weighing sensor has advantages such as a clear stress streamline distribution, high output sensitivity, an integral elastic body, a simple structure, a stable stress state, and easy processing. It still accounts for a large proportion in sensor production, but the design formulas for this type of sensor are not yet perfect.
6. Vibration Type
After the elastic element is subjected to force, its natural vibration frequency is proportional to the square root of the applied force. By measuring the change in the natural frequency, the force acting on the elastic element can be obtained, and then its mass can be obtained. There are two types of vibration sensors: vibrating string type and tuning fork type.
7. Gyroscopic Type
Gyroscopic sensors have a fast response time (5 seconds), no hysteresis, good temperature characteristics (3ppm), small vibration influence, and high accuracy of frequency measurement, so high resolution and high measurement accuracy can be obtained.
8. Photoelectric Type
Includes grating type and encoder type. Photoelectric sensors were mainly used in electromechanical combination scales.
Disadvantages of Weighing Sensors
Weighing sensors are not perfect; they also have insurmountable disadvantages.
1. Non-linear Output Characteristics
The output characteristics of capacitive weighing sensors are non-linear. Although a differential type is used to improve this, it is impossible to completely eliminate it. For other types of capacitive sensors, the output characteristics are linear only when the edge effect of the electric field is ignored. Otherwise, the additional capacitance generated by the edge effect will be directly superimposed on the sensor capacitor, making the output characteristics non-linear.
2. Significant Influence of Parasitic Capacitance
The initial capacitance of capacitive weighing sensors is small, while the lead capacitance connecting the sensor and the electronic circuit, the stray capacitance of the electronic circuit, and the capacitance formed by the inner plate electrode and the surrounding conductor, the so-called parasitic capacitance, are relatively large. This not only reduces the sensitivity of the sensor but also makes these capacitances often change randomly, making the instrument work unstable and affecting the measurement accuracy.
3. High Output Impedance, Poor Load Capacity
The capacitance of capacitive weighing sensors is difficult to make very large due to limitations such as the geometric dimensions of their electrodes, generally ranging from tens to hundreds of microfarads, or even only a few microfarads. Therefore, capacitive weighing sensors have high output impedance and poor load capacity, are easily affected by external interference, and may become unstable, or even fail to work in severe cases.
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