Types of sensors in automatic control (PT3): Loadcell

Loadcell is a sensor used to convert force or weight into electrical signals.
The term “strain gage”: a structure that can deform in an elastic form when subjected to the force that generates an electrical signal proportional to this distortion.

Loadcells are often used to induce large, static, or slow-to-force forces. Some loadcell cases are designed to measure the impact force depending on the design of the loadcell.

2. Structure and principles of operation
2.1. Structure
The loadcell is made up of two components, the first being the “Strain gage” and the other being “Load”. Strain gage is a special resistor that is only small with fingernails, with variable resistance when compressed or stretched and fed with a stable power supply, dies to “Load” – a loaded metal rod. elasticity.
2.2. Operating principle
It is based on the Principle of balanced Wheatstone balance. The force acting is proportional to the change in the induced resistance in the resistor bridge, and hence the voltage on the signal.

3. Basic Specifications
– Accuracy: indicates the exact percentage in the measurement. Accuracy depends on non-linearity, latency, repeatability.
– Rated power: the maximum mass value that a loadcell can measure.
– Temperature compensation band: This is the temperature range at which the loadcell output is compensated. If it is out of this range, the output is not guaranteed to comply with the specified specifications.
– Protection level: measured in IP scale (IP65: moisture and dust resistant).
– Voltage: The working voltage value of the loadcell (normally the maximum value and the minimum value 5 – 15 V).
– Delay: Delayed results display results in errors in results. Usually given as% of load.
Input impedance: The impedance is determined via S- and S + when the loadcell is not connected to the system or in idle mode.
Insulation resistance: Normally measured at DC 50V. Insulation value between the metal sheath of the loadcell and the current connected device.
Mechanical degradation: The load value that the loadcell may break or deform.
– Output: measured results (unit: mV).
Output impedance: given as the impedance measured between Ex + and EX- in load cell condition is not connected or in idle mode.
– Overload safety: is the capacity that the loadcell can exceed (for example, 125% of capacity).
– Temperature effect coefficient: The quantity measured in load mode, is the change in capacity of the loadcell under the temperature change, (eg, 0.01% / 10 ° C means that if the temperature rises By 10 ° C, load capacity of Loadcell increases by 0.01%).
– The coefficient of action of temperature at point 0: same as above but measured in idle mode.

4. Classification
Loadcells can be classified as follows:
Loadcell classification according to shear loadcell, compression loadcell, bending), TensionLoadcell
– Classification according to shape: disk, rod, cylindrical, spherical, S type
– Sort by size and load capacity: small, medium, large.

 5. Application of Loadcell.
A commonly used application of Loadcell is the use of electronic scales.

From application in high-precision engineering scales to heavy duty industrial scales such as truck scales.
Some other applications:
– In high technology:
With today’s advanced science and technology, small Loadcell type also improves technology and application higher. This type of loadcell is attached to the tip of the robot finger to determine the tensile strength and compression force that affect the objects when they are gripping or lifting.

– Even weight distribution in industry:
Technology to use:
Loadcell LSB and LCF Series combined with data acquisition and direction devices via PC or PLC
Activity summary:
Load cells are designed to be suitable for industrial automation applications to distribute product weight evenly. As shown in the diagram below, Loadcell is installed in the automation line, which monitors the delivery of mass into each package precisely.

Operating System:
+ A load cell is connected to the necessary measuring device.
+ When the product volume for delivery to the bin is sufficient, the loadcell will emit a signal to the conveyor controller so that the conveyor belt stops working.
Signals when the conveyor stops are passed to the container distribution system for the container release.
+ Once the container is distributed, it will signal the distribution system continues to operate.
– Application in the road
The loadcell is used in the suspension safety warning. Loadcell is installed on cables to measure cable tension and foot pressure in various weather and weather conditions. The data collected will be sent to a data acquisition and processing system. Then the data will be exported via access device such as phone, computer, LCD. Since there is a warning about the safety of the bridge. From there find the necessary measures to repair in time.

6. Basic loadcell types
6.1. Similar loadcell
A) Concept
Loadcell senses tension, converts it into an electrical signal called a similar loadcell. This attribute is transformed into useful information by measuring devices such as indicators.
Each loadcell loads an independent output, typically 1 to 3 mV / V. Output matching is based on the output of each loadcell output. Input or output power amplifiers, via the CAD converter, microprocessors with built-in software perform calculations and read the results on the screen. Most modern displays or displays allow communication with external devices such as computers or printers.
B) Advantages and disadvantages
The main advantage of this technology is that it comes from the actual requirements, with predefined parameters, that will have the right product design for each user application. There are touch elements of different sizes and shapes that fit the requirements of the application.
Common forms: shear, bending, compression
The output voltage signal of the loadcell is very small (usually no more than 30mV). Such small signals are easily affected by many types of industrial noise such as:
Electromagnetic interference: generated by the transmission of electrical signals in the surrounding environment, transmission of radio signals in space or by the switching of high-power switches
Resistance to the cable resistance of the signal conductor: due to erratic changes in environmental temperature affecting the transmission cable.
Therefore, to make the system more accurate, the shorter the distance between the loadcell and the measuring device, the better. The usual workaround is to minimize the Loadcell output tolerance. However, the limitation of technology does not allow to exceed the desired number too small. While multiple loadcell loads are connected to each loadcell, each loadcell loads with one output independent of the other loadcells in the system, thus ensuring consistent, stable readings regardless of location and system. Require output tuning with individual load cells. This work is time-consuming, especially for systems requiring high accuracy or in applications that are difficult to load such as tank weighing, silo balances.
The general output signal of a multiple loadcell system is based on the initial output signal of each loadcell. That makes it easy to see if the faulty Loadcell is unrecognized. Once recognized, it is also difficult to determine which loadcell is defective, or whether it is difficult to load test requirements, or whether to use measuring devices such as volt-amps with high accuracy. , Especially in the condition that the factory is operating continuously.
In fact, there are many other factors related to the accuracy of the weighing system:
· System tuning process.
· Noise and vibration noise.
· Due to the effect of radial force in the tubular body.
· Analytical error detection analysis.
· Replace components in weighing systems or related systems.
· Take the long signal cable.
· The operating environment is too tight.
It is impossible to calculate in advance these influencing factors for modeling during analysis and design. While the working conditions vary from location to location, the device is located away from the sensor, the signal is weak, it is prone to loss and many types of interference, especially in the harsh working environment in the home. Machines and factories. The signal sent to the measuring device is difficult to accurately reflect the actual value.
Meanwhile, current displays often use low-speed microprocessors, low computing power, less equipment that integrates algorithms to process the data collected, or if any. At a simpler level. Because displays are used with a variety of loadcell types, tuning algorithms are relatively relativistic, especially since no device incorporates temperature compensation. The electromagnetic interference filter function of these devices is very poor. Another downside is that the sampling frequency is low, so it can not be applied in applications where fast-acting forces such as continuous-

6.2. Digital Loadcell
A) Concept and its Background
Time of founding: From the late 1970s
Basically Digital Loadcell is the integration of load cell analogue with modern electronic technology.
Initially, when the concept of the new Digital Loadcell was introduced, many people mistakenly believe that digital cell load with low power elements can be used to convert a low quality load cell to a high quality loadcell. In fact, on the contrary, each Digital Loadcell simply carries a rather complicated structure.
– First: There must be a basic loadcell with high accuracy, stability and repeatability in all working conditions.
– Second: There must be a 16 to 20 bit high speed analog-to-digital converter (ADC) to convert analogue electrical signals to digital ones.
– Thirdly, there must be a processor chip to carry out the entire process of converting from the measured force signal into the best digital data and communicating with other devices to exchange information. .

B) Activities:
The voltage signal from the resistor bridge of the high precision loadcell is fed to the input of the integrated circuit, including the amplifier, demodulator, a 20 bit high speed ADC and digital filter. An integrated temperature sensor is used to measure the actual temperature of the loadcell for temperature compensation. Data from the ADC, temperature sensor with software algorithms and some additional built-in hardware optimizes the handling of non-linear errors, The ability to recover the state and effects of temperature is handled by high-speed processors. Output data is transmitted remotely via a protocol interface. These electronic modules can be placed in the loadcell, load cell cable or in the junction box. The critical characteristics of each load cell placed in the EEPROM are in the loadcell module, which also means that all error handling issues are performed at the load cell, with the same loadcell, which also means Compensation tolerance is fairly well done.
A typical digital system consists of a number of digital loadcells connected to a computer, a PLC, or a measuring device such as a display. Within the system, each loadcell can be identified by its working address. That work address can be set by the programmer through one or more addresses provided by the factory. Typically, the address “0” is used as an address for all load cell responses, while serial numbers for the loadcell can be used to request a specific address.
Digital Loadcell operates on a Master / Slave control program, which defines a device (usually a PC or indicator) as a master on the network. There are two main modes of operation: The master monitors all transmissions by communicating with each slave sequentially, or the master sends data that requests the slaves to respond at sequential addresses. The first mode has the advantage of being flexible and catching errors, while the two modes reach the speed of communication. Most Digital Loadcells are RS485 or RS422 compliant. Both protocols have similar features providing a multi-drop environment. The communication between the connected devices on the network is based on the protocol specified by the manufacturer. Perhaps the most important difference between analogue and digital loadcell systems is that although they are connected together, each Digital Loadcell operates as Is a standalone device.
C) Advantages and disadvantages
– The “strong” output signal, very little affected by electromagnetic interference or erratic temperature change on conductor cable.
– Cable distance can extend up to 1200m.
– Easily replace Loadcell.
– Digital data can be processed directly by computer, PLC, or on display when needed.
– Each Loadcell is a stand-alone device in the system, so it is easy to expand the structure.
System optimization can be easily accomplished through integrated component analysis.
– Balances of scales can be made by the device. Changing, fixing a Loadcell does not affect the other Loadcell. Work done easy and simple, saving time.
– With low and medium accuracy requirements system can be automatically adjusted without dead load.
– Loadcell can be replaced without re-adjustment.
– RS485 / 422 devices can participate in the system.
– Many systems can be connected and controlled by a station. Simply expand the cable line. Saving the hardware. The software is easy to develop.
The advantages of the Digital Loadcell system allow for high accuracy and noise suppression applications, especially in applications that require scattered measuring points on a large scale.

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