I. Development Trends and Related Products of Dynamic Weighing Technology
Weighing technology integrates sensor technology, microelectronics technology, computer control and measurement technology, mechanical automation technology, logistics transportation and management technology, and forms the technological foundation of modern weighing, measurement and control system engineering. It permeates various industries, and its function has evolved beyond simple measurement to encompass multi-parameter, professional integrated control, forming diversified intelligent management and control systems. These systems can be broadly summarized by the following characteristics:
1. High speed, high precision, and high performance;
2. Miniaturization, modularization, intelligence, and integration;
3. Stability, reliability, long lifespan, self-diagnosis of faults, and maintainability;
4. Networked management enables online communication and remote control;
In conclusion, intelligence, automation, and multifunctionality will become the development direction of modern weighing technology.
The main products utilizing dynamic weighing technology include:
Continuous cumulative automatic weighing scales (belt scales); automatic sorting weighing scales; gravity-type automatic loading weighing scales; automatic track weighing scales;
Dynamic vehicle scales, etc.
II. Application of Electromagnetic Force Balance Weighing Sensors in Dynamic Weighing Technology
01 Basic Principles and Theoretical Basis of Electromagnetic Force Balance Sensors
Electromagnetic force is the general term for the force exerted on electric charges and currents in an electromagnetic field. It is also called the electrostatic force on stationary charges in an electric field, while the force exerted on a current-carrying conductor in a magnetic field is called the Ampere force. For a current- carrying conductor of length L , the magnitude of the Ampere force in a uniform magnetic field is: F = ILBsinθ
Where θ is the angle between the direction of the current and the direction of the magnetic field strength, I is the current flowing through the straight conductor, B is the strength of the magnetic field where the straight conductor is located, and L is the length of the straight conductor within the magnetic field region. When the direction of the current is the same as or opposite to the direction of the magnetic field, the current is not subjected to magnetic force. When the direction of the current is perpendicular to the direction of the magnetic field, the Ampere force on the current is the largest, which is: F=ILB
The direction of the Ampere force is determined by the left-hand rule. With your left palm flat, allowing the magnetic field lines to pass through your palm, and your four fingers indicating the direction of current movement, the direction your thumb, perpendicular to your four fingers, points is the direction of the Ampere force. The Ampere force is essentially the resultant force of the Lorentz forces acting on the directed movement of charges that form the current. See Figure 2-1 below:
The weighing process of the electromagnetic force balance sensor is based on the Ampere force. The force exerted on the current- carrying conductor in the magnetic field is shown in Figure 2-2. A weighing pan is added to the conductor. The weight of the weighing pan and the conductor itself is downward, while the current-carrying conductor is subjected to an upward electromagnetic force. When the current value through the conductor reaches a certain value, the two forces are balanced, and the sensor is in a balanced state, as shown in Figure 2-3 .
Figure 2-2 Schematic diagram of the forces acting on a current-carrying conductor in a magnetic field
Figure 2-3 Schematic diagram of the sensor in an unloaded state
When a weight is loaded onto the weighing pan, the downward force of gravity increases, causing the pan to move downwards. This results in an unbalanced state for the sensor, as shown in Figure 2-4. To restore balance, the current flowing through the wires must be increased so that the magnitude of the electromagnetic force generated by the current equals the force of gravity exerted by the added object. Only then will the weighing pan return to its original balanced position, as shown in Figure 2-5 .
Figure 2-4 Schematic diagram of sensor load conditions
Figure 2-5 Schematic diagram of the sensor returning to equilibrium after bearing load.
Because the current flowing through a single wire is very small, the electromagnetic force generated is also small. In practical applications, the wire is wound into a loop; one turn of the coil constitutes one wire . The electromagnetic force generated by an N-turn coil is N times that of a single wire. This method is used to generate electromagnetic force in electromagnetic force balance sensors. For a looped wire, the force F is: F = ∫ dF = ∫ BIdLsin θ
When θ = 90 °, F = BIL; when there are N turns of coil, F = NBIDL .
Assuming that in a uniform magnetic field, where B is constant and DL is a constant value, the magnitude of the electromagnetic force F is proportional to the current.
F ∝ I, F=mg, m ∝ NI
From the above formula, we can conclude that the weight of an object is directly proportional to the magnitude of the electric current.
02 Several factors affecting the performance of electromagnetic force balance sensors
The principle of an electromagnetic force balance sensor seems relatively simple, and its implementation appears straightforward, but in reality, it faces numerous challenges. Theoretically, the mass of the object being measured is directly proportional to the current flowing through the conductor. While it's assumed the conductor is in a uniform magnetic field (a constant value), the actual magnetic field is generated by a permanent magnet. Even two magnets of identical material, shape, and mass will not have identical magnetic fields. Regardless of the material used to cast the magnet, they all possess a temperature coefficient. The magnetic induction intensity of a molded permanent magnet varies with temperature, time, and the surrounding magnetic field. Furthermore, corrosion, impact, and vibration also affect the stability of the permanent magnet's magnetic induction intensity.
03 Key Technologies of Dynamic Weighing Systems
A good control system hinges on resolving the relationship between dynamic characteristics and steady-state error. In dynamic weighing systems, two key issues are accuracy and speed: achieving the highest possible accuracy at the fastest possible speed. The adjustment of the load cell primarily involves controlling the current in the sensor coil. The current is adjusted according to the weight of the object, while the coil current regulator, in turn, controls the current magnitude to keep the sensor in a balanced position.
Weighing a moving object requires considering not only its static characteristics but, more importantly, its motion characteristics. Currently, dynamic weighing is primarily achieved through data processing. This processing relies on a corresponding kinematics model and algorithm, and must address weighing errors caused by factors such as the object's velocity, acceleration, direction of motion, vibration frequency, and vibration amplitude.
