Several grounding methods and case analysis of electronic truck scales

With the overall progress of technology in the field of automotive electronics and electrical appliances, electronic truck scales have been widely and deeply popularized and applied in various industries. However, most electronic truck scales are now used and maintained outdoors, so they often inevitably encounter electromagnetic interference, thunderstorms, lightning and other external factors that affect system operation. Common phenomena include: instrument jump count, drift, etc. Grounding is an important method to suppress electromagnetic interference and avoid lightning strikes. It is also an important part of interference shielding and improving system safety. It plays a vital role in electronic truck scales.

1. The concept of “land”

Usually, when we say "ground", we mean the reference potential point, which is not necessarily the potential point of the earth. Since the earth itself is a conductor, its capacitance is extremely large, so the potential of the earth is constant. Therefore, the potential of the earth can be regarded as the reference potential - zero potential.

2. Methods for grounding electronic truck scales

(1) Suspended grounding: refers to the mutual insulation between the working ground of the electronic truck scale and the earth system, so that the interference on the grounding system will not enter the equipment, improving the working reliability of the equipment. This method is most commonly used in electronic weighing instruments.

(2) Single-point grounding: A certain point in the entire circuit is used as the grounding point, and all other units are connected to this grounding point.

(3) Multi-point grounding: refers to each grounding point in a device or system being directly connected to the grounding plane closest to it.

3. Application of grounding of electronic truck scale

In the design and installation of fully electronic truck scales, from the perspective of actual engineering structure layout, anti-interference and safety, mixed grounding is often used, that is, according to the power and level of each device in the scale and the sensitivity of the device, a mixed use of floating grounding, single-point grounding and multi-point grounding is used to give full play to the advantages of each grounding method. To achieve better results. The following is an example of a fully electronic truck scale to illustrate.

Assume an electronic truck scale consisting of a weighing platform, multiple weighing sensors, and connected external devices such as printers, computers, and large screens. According to the use of one or more grounding methods, we will analyze and explain them one by one.

3.1 Scale Platform Grounding

Since the weighing platform of the truck scale is generally outdoors, it is easy to be charged and struck by lightning. It is also easy to introduce static electricity when the vehicle gets on and off the weighing platform. It is also easy to be charged when the insulation of the sensor and other connecting wires is damaged. In addition, the weighing platform is also the gateway for the sensor, junction box and sensor transmission line to be free from direct interference. The good grounding of the weighing platform plays a great role in the safety and working reliability of the sensor and junction box. Therefore, multi-point grounding is a relatively ideal grounding method. In addition, since the sensor and the weighing platform are in direct contact, and the metal part of the sensor is connected to the ground, multi-point grounding is achieved. In this way, the accumulation of static electricity on the scale body can be effectively eliminated, and equipment damage caused by lightning induction can be prevented.

3.2 Sensor Grounding

The sensor is a highly sensitive unit, which is easily affected by static electricity and electromagnetic radiation. The sensor is also outdoors like the scale platform, and is very likely to be struck by lightning. Therefore, for the above reasons, the grounding of the sensor is generally multi-point grounding. The elastic body and the mounting structure parts are grounded nearby, and one end of the shielding wire of the signal transmission line is grounded through the elastic body, and the other end is connected to the junction box shell in the junction box, and connected to the scale platform through the junction box shell or lap grounding. Since the connection between the junction box and the sensor is relatively short and the grounding point is close, there is basically no large ground loop at the grounding point of the sensor, so interference is effectively suppressed while ensuring the safety of the equipment.

3.3 Junction Box Grounding

The junction box is the intermediate link between the scale body and the weighing instrument. The reliability of grounding directly affects the signal transmission between the scale body and the instrument. Therefore, the grounding of the junction box is also a very important link. The junction box can generally be connected to the scale body through the shell. Sometimes, it is also considered to overlap the junction box with the scale body to improve the grounding reliability, increase the safety of the equipment and the shielding effect of the junction box shell.

3.4 Grounding of shielded cables

Sensor transmission lines, computer communication lines and connecting lines of large screens. Since these transmission lines are long, the shielding effect of the shielded wire is very effective in preventing interference caused by electromagnetic induction. The shielding effect of the shielded wire is completely dependent on grounding. If it is not grounded or the grounding is unreliable, the shielded wire is not only useless, but also becomes a source of interference. Shielded wires are generally connected to the earth using a single-point grounding method; this can prevent the two devices from forming a ground loop through the shielded wire and interfering with the transmission line. However, it is also feasible to connect the shielded wire at two points. First, it increases the discharge path of the interference signal on the shielded wire; in addition, when the grounding effect at both ends is good, due to the increase in the release loop, the generation of interference sources such as static electricity accumulation is greatly reduced. Even if a ground loop is formed, the current on the ground loop is extremely weak, and it is difficult to cause interference to the transmission line. Two-point grounding actually increases the grounding reliability of the shielded wire and enhances the shielding effect.

3.5 Instrument grounding

The instrument grounding is actually divided into two parts: first, the instrument housing and metal structural parts and the metal shell of each socket are grounded. Generally, single-point grounding is used to connect the ground wire to the ground wire of the power supply, and then to the earth through the power ground wire. Secondly, the internal circuit board, components, etc. of the instrument are generally suspended. This connection method ensures that external interference will not enter the instrument, and at the same time prevents the instrument housing from being charged and causing safety hazards.

As mentioned earlier, the grounding of the instrument housing and other equipment is done by the power cord. This requires that when laying the power cord of the weighing instrument, you must pay attention to the connection of the ground wire. The practice of ungrounding or unreliable grounding of the power supply is extremely wrong. Not only does it cause safety problems, it will also cause great problems for the interference protection of the instrument.

3.6 External Equipment Grounding

It is very common to connect electronic truck scales to external devices such as large screens, computers, printers, etc. When these external devices form a weighing system, they must be well grounded. When connecting these external devices, especially printers and computers, poor grounding will directly lead to problems such as digital jumps, drifts, and error message prompts on the weighing instrument. This is because the internal signal ground and the earth (power ground) of devices such as printers and computers are connected together. If the power grounding is not done well, the interference signal coupled on the power line ground will be introduced into the signal ground inside the instrument through the signal ground of the printer or computer, destroying the original suspended ground state of the instrument, causing instrument ground interference and affecting the normal operation of the instrument. Therefore, when connecting external devices, the power supply of the external device must be well grounded.