What is a weighing sensor? How to connect with PLC? How to use it most accurately?

What is a weighing sensor? How to connect with PLC? How to use it most accurately?

Weight is an important unit in daily life, and it is even more crucial in industry. How many kilograms of raw materials are added in the production process? How many kilograms of waste are generated? These data are essential elements supporting the smooth operation of the entire factory.

So, how is weight measured in industry? We need to use weighing sensors!

The principle of a weighing sensor is to convert the gravity acting on the measured object into a quantifiable output signal in a certain proportion.

Common industrial sensors are categorized as 4-wire and 6-wire systems:

For a 4-wire sensor, there are only the four black cables shown in the diagram below. Among them, EXC+ and EXC- provide excitation from the weighing module to the sensor, while SIG+ and SIG- are the outputs from the weighing sensor. For a 6-wire sensor, two additional red wires, SENSE+ and SENSE-, are present. What is their purpose? These two wires are used for compensation. When the distance between the sensor and the weighing module is long, these wires provide compensation, thereby making the measurement results more accurate.

In practical field applications, due to complex working conditions, the distance between the weighing sensor and the weighing module can be considerable. If the supply voltage provided by the weighing module to the sensor is, for example, 6V, due to line losses, this excitation signal will be less than 6V when it reaches the weighing sensor. Under the same load conditions, the output signal of the sensor is proportional to the supply voltage. Therefore, the weighing module uses a high-impedance circuit to feed back the actual supply voltage at the sensor side to the module itself. The weighing module then adjusts its output supply voltage via an internal comparator.

The weighing module can connect to both 4-wire and 6-wire sensors. If connecting a 4-wire sensor, it is necessary to short SENSE+ to EXC+ and SENSE- to EXC- at the junction box or on the weighing module side.

For connecting weighing sensors to a PLC in a factory setting, we typically have two methods:

1. **Using a Weighing Sensor + Signal Conditioner/Transmitter + PLC Analog Input**
Why is a secondary transmitter needed? Because the sensor generates a millivolt-level voltage signal when deformed by the object's weight, which the PLC cannot read directly. Therefore, a secondary transmitter is required for conversion. Whenever conversion is involved, everyone knows there will be some loss. This loss isn't very large, but it can have a significant impact on precise measurements. I have previously worked on equipment requiring 10g accuracy, where this loss substantially affected the results. The characteristic of this method is its low cost and minimal requirements on the PLC system, as over 98% of PLCs have analog input functionality. However, the drawback is that it cannot be used in situations requiring high accuracy.

2. **The Most Accurate Method (and Most Expensive): Using a Weighing Sensor + PLC Weighing Module**
This method is the most precise because there is no loss when the PLC is connected to its dedicated module. Furthermore, the speed of this method is very fast, making it suitable for various complex applications.

**Its disadvantage is the high requirement it places on the PLC system.** Firstly, the PLC must use a weighing module, which is generally quite expensive. Furthermore, a new weighing system requires calibration before use. If a weighing sensor is damaged and replaced, recalibration is also necessary. The calibration process for various series of Siemens weighing modules is not straightforward and requires a certain level of experience and technical skill, which we will discuss later. Therefore, this technology is not suitable for beginners.

**As a unit for precise measurement, its usage method must be carefully followed; otherwise, it could lead to inaccurate measurements or even damage.**

**I. Extreme care during installation is crucial to avoid damaging the sensor.**

The elastomer of a weighing sensor is primarily made of alloy aluminum material. Therefore, any impact from vibration or accidental dropping can cause significant errors in data output. For example, a sensor with a 10kg capacity could be permanently damaged if subjected to a force of around 15kg. Hence, handling the weighing sensor with utmost care during installation is essential.

**II. Installation must strictly follow its design requirements.**

Depending on the equipment process, it may be installed horizontally or vertically. However, the load-bearing part of the sensor must fit snugly against the measured object. Importantly, the load-bearing part and the measured object must not contact any other force-bearing points, as this will lead to inaccurate measurements.

**III. Sometimes, three or even multiple sensors are used to measure the same object. The contact surfaces between these sensors and the weighing assembly must be level.** Otherwise, uneven force distribution will affect the measurement results. While slight level misalignment can be compensated for using resistors in the junction box, this adjustment capability is limited and is also not ideal for calibration. Therefore, strive to maintain a level installation whenever possible.

**Other Precautions:**

**I. Junction Box:** When using multiple weighing sensors, a junction box is a necessary tool. It consolidates the data from multiple sensors, reducing the workload on the weighing module.

However, it is also a component prone to issues during use.

First, pay attention to dustproofing, waterproofing, and preventing static electricity. Second, during use, be mindful of the function of each terminal and avoid incorrect wiring.

**II. Cable Connection and Shielding**

The sensor's signal is ultimately transmitted through cables. Given the high precision of this signal, even minor interference can cause data instability.

Therefore, connections must be strictly made according to the diagram provided during installation.

**III. Programming Considerations: Is Filtering Acceptable?**

In actual industrial environments, numerous interferences exist. While many are accustomed to using programming methods for filtering, this can also be applied to weighing applications. However, it is crucial to note that filtering introduces signal lag. If the purpose is merely observation and measurement, this might not pose a problem. But if the data is used for calculations or to control processes, filtering can lead to inaccurate weighing results. This is an important point to keep in mind.