How does a capacitive sensor detect non-metallic materials?

What is a Capacitive Proximity Sensor and How Can it Work with Non-Metals?

A capacitive proximity sensor is a non-contact sensor used in flexible packaging. It can sense non-metallic materials. It senses materials like glass, paper, or plastic. It can detect anything dielectric up to 2 m in any direction. The only limitation is packaging materials with a dielectrical constant of 2 or more. This sensor is able to detect dielectric materials without identifying any metal with the packaging materials and therefore can highlight materials above the 2 constant.

A Capacitive Proximity Sensor has two small metal plates that form a weak electric field at the front of the sensor. When nothing is nearby, the sensor's internal circuit maintains the electric field at a steady state. Once non-metallic materials, like plastic bottles, come close enough to enter the electric field, the electric field stabilizing circuitry gets changed. The sensor circuitry picks up that change, locks onto it, and converts it to a signal that says, "Yes, I see something." This sends an alert to whatever machine the sensor is connected to.

Chengwei's Capacitive Proximity Sensors are designed with the intention to be the best in the industry. There are small details that are addressed, such as the difference in the size of the metal plates, and the change in circuit sensitivity so that the sensor can even detect thin non-metals like paper and those with lower dielectric constants such as foam. In comparison to cheaper sensors available in the market which tend to pick up air and lose non-light plastics, Chengwei's are much advanced. Chengwei focuses on the non-metals that you need detected which is crucial on busy production lines as there are no false positives.

To clarify, imagine you are working with a Chengwei Capacitive Proximity Sensor on a filling line that pours yogurt into plastic cups. There is no need to explain this process in overly complicated engineering terms, so, as simply as possible, here is the process in steps.

First, the sensor activates and its electric field begins forming. The face of the sensor is aligned where the plastic cups pass and remain a few millimeters apart, never touching. At this point, the electric field is only focused on the surrounding air, and the sensor's capacitance remains at "base level." This is the "no-target state" as termed in their manuals by Chengwei.

Next, a plastic cup passes through the electric field. Compared to air, plastic has a higher dielectric constant, thus the sensor "disturbs" the field. Think of a pond, calm and still, and you throw a stone. The sensor in this case is detecting the change in capacitance. The cup would cause the sensor to increase capacitance and the sensor would record a change. Lightweight plastic cups won't pass without being registered, because Chengwei's sensors are designed to detect small increases in capacitance.

Now, the inner workings of the sensor's circuit start to activate. Each microsecond, the sensor checks the level of capacitance. When the level of capacitance increases, the sensor record the increase, and then, activates the the sensor to communicate to the line's controller. Then, the sensor sends an electrical signal to the machine with the command, "Hi, a cup is here. It's time to fill yogurt." This command is set to activate when the cup is at the machine. The user of Chengwei's sensor, designed, gets complete control of the capacitance level sensor, so it is not set to trigger on large non-metal blocking objects, like thick cardboard. There is a dial to set to the capacitance level whether it's thick cardboard or thin glass.

Once the filling process is complete, the cup is transferred to the next station. The sensor checks the capacitance and gets the cup has moved, then immediately checks the the level of capacitance again with the command to freeze the machine. When the level of capacitance dropped down to baseline is when the machine is instructed to wait for the next cup. The speed of the sensor is the reason the fast moving lines don't get backed up.

Chengwei's sensors can detect these in milliseconds, which is pretty impressive for a line producing 100 plastic cups every minute.

Real World Applications for Non Metels.

Capacitive proximity sensors are among the most versatile non metallic sensors, expanding well beyond labs into the real world. Chengwei designs non metallic sensors that are specifically made to meet safety and operational efficiency standards in non metallic factories to improve efficiency in daily life.

In packaging lines, these sensors are incredibly valuable. In a plastic bag manufacturing factory, inline sensors scan the sealing system for the sealing roll of plastic film and signal operators before it runs out for bag sealing continuity. Chengwei's sensors and packaging technology can identify even thin, modern, translucent plastic rolls. In warehouses, these sensors are equipped to cardboard box packaging lines. For example, inline sensors detect a shipping container placed on the conveyor belt and trigger the automated sealing system.

Why do food and beverage industries choose these kinds of sensors? Think about a juice bottling line for example. Each of the containers (be it a plastic container or glass) are non-metal, and the sensor has to determine if each bottle is positioned under the filler before the juice flows. Chengwei sensors are reliable even when cleaning or when containers have labels. Their sensors can withstand moisture and small debris, and will not false trigger. Other applications include detection of plastic lids on jars and ensuring a jar is capped before removing the lid.

Chengwei also focuses on logistics and material handling. For instance, Chengwei Proximity Caps are Used for Capsule Automatic Detection on Conveyor Systems in Warehouses. The sensors adjust for delays in the process when they auto determine stuck/untouched containers aiding in time loss. For example, when foam padding is placed in containers to protect fragile items for shipping, the sensors determine if there is a container placed on the conveyor, if packaging foam is placed in the container and if the container is left on the conveyor for too long.

The best part about this sensor is that it can detect non-metal objects without having to touch them. This can be particularly useful in cases where touch can scratch non-metal objects, like glass, or obstruct movement. Chengwei has designed their sensors to be compact enough to be mounted in tight spaces like the center of conveyor belts, or next to filling nozzles, for seamless integration into the existing setup.

Getting The Best Results with Non-Metals

To optimize performance with non-metals, every Capacitive Proximity Sensor requires a few adjustments. The following recommendations focus on non-metal detection, and improving performance to best detection limits provided in Chengwei's technical documentation.

First, choose a sensor according to the type of material. Non-metals exist in various forms. For example, thick plastic and thin paper have different dielectric constants. Chengwei's sensor models differ in design for various dielectrics. Some models are designed for high-dielectrics like water and thick plastic, and others, thin film and foam, are for low-dielectrics. If a sensor designed for thick plastic is used on thin paper, detection will likely fail. They have a guide on their site to help you match a sensor model to a material. For detection, you will need the dielectric constant of the material, and then you can choose the appropriate sensor.

Let's figure out how to set the distance properly. "Detection range" is the term used to describe the area where the sensor can identify a non-metal object. Chengwei's sensors have a range of 1mm to 20mm based on the specific model you are using. You want to make sure to not set the range too far (the sensor won't detect anything) or too close (the object might hit the sensor). A good rule of thumb is to set the distance to 70% of the detection range to the non-metal object. If the sensor's range for plastic is 10mm, set it to 7mm. You can test this with a piece of the non-metal to see how close you can get before the sensor activates, and then pull it back a little for a safe distance.

Adjusting Sensitivity can be a tricky task, to begin with Chengwei's sensor can be customized, When trying to detect a smaller non-metallic object like a plastic lid, sensitivity can be adjusted upwards to grab hold the object. For larger objects such as a cardboard box, it is better to lower the sensitivity to avoid the box triggering the system due to dust or other small particles. Users often make the mistake of setting the sensitivity way too high which in turn will cause false triggering. Always test with the material you intend to use in practice, not a sample. Finally, if the line is running different materials, for example small and large plastic parts, then use sensors with adjustable rapid changeable sensitivity.

Environmental factors need to be monitored closely. Dust, moisture, and temperature are some of the factors that affect the sensors.

Chengwei's sensors have IP ratings which prove they are dust and waterproof, for example IP67, however, you still need to make sure to install these away from direct streams of cleaning water or extreme temperatures (greater than 80°C). If you are working in a dusty environment, for example a paper mill, you will need to clean the sensor front face every few days. If dust accumulates, it will block the electric field.

Do regular checks. After setup, things can still shift, like the sensor moving a little from vibration. Once a week, test it with your non-metal to make sure it triggers every time the material passes and no false triggers when there's nothing. Chengwei's support team can help if you're having trouble. A little maintenance goes a long way to keeping the sensor working well for years.