Description

Description

This application note describes how to use capacitance for the non-contact, wireless measurement of very small gap changes (distances) between a device and a grounded target. It uses the textile industry as an example, but any industry that uses machinery with continuously rotating drums or motors can benefit from this approach to remote measurement.

Capacitance is highly accurate and can support the use of probes that are wireless. In our example, a capacitive sensing system from MTI Instruments eliminated concerns about establishing and maintaining wired connections inside a machine where the device is hard-to-reach and where it and the grounded target spin continuously. Moreover, wireless capacitive sensing supports remote monitoring for reduced downtime.

Problem

In the textile industry, carding machines are used to clean, detangle, and align fibers in a continuous web for subsequent processing. This mechanical process occurs between a device (a carding flat) and a grounded target (the surface of a spinning drum). The flat spans the cylinder’s width to clean and comb passing fibers. Saw-tooth wires that are wound tightly around the cylinder hold the fibers in place.

Figure 1 - Gap Changes

Figure 1: Carding Machine

To ensure high-quality yarn, a carding machine needs to maintain a proper gap between the surface of the cylinder and the carding flat. Different types of materials require different gap sizes. The consistency of the gap affects quality, too. Traditionally, operators checked changes in gap sizes through frequent machine shutdowns and manual inspections. This added labor costs and reduced machine productivity.

To ensure high-quality yarn, a carding machine needs to maintain a proper gap between the surface of the cylinder and the carding flat. Different types of materials require different gap sizes. The consistency of the gap affects quality, too. Traditionally, operators checked changes in gap sizes through frequent machine shutdowns and manual inspections. This added labor costs and reduced machine productivity.

Solution

MTI’s capacitive sensing system includes a cylindrical device (the probe with built-in low-power amplifier), a battery and battery holder, and a wireless transceiving base station. The cylindrical enclosure provides a way to mount the sensor. Not pictured is a plastic end cap that screws onto the two holes in the circuit board that extends through the cylinder’s body. The total profile is about 60 mm long and 10 mm in diameter.

Figure 2: Cylindrical probe, battery holder, and base station

The sensor unit has two connectors, one of which is for a five-pin cable that connects to a circuit board in a battery holder that houses a 3.6 V lithium battery. The other connection is a micro coaxial attachment for the antenna.

On the base station, the connector is wired using a RS-485 serial connection to a client’s programmable logic controller (PLC), a ruggedized computer that record machine data. For field calibration, a USB cable that converts to RS-485 may be used for calibration or communicating with the sensor.

Figure 3 - Gap Changes

Figure 3: Five-pin cable, battery holder and battery

The low-power amplifier that was used with the carding machine was a custom-designed Accumeasure from MTI Instruments. This amplifier converts a highly reliable capacitive electric field measurement (displacement) into a highly precise digital reading to accurately measure position. Technically, the sensor used in wireless deployments has 32-bit results. The digital Accumeasure™ also eliminates the need for an analog-to-digital converter.

Figure 4 - Gap Changes

Figure 4: Standard Digital Accumeasure

MTI’s capacitance-based sensing system for a textile carding machine demonstrates how this wireless, non-contact measurement system can be used with other machinery where precise measurements of very small gaps are required.