Capacitance Guide for Industrial Applications

Accumeasure Digital Capacitance System D Series Gen 3

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Capacitance Displacement and Gap Measurement Probes

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Accumeasure Analog Capacitance System

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Purpose

This guide from MTI Instruments explains what you need to know about using electrical capacitance for measurement in industrial applications, including advanced manufacturing.

Basic Principles

Capacitance is the ratio of the change in an electric charge to the corresponding change in its electrical potential (i.e., voltage). Capacitors, components that that have the “capacity” to store an electric charge, consist of conductive parallel plates that don’t touch or connect with each other. Instead, these metal plates are electrically-separated by air or a layer of insulating material (i.e., the dielectric).

MTI’s technology is based on the principles of parallel plate capacitor measurement and uses a probe as one of the plates and a grounded target as the other. The electrical capacitance that’s formed between the probe and the target varies as a function of the distance (gap) between these two surfaces. An amplifier converts the capacitance of the gap into an output voltage that’s proportional to the gap.

Why Capacitance?

Because capacitance doesn’t require physical contact between the sensor and the target, there’s no mechanical loading, probe or target wear, or target distortion. Capacitive probes also cost significantly less than laser interferometers while matching or exceeding them in terms of stability, precision and resolution.

Importantly, capacitive probes provide reliable performance in extreme environments. For example, they can withstand temperatures as high as 1200°F (650°C) and as low as 4°K (-269.15°C). Capacitive probes also work in very strong magnetic fields (2 Tesla) and operate under vacuum conditions (10-7 Torr). They withstand humidity, too.

Getting Started

Now that you know some basics, it’s time to examine uses and applications. You’ll also benefit by reading MTI’s case studies from different industries. Before using capacitance for your own application, however, you need to account for design considerations. Comparing the capacitance measurement systems that are available from MTI Instruments can also support your efforts.

Uses and Applications

Capacitance is used in a variety of industrial applications. Examples include measuring position and vibration. Whether you need capacitance for standard or advanced uses, you’ll benefit from its accuracy and reliability.

Standard Uses

Position Sensing
- This is probably the most common application for non-contact capacitive sensors.
- The output of a capacitance sensor is proportional to the distance to the target.
- Precise positioning is now required in many advanced manufacturing applications.
Dynamic Measurements
- Unlike tripwires, capacitance sensors can provide continuous, real-time measurements.
- Non-contact sensors are ideal for moving targets because they don’t add mass.
- Capacitive sensors also have high frequency response for greater precision.
Thickness Measurements
- On-line monitoring is better than periodic, post-production sampling.
- Thickness measurements may need to be performed at high speeds.
- Multi-channel measurement supports more than one thickness channel.

Position Sensing
- Supports closed-loop feedback systems for micron and sub-micron precision.
- Piezo motors support high precision but are less accurate with open-loop linear response.
- Capacitance eliminates mechanical loading because there’s no physical contact.
Dynamic Measurements
- Real-time data instead of data logging.
- Frequency and amplitude measurements with component vibrations.
- Measuring thermal expansion across heating and cooling cycles.
Thickness Measurements
- Capacitance changes when an insulating material is inserted into the gap.
- This change occurs even if the distance between the probe and target remains constant.
- The amplifier’s voltage output changes and is proportional to the dielectric’s thickness.

Design Considerations

Before using capacitance with your application, you’ll need to ask and answer a series of questions:

What are you trying to measure?
What is the level of accuracy that you need?
What is the application environment?
What are the risks of grounding the target?
Do you need continuous, automatic monitoring?

This is just some of what you’ll need to know. That’s why it’s important to choose the right partner, too.

Get Started

Industries & Case Studies

Capacitance is used in industries ranging from aircraft maintenance to tire manufacturing. As the following case studies explain, capacitance is also used in consumer electronics and semiconductor manufacturing, wind and solar power, industrial processing, and in the automotive industry.

Consumer Electronics

Electronic manufacturing requires the fast, precise positioning of miniature components. On many assembly lines, process control now demands micron or even sub-micron accuracy. For precision positioning with piezoelectric actuators or voice coil motors, digital capacitance sensor systems combine closed-loop feedback with speed and accuracy.

Semiconductor Manufacturing

Semiconductor manufacturers are building three-dimensional (3D) integrate circuits (ICs) that feature vertically-stacked silicone wafers and dies. This helps boost device performance, but 3D ICs need coplanar surfaces that make contact with all pins, pads and pillars. To achieve high-resolution position control, capacitance probes are used. The gap measurements are processed with a high-precision digital amplifier.

Power Generation

The power generators for wind turbines need an optimal gap to ensure peak efficiency. They also need a gap measurement solution that withstands harsh environments and magnetic fields. Corrosion-resistant capacitance probes made of non-magnetic materials are durable and highly-precise. They integrate with digital amplifiers and support the use of custom cables that prevent magnetically-induced currents.

Solar Technology

The solar industry needs higher-density chips with smaller critical dimensions. Capacitance enables precise measurements, but grounding can damage wafers or interfere with sensing. Pull capacitance systems that measure ungrounded semiconductor wafers remove these risks. They also eliminate inaccuracies by supporting poorly-grounded targets with sub-micron precision.

Automotive

Brake rotors are now thinner and lighter, but they’re still exposed to conditions that can cause distortion or failure. That’s why the automotive industry needs continuous data collection during testing. Multi-channel measurement systems that withstand high temperatures and provide 24-bit digital readings eliminate errors from analog filtering, linearization, range extension, and the summing of channels.

Industrial Processing

Oil System Contamination

Water-in-oil contamination can reduce the life of industrial lubricants and increase the cost and frequency of repairs. In-line capacitance probes support a proactive approach to maintenance and, when used with a digital amplifier, can trigger a CPU alarm if the dielectric field reading for an oil flow channel differs from the reading for a sealed oil sample. Monitoring is both continuous and automatic.

Uses and Benefits of Digital Capacitance Whitepaper