Experiment 6 - Capacitive II: Acceleration sensor

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Usually, one does not design and build a sensor for their applications. Exceptions are application specific sensors that have to provide very specific specifications with regard to e.g. energy management, accuracy, measurement range. Those sensor typically require complex ASIC design and sophisticated and expensive design processes. Normally, off-the-shelf sensors  that provide an either digital or analog way of reading out sensor data are chosen. The challenges then are finding a suitable sensor, that is suited for the desired application by matching requirements for e.g. output voltage, data rate, precision, size, price or measurement principle. Therefore, this experiment focuses not on the design of a sensor but on choosing suitable sensors, working with measurement data from said sensors and observing the consequences of choosing a wrong sensor for an application.

In the previous experiment, the concept of capacitance measurement was introduced. The principle is based on measuring changes in capacitance or the influence of a changed capacitance on an electrical circuit. One field of application is acceleration measurement, which are covered in this experiment.

This experiment is dived into three parts, with the first part focusing on choosing a suitable sensor for different applications. In the second part, a digital sensor is provided and to be evaluated with an Arduino mirco-controller. The data shall give insight and understanding of the sensor's advantages and drawbacks. In the third part, a measurement of the often only theoretically discussed pendulum is to be conducted and the data to be analyzed.

Similar to already conducted experiments, questions concerning knowledge, understanding and your results have to be answered correctly, to pass this experiment. The questions are VIPS only and can be answered during and after the experiment.

• Knowledge:

  • Understanding sensor principle

  • Learning about limits of capacitive sensors due to their principle of operation

  • Selecting a suitable sensor according to predefined requirements

• Skills

  • Skimming through datasheets for desired information

  • Working with an Arduino micro-controller for data acquisition of digital data

  • Processing of measurement data in Matlab

      [1] Kern, A. Abbasimoshaei, C. Hatzfeld, Engineering Haptic devices, 3rd edition, Springer, 2022.

      [2] Craig Aszkler, CHAPTER 5 - Acceleration, Shock and Vibration Sensors, Editor(s): Jon S. Wilson, Sensor Technology Handbook, Newnes, 2005, Pages 137-159, ISBN 9780750677295,https://doi.org/10.1016/B978-075067729-5/50045-8.

      [3] Lectures: Electrical Measurement Technology, Capacitive Sensors

A fundamental part of mechanical dynamics is acceleration. It is used to measure vibrations, impulses and can be further used to get velocities and positions. Acceleration is sensed by accelerometers, where several different types of accelerometers are commercially available. These types can be distinguished by their principle of operation. There are for example, piezo-electric, piezo-resistive, and capacitive sensors.

The capacitive accelerometer provides an output voltage according to the change of an internal capacitance, where Fig. 1 shows the basic structure of one type of capacitive accelerometer. Inside, there are two fixed and one movable electrodes that make two capacitors in total. The capacitance of each capacitor depends on the distance between the movable and the outer electrode, where the movable electrode is influenced by its mass and moves according to forces affecting the mass.

From this point it is clear, that the mass is affected by a multitude of forces such as friction, material elasticities, friction and gravitation to just name a few. The most important ones are dynamic forces, such as accelerationLaTeX Math Inline.

 

Fig. 1 Structure of one type of capacitive accelerometer

 

By knowing the mass, the relationship between displacement and force and the relationship between displacement and capacitance, the acceleration can be calculated. An electronic circuit is used to convert the change of capacitance into a useful voltage signal as sensor output. The output signal can then be used directly (analog sensor) or be transformed into a digital signal (digital sensor). In analog-sensors, an analog-to-digital converter is required to digitalize the sensor values. This process typically includes filtering noise and amplifying the signal. On the other hand side, a digital sensor consists of a small micro-controller, that filters and digitalizes the sensor signals and communicates with the main processing unit (Computer, Main controller) transmitting the sensor output as already digital signals.

There are several specifications for capacitive accelerometers. The specifications are usually listed in the datasheet of the sensor. Some of them are:

The output acceleration is usually scaled to multiples of

and typical capacitive accelerometers range from 

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to

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.

Sensor sensitivity defines, how sensitive the sensor output (in LaTeX Unit)  is to changes in acceleration. The sensitivity depends on the excitation frequency and is typically higher for smaller sensor ranges. Usually, the sensitivity is given in

.

Depending on the application's requirements, a suitable accelerometer has to be chosen. Typical parameters to respect are:

• The range of acceleration to be measured

• The required sensitivity

• The number of required axes of measurement

• Maximum allowed error

• The operational ranges (pressure, temperature)

• Linearity-factor (how reliable can values be reproduced

• In the following are some aspects to be considered before starting a measurement

  • What is supposed to be measured → What values are to be expected (sine-wave-like shape, spikes etc.)?

  • What measuring device can be used for measuring → Does it need to measure analog values or digital protocols? Does it provide the necessary accuracy?

  • What are the possible (systemic) errors and external limitations that exist in the setup?

  Preparations:

  For the first part:

  • Check the following datasheets:

 

In this section, different applications are introduced and a suitable accelerometer are to be chosen for each application. The decision on the sensor for this shall be based on the conditions and circumstances of each application.

For each application, one sensor is considered the optimal solution. The task is therefore, to find the optimal sensor.