People’s Democratic Republic of Algeria Ministry of Higher Education and Scientific Research University M’Hamed BOUGARA – Boumerdes Institute of Electrical and Electronic Engineering Department of Power and Control Mini Project Title: A simple capacitive proximity sensor experiment for exploring the effects of body capacitance and earth ground Presented by: BENRABAH Younes Supervisor: Mr. Ouadi Abstract Abstract Capacitive proximity sensors are well-suited for educational projects due to their low cost and simple design. Traditional undergraduate textbooks and lab exercises rarely highlight the fact that the performance of capacitive proximity sensors can be quite sensitive to ground loading. This paper presents a simple classroom demonstration for exploring this topic in detail. The capacitive proximity sensor for this demonstration is a hand-held LCR meter connected to a homemade capacitor composed of two strips of aluminum foil. Students explore the operation of this sensor for two different system ground configurations. In the first case the LCR meter is battery powered (floating ground referenced) and in the second case the LCR meter is powered by AC mains supply (earth ground referenced). When a student positions their hand near the foil strips, the batterypowered sensor measures an increase in capacitance. Conversely, the AC-mains-powered sensor measures a decrease in capacitance. The instructor guides students to discover for themselves the reason for this seemingly puzzling difference by modeling parasitic capacitance and ground loading using simple circuit models. i Acknowledgement Acknowledgement First, I am thankful to ALLAH almighty, for showing heavenly blessing upon me, as without that nothing would have been possible. I would like to express our sincere gratitude and appreciation to my supervisor, Mr. OUADI for his considerable assistance, guidance and encouragement. ii General Introduction Table of Content Abstract i Acknowledgements ii Table of content iv General Introduction 1 Chapter 1: Generalities about proximity capacitive sensors 1.1. Introduction 3 1.2. Proximity Capacitive Sensor Overview 3 1.3. Capacitance Sensors in Touch Sensing Applications 4 1.4. Conclusion 5 Chapter 2: Implementation of proximity sensor 1 2.1. Introduction 6 2.1. Principle of auto-balancing bridge 7 2.1. The hidden schematic 7 2.1. Conclusion 8 General Introduction General Introduction The functioning of electronics can be sensitive to the presence of earth ground and nearby objects and bodies. For example, a student handling a circuit can unknowingly affect the circuit’s operation via stray capacitance between the student’s body and the circuit itself. Sometimes this body capacitance can be a nuisance, such as when encountering unwanted electrostatic discharge (ESD). On the other hand, capacitive proximity sensors in the form of touchscreens, touchpads, pushbuttons, and Theremins intentionally utilize body capacitance. Capacitive proximity sensors (and capacitance measurement devices in general) are particularly prone to ground loading effects. For instance, a capacitive sensor device connected to a battery with no other connections to the external world may perform very differently compared to the same sensor powered by an AC wall outlet and referenced to mains supply earth. An effective designer of such devices requires an understanding of appropriate system ground design. A key design skill knows how to visualize and model parasitic capacitance between the various bodies and objects in the system, such as the local ground plane, earth ground, capacitor electrodes, and the user. Capacitive proximity sensors are well suited for educational projects and rapid prototyping. Simple capacitive sensors can be made with inkjet printing technology or by hand using inexpensive and recycled materials. Traditionally, however, textbooks and the educational literature pay relatively little attention to the fact that capacitive proximity sensors can be highly sensitive to ground loading. Earth ground is largely discussed in the context of power distribution, safety, and EMI shielding, with little attention to capacitive proximity sensors. This paper presents an interactive classroom demonstration for directly exploring body capacitance and the effects of ground loading on a capacitive proximity sensor. The activity was developed to give students a simple and practical example of how ground loading can affect a real capacitance measurement. The activity follows established strategies for effective demonstrations, such as stimulating discussion, challenging an assumption, and asking students to make predictions on the most probable outcome. For the demonstration, the class is presented with a hand-held LCR meter connected to a pair of homemade capacitive sensor electrodes composed of two strips of aluminum foil. Students compare the sensor’s performance for two system ground configurations. In one case the LCR meter is battery powered, and in the second case, the LCR meter is plugged into a three prong wall outlet. This experiment 2 General Introduction combines ideas from two recent publications in the literature. First, the test setup has features in common with a method presented in Aliau Bonet and Pallas-Areny8 that uses commercial impedance analyzers to estimate stray body capacitance to ground. Second, the measurement procedure is similar to an EMC experiment presented in DuBroff and Drewniak9 that uses a benchtop LCR meter to measure and compare the capacitance between two metal pipes in the presence of a grounded and floating aluminum sheet. 3 Chapter 1 Generalities about proximity capacitive sensors Chapter One: Generalities about proximity capacitive sensors 1.1 Introduction In 1831, Michael Faraday discovered electro-magnetic induction. Essentially, he found that moving a conductor through a magnetic field creates voltage that is directly proportional to the speed of the movement the faster the conductor moves, the higher the voltage. Today, inductive proximity sensors use Faraday’s Law of Electromagnetic Induction to detect the nearness of conductive materials without actually meeting them. The primary deficiency of these sensors, however, is that they only detect metal conductors and different metal types can affect the detection range. Proximity capacitive sensors, on the other hand, adhere to the same principle but can detect anything that is either conductive or has different dielectric properties than the sensor’s electrodes’ surroundings. Proximity capacitive sensors have become increasingly popular, as more user/machine interfaces are designed using touch panels to reliably respond to commands. Freescale has advanced MPR083 and MPR084 proximity capacitive touch sensor controllers can be used to replace switches and buttons on a wide variety of control panel applications. The MPR083 device supports an 8-position rotary interface while the MPR084 device controls up to eight touch pads 1.2 Proximity Capacitive Sensor Overview Proximity capacitive sensing is a technology that enables touch detection by measuring capacitance, exhibiting a change in capacitance in response to a change in surrounding materials. Certain sensors gauge the change by generating an electric field (efield) and measuring the attenuations suffered by this field. Unlike inductive sensors, a proximity capacitive sensor can detect anything that is either conductive or has different dielectric properties than the sensor’s electrodes’ surroundings. They are excellent touchpad enablers because we, humans, being mostly water, have a high dielectric constant, and we contain ionic matter, which makes us good electric conductors. Freescale uses multiple technologies in its proximity capacitive sensors. The portfolio of MC33794, MC33941 and MC34940 products contains oscillator circuitry in the sensor integrated circuit (IC) to generate a high purity, low frequency 5V sine wave, tunable by an external 39k ohms load resistor. This AC signal is fed to a multiplexer, which directs the signal to a selected electrode or reference pin or to an internal measurement node. The IC automatically connects the unselected nodes to the circuit ground, and these act as the return path needed to create the e-field current. When an object is brought close to a metal 3 Chapter One: Generalities about proximity capacitive sensors electrode, for instance a finger from our highly dielectric and conductive human subject, an electric path is formed, producing a change in the e-field current. Normally, the sensor measures the AC impedance of the generated e-field and translates that measurement into a DC output voltage. An external microcontroller with an analog-to-digital controller (ADC) then processes this information to perform any number of functions, such as those that are associated with a touchpad control panel. However, our more advanced MPR083 and MPR084 proximity capacitive touch sensor controllers generate digital output through an inter-integrated circuit (I2 C) with custom addressing, thus eliminating the need for an external ADC. Figure 1. Proximity capacitive sensor E-field concept. 1.3 Capacitance Sensors in Touch Sensing Applications Proximity capacitive sensing technology is finding application in a wide variety of industrial and consumer products. The MPR083 and MPR084 devices offer designers a cost-effective alternative to mechanical push buttons and switches for control panel applications. Both utilize touchpad technology, though in differing form factors. Touchpad this is simply a contactless “area” which detects the presence or absence of a finger. The raw detection output is a single bit giving a touch condition. There are three important considerations when developing a touch panel: 4 Chapter One: Generalities about proximity capacitive sensors 1. Touch pad electrode design and layout 2. The different dielectric materials for the surface of the panel 3. The effect on e-field measurements of various environmental conditions The relationships among these three considerations are described in the following equation. Figure 2. proximity capacitive touch sensor. 1.4 Conclusion Capacitive sensors are both commercially important and well suited for education al projects on the undergraduate level. A valuable design skill involves modeling and understanding how different system ground configurations impact sensor performance. To illuminate this topic, this paper presented an interactive classroom demonstration for directly exploring body capacitance and the effects of ground loading. For the demonstration, students are presented with an LCR meter that is connected to a homemade capacitor composed of two strips of aluminum foil. Students position their hands near the foil strips and measure the change of capacitance for two separate cases. In the first case the LCR meter is battery powered (floating ground referenced), and in the second case the same LCR meter is plugged into a three-prong wall outlet (earth grounded referenced). The class is asked to predict what will happen in each case. Curiously, the two cases yield different results, which may seem puzzling at first and peak student interest. Specifically, in the battery powered case the measured capacitance increases in response to a subject’s hand, and in the AC mains powered case the capacitance decreases in response to a 5 Chapter One: Generalities about proximity capacitive sensors subject’s hand. The instructor guides students to dis cover for themselves the reason for these results by modeling parasitic capacitance using simple circuit models. In doing so, students gain perspective on how parasitic capacitance and grounding affects a real-world capacitive measurement. The demonstration was conducted in a traditional lecture-based course in electromagnetics. The majority of students in this course reported that short inclass demonstrations helped improve learning. The proposed experiment is expandable, and can be used as a basis to explore related topics, such as isolation transformers, shielding and EMI, and proper grounding practices for safety. 6 Chapter 2 Implementation of proximity sensor 2.1 Introduction The LCR meter is powered by an AC switching adapter (model DSA-42D-12), which is plugged into a three-prong AC wall outlet, and the experiment is repeated. Students are again asked to predict how the measured capacitance will respond to someone’s hand in this new case. Curiously, student volunteers will find that moving their hands closer to the capacitor now has less influence on the measured capacitance and will likely decrease the measured capacitance, depending on one’s position, orientation, and how physically close one is to earth grounded equipment and wiring. When the author closely cupped his hands over the foil strips in this case, he measured a 4.9% decrease in capacitance. Many students may find it puzzling that a hand increases the capacitance for the battery-powered meter, but decreases the capacitance for the same meter powered by AC mains. It should provoke students to question their assumptions and understanding of the problem, and lead to a class discussion as to why the LCR meter’s power source has such an effect on the measurements. 2.2 Principle of auto-balancing bridge The LCR meter’s response to the presence of an external body clearly depends on if the meter is battery powered or plugged into the wall. To understand the reason for this, it is helpful to review the basics of how LCR meters operate. The auto balancing bridge method is commonly used to measure impedance in low frequency, hand-held LCR meters like the Agilent U1733C LCR meter. depicts a simple impedance measurement using an ideal amplifier with negative feedback. The device under test or DUT (in this case the DUT is the homemade capacitor) is excited by a time-harmonic voltage source with known frequency x. The current Ix through the DUT is approximately equal to the current Ir through the feedback resistor Rr. Therefore, if the measured AC voltage across the DUT is Vx and the measured AC voltage across Rr is Vr, then the impedance Zx of the DUT can be determined Zx ¼ Rr Vx Vr (1) where Vx and Vr are taken to be phasor quantities. Assuming the DUT is a capacitor, the capacitance C can be found from taking the imaginary part of the impedance C ¼ 1 xIm½ Zx (2) Given the above description of the auto-balancing bridge method for determining capacitance, the class is again asked to explain why the LCR meter’s response to the presence of an external body depends on whether the meter is battery powered or plugged into the wall. Students are encouraged to contemplate this question for themselves and offer a simple and clear explanation using schematic diagrams 2.3 The hidden schematic The experimental setup shown in Figure 1 is can be modeled by the circuit diagram shown in Figure 2, with the DUT replaced by a lumped element capacitor that has the same capacitance as the physical aluminum foil capacitor. Such schematics are presented in Figure 3(a) and (b) for the battery-powered and AC-mains-powered cases, respectively. In these schematics, C0 F denotes the mutual capacitance between
