ECE5320 Mechatronics Capacitve Sensors

1. ECE5320 MechatronicsCapacitve Sensors Prepared by: Alberto Garcia Dept. of Electrical and Computer Engineering Utah State University E: alber.g@aggiemail.usu.edu ; T: (435)363-0564; F: (435)797-3054 (ECE Dept.) W: http://mechatronics.ece.usu.edu 3/6/2009

2. Slide-2 Outline • Reference list • To probe further • Major applications • Basic working principle illustrated • A typical sample configuration in application (application notes) • Major specifications • Limitations 03/06/09 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

3. Slide-3 Reference list • http://www.lionprecision.com/tech-library/technotes/tech-pdfs/cap-0020-cap-theory.pdf • http://www.capsense.com/capsense-wp.pdf • http://www.analog.com/library/analogDialogue/archives/40-10/cap_sensors.html 03/06/09 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

4. Slide-4 To explore further • http://www.planetanalog.com/showArticle.jhtml?articleID=189602704 • http://www.capacitance-sensors.com/capacitive_sensor_tutorial.htm • http://www.controleng.com/article/CA6637435.html?rssid=130 • http://www.controlengeurope.com/article.aspx?ArticleID=19714 03/06/09 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

5. Slide-5 Major applications • Dynamic Motion : Measure the dynamic of constantly moving targets. • Position Measurement/Sensing : Measure changes in the position of the target. • Liquid Level 03/06/09 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

6. Slide-6 Major applications • Thickness Measurement • Nonconductive Thickness • Assembly testing • Pressure measurment • Machine interface • Limit Switch 03/06/09 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

7. Slide-7 Basic Working Principle • Capacitive sensors work using the capacitance property that exists between any two conductive surfaces. • One of the surfaces is fixed and is part of the sensor, the other surface is the sensing surface that will move. • The capacitance is inversely proportional to the distance between the surfaces. 03/06/09 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

8. Slide-8 Basic Working Principle • As someone or something approaches the sensor, the capacitance sense by the sensor changes. The sensor uses this change in capacitance to determine its output, depending on the application and type of output. 03/06/09 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

9. Slide-9 Basic Working Principle • When a sensing voltage is applied to the Sensing Area of the sensor an electric field eliminates from the surface of the Sensing Area. This is the electric field used to determine the capacitance. • The target must be inside the electric field in order to be sensed. • Is there any problem with this sensing Area? 03/06/09 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

10. Slide-10 Basic Working Principle • To solve this problem, a Guard is used. • The Guard is another conductor surrounding the back and sides of the Sensing Area. • The Guard is kept with the same voltage as the Sensing Area itself. • Since there is no difference in the Guard and Sensing Area voltages, there is no electric field in the areas common to both. This give a directional field, pointing to the front of the sensor. 03/06/09 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

11. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Sample Configuration • Human Interface to Electronic Equipment • In this configuration, there is a RX and TX plate and the electric field is being generated by TX . When a person press the button, the distance between the plates changes producing a different capacitance value that can be sensed using the capacitance to digital converter .

12. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Major Specifications • Sensitivity • This parameter indicates the ratio of change of the output voltage as result of a change in the gap between the target and the sensor. • The graph show this relation. The slope of the graph give us the sensitivity of the sensor. • A common sensitivity is 1V/0.1mm.

13. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Major Specifications • Offset Error • This effect occurs when the output voltage of the sensor is not zero for a zero input. • Capacitive sensors are usually calibrated to have zero offset error, but this can change after the system has been calibrated. • Changes in temperature is the primary factor in offset error.

14. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Major Specifications • Linearity Error • The variation of sensitivity between any two points of data is called linearity error. • To calculate the linearity error, the straight line that would best fit the point is compared with the information obtained in the calibration process

15. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Major Specifications • Resolution • Resolution is defined as the smallest measurement the sensor can make. • Electrical noise is a primary factor when determining the resolution of the sensor. Electrical noise can appear in the output voltage causing small errors to the measurement.

16. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Major Specifications • Resolution • This noise can be found in many electronic components. It can be reduced but never eliminated • The specified bandwidth is critical when examining resolution specifications.

17. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators Limitations • The high impedance capacitive sensor needs special handling compared to other sensor types • The type of material that can be sensed • The range of detection.