1. ECE5320 MechatronicsAssignment#01: Literature Survey on Sensors and Actuators Topic: Servo Accelerometer Prepared by: Anthony Meeks Dept. of Electrical and Computer Engineering Utah State University E: ; T: (435)797-; F: (435)797-3054 (ECE Dept.) W: http:// 3/11/2010

2. Outline • Reference list • To probe further • Major applications • Basic working principle illustrated • Detailed description • Analysis • A typical sample configuration in application • Major specifications • Limitations • Where to buy • Conclusion ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

3. Reference list • United States Patent: 5488865 • www.sensorland.com • http://www.jae.co.jp/e-top/eproduct/jpeg/JA-25H200A.pdf ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

4. To explore further • Visit the follow websites to learn more about servo accelerometers: • ttp://www.sensorland.com/HowPage019.html • U.S. patent #: 5488865 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

5. Major applications • Measuring the acceleration on an object to find the forces acting on it. • Use to be able to measure the frequency of a vibrating object. • Needed for controlling the acceleration and velocity on moving parts of all sorts of machines and robots. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

6. Major applications • Good for applications of +/- 4 g • Good for applications needed 1 micro g of resolution • Good for all applications vibrating and less than 100 Hz ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

7. Basic working principle illustrated • A pendulum mass is magnetically connected to a coil. When the mass moves, it is detected by a motion sensor, then a current is applied to correct the position. The current is directly related to the force acting on the accelerometer. http://www.jae.co.jp/e-top/eproduct/sample1.asp?SEQNO=5037 http://www.sensorland.com/HowPage019.html ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

8. Detailed Description Assembly This basic assembly of a servo accelerometer shows the proof mass, held in position by the flat leaf flexure. This proof mass assembly is encased by two magnet assemblies. Flat Leaf Flexure Proof Mass • Proof Mass-Mass that is displaced when an applied acceleration acts on the accelerometer • Flat Leaf Flexure-Flexible material that acts as a torsion spring for the proof mass. This allows the mass to be displaced U.S. Patent #:5488865 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

9. Detailed Description Displacement Detection A conductive material is placed on the proof mass. The magnetic assembly is encased with an excitation ring. The conductive material forms a capacitance element with the flat surface on the excitation ring. When there is no applied acceleration, the distance of the mass between the top excitation ring, and the bottom excitation ring is equal. When this is the case, capacitance C1 and C2 are equal. When a displacement happens, C1 does not equal C2. The difference in these values represent the displacement of the mass. Conductive Material • Conductive Material-Applied to the proof mass to form a capacitance element with the excitation ring • Excitation Ring-Acts as a magnetic return path. • Flat surface of excitation ring-forms a capacitance element with the proof mass Flat surface of excitation ring Excitation Ring C1 Conductive Material C2 Flat surface of excitation ring Proof Mass Cross section of proof mass U.S. Patent #:5488865 U.S. Patent #:5488865 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

10. Detailed Description Displacement Correction When a displacement is detected by the change in capacitance, the change is sent to the servo, which sends a current through the torquer coils attached to the mass. This couples with the permanent magnet and restores the displacement to zero. Torquer Coils Permanent Magnet • Torquer Coils-is magnetically coupled to the permanent magnet when a current is passing through it. • Permanent Magnet-Couples with the coils when there’s an applied current • Excitation Ring-Acts as a magnetic return path. Excitation Ring U.S. Patent #:5488865 ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

11. Analysis Free Body Diagram of an ideal servo accelerometer (Source: U.S. Patent: 5488865) Torsion spring representing the flat leaf flexure Position detector Proof mass KD-position detector constant C-The damping constant KF-The spring constant M-mass of the proof mass L-distance between the center of mass of the proof mass and the effective pivot point of the flexure Kt-the torquer constant J-the moment of inertia of the proof mass ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

12. Analysis Block diagram for the ideal servo accelerometer, where the input is an acceleration, and the output is the current to return the proof mass to the null position. (Source: U.S. patent: 5488865) The vertical acceleration input (s^2*z), gets amplified by the moment arm (ML) of the system. After the summation point is the transfer function on the system, which is a typical second order spring mass damper system with the gain KD coming from the position detector. A(s) is the controller that converts the position to current. The feedback loop is the torsion spring which linearly increases with displacement. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

13. Analysis The required current to return the proof mass to the null position is found by this equation (source, U.S. patent 5488865) which is derived from the block diagram on the previous slide. A-The gain of the feedback amplifier KD-position detector constant C-The damping constant KF-The spring constant M-mass of the proof mass L-distance between the center of mass of the proof mass and the effective pivot point of the flexure Kt-the torquer constant J-the moment of inertia of the proof mass ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

14. Analysis The link between current and acceleration The acceleration is represented by in the laplace transform notation. So from this equation, we can see that the current is directly proportional to the acceleration. So by measuring the current drawn from the accelerometer, we can find the acceleration that was required to draw that current. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

15. A typical sample configuration in application Accelerometers act as the feedback on vibration tables. This will ensure that the operating frequency is correct. These tables can ensure that expensive parts for aircraft and rockets wont fall apart in mid-flight. http://www.protos-eci.es/Vibration_Shaker_SlipTable.jpg ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

16. A typical sample configuration in application Can be used to measure the force that’s acting on an object You can attach an accelerometer to a crash dummy and find out exactly what the force is acting on the dummy. This will tell you if a real person would’ve survived the crash or not. http://static.howstuffworks.com/gif/human-crash-test-dummy-1.jpg ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

17. Major specifications Servo Accelerometers are capable of being very sensitive to small changes. Since the changes are small, there are many specs that need to be known to the designer so as to accurately read the data. The following criteria's should be known to the technician/engineer before using the accelerometer. • Range • Scale Factor • Threshold • Resolution • Linearity • Bias Error • Natural Frequency • Damping Ratio • Temperature Range • Interface dimensions • Input Voltage • Input Current ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

18. Limitations • Need to be calibrated often • Forces higher than +/- 4 g will give inaccurate results and/or mechanical failure. • The frequency response bandwidth is typically less than 100Hz. • Only capable of one degree of freedom ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

19. Where to buy? • Japan Aviation Electronics • Sundstrand Hydraulics (phone orders only) • Sherborne Sensors • Direct Industries Servo accelerometers are harder to come by than most accelerometers. Few companies carry them and searching servo accelerometer in a search engine will bring up many companies that don’t sell servo accelerometers. Here is a list that companies that will sell servo accelerometers. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

20. Conclusion • Accelerometers are incredibly important in todays world. Since servo accelerometers draw current when the mass is displaced, they are very sensitive and can detect the smallest change in motion and vibration. Which makes the servo accelerometer a very desired sensor in today’s industry. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators