MECHATRONICS SYSTEM DESIGN (MTE-401) 2 hr Lecture on 15 th Sep 2014

Transcription

1 MECHATRONICS SYSTEM DESIGN (MTE-401) 2 hr Lecture on 15 th Sep 2014

2 COURSE DETAILS 3 Credit hour theory = 3 hours teaching per week Course Book Mechatronics Electronic control systems in Mechanical and Electrical engineering (3 rd Edition) by W. Bolton Available in Mechatronics Library Other useful books (For in depth study) Mechatronics System Design (2 nd Edition, SI) by Devdas Shetty and Richard A. Kolk The Mechatronics Handbook by Robert H. Bishop I will soft copies to CRs. Course website All course-works will be submitted via website All lecture notes will be available on course website

3 TODAYS LECTURE Introduction to Mechatronics System Design Real world examples Mechatronics Design Approach Sensors and transducers Performance Terminology

4 INTRODUCTION TO MECHATRONICS SYSTEM DESIGN

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6 SYSTEMS

7 OPEN LOOP VS CLOSED LOOP SYSTEM

8 MECHATRONICS KEY ELEMENTS

9 WASHING MACHINE EXAMPLE

10 POINT AND SHOOT CAMERA EXAMPLE

11 MECHATRONICS DESIGN APPROACH

12 SENSORS AND TRANSDUCERS A sensor is a device which produce a signal relating to the quantity being measured. A transducer is a device which experience a related change when subjected to some physical change. The term sensor and transducers used interchangeably. Some measurement systems may use transducers

13 PERFORMANCE TERMINOLOGY Range and Span The range of a transducer defines the limits between which the input can vary Span is difference between the maximum and minimum value A load cell can measure the force between 10kN- 100kN. Its range is 10kN to 100kN while its span is 90kN Error Error is the difference between the result of a measurement and the true value of the quantity being measured. Error = measured value - true value

14 PERFORMANCE TERMINOLOGY Accuracy Accuracy is the extent to which the value indicated by a measurement system might be wrong It is the summation of all possible errors that are likely to occur For example, a temperature measurement system be specified as having an accuracy of ±2 C Accuracy is often expressed as percentage of full scale reading (%FSR) For example a sensor have an accuracy of ±5% of full scale reading, with the range of C, then the reading given can be expected to be within + or -10 C of the true reading.

15 PERFORMANCE TERMINOLOGY Sensitivity Sensitivity is the relationship which relates a unit change in output to a unit change in input. For example a resistance thermometer may have a sensitivity of 0.5Ω/ C The term is often used to measure the sensitivity to other inputs other than the one being measured. For example a pressure sensor might have sensitivity to temperature.

16 PERFORMANCE TERMINOLOGY Hysteresis error Different outputs from a sensor for the same input depending on whether the value has been reached by continuous increase or continuous decrease. Defined as maximum difference in output for increasing and decreasing value

17 PERFORMANCE TERMINOLOGY Non-linearity error A linear relationship assumed between the input and output of a transducer for its working range. This assumption lead to non-linearity error Different methods to find non-linearity error include End range value method Best straight line for all values Best straight line through zero point

18 PERFORMANCE TERMINOLOGY Repeatability and Reproducibility Defines as the ability of transducer to give the same output for repeated application of the same input value The error arising from the non-repeatability is often expressed as a percentage of full scale reading Stability It is the ability of a transducer to give same output when used to measure a constant input over a period of time. The term drift is often used to describe the change in output over the period of time.

19 PERFORMANCE TERMINOLOGY Dead band/time It s a range of input values for which there is no output. The dead time is a length of time from the application of an input until the output begin to respond and changes. Resolution Resolution is the smallest change in the input value that will produce an observable change in the output. For example a wire-wound potentiometer might have a resolution of 0.5 degree or as a percentage of full scale reading. For digital sensors a smallest change is 1 bit. Thus for a sensor giving a data word of N bits, (a total of 2 N ) the resolution is 1/2 N

20 PERFORMANCE TERMINOLOGY Output impedance The output of a sensor needs to be connected with the other systems It is essential to know the output impedance of the sensor so that impedance can be matched with the interfacing circuit. Non-matching of impedance can lead to a significant change in the behaviour of overall system.