Instrumentació I Bioenginyeria. Bloc Digital

E. T. TELECOMMUNICATIONS Instrumentation and Bioengineering Digital Block 31/05/2007 Prof. Josep Conesa and F. J. Sànchez - Minimum control: From May 30th to June 5th - Grades will be available on June 14th - Questions about the examination: Monday 11:00 - 13:00; Tuesday 15:00-19:00

VERY IMPORTANT: Draw a general schematic or plan, develop the exercise and justify the results always explaining what are you doing Minimum 5: Microcontrollers applications in assembler Task description: Designing a rotation speed meter (tachometer) a) The speed sensor We want to build a tachometer to measure the speed of rotation of the electrical motor shown in Fig. 1 which revolves at a maximum speed of 1000 min-1 (or rpm: revolutions per minute). A geartooth joined to the motor axis turns while a magnetic sensor generates a digital pulse waveform1. The internal design of the magnetic sensor can be seen in Fig. 2. An alternative sensor, also very common but based on photodiodes and LED or optocouplers, can be seen in Fig. 3.

sensor output

Fig. 1 Tachometer placement attached to the motor axis and detail of the magnetic Hall effect sensor

The metallic geartooth has 36 teeth and we to build a 3-digit 7-segment display to represent speeds up to 999 min-1 as sketched in Fig. 4. 1 References for documenting the problem: - http://archive.chipcenter.com/knowledge_centers/embedded/todays_feature/showArticle.jhtml? articleID=10817759 - http://www.freescale.com/files/microcontrollers/doc/app_note/AN1857.pdf - http://www.cherrycorp.com/english/sensors/pdf/GS1012_Series.pdf - http://dkc3.digikey.com/PDF/T072/P2115.pdf

Fig. 2 Magnetic Hall effect sensor (SD1012 from www.cherrycorp.com)

Fig. 3 Speed sensor based on optical sensor build using optocouplers

VCC h_L0 t_L0 u_L0 h_L1 t_L1 u_L1 h_L2 t_L2 u_L2 TACHOMETER h_L3 t_L3 u_L3 SENSOR h_L4 t_L4 u_L4 h_L5 t_L5 u_L5 (600 Hz max) IN h_L6 t_L6 u_L6

h_L[6..0] h_L[6..0] t_L[6..0] t_L[6..0] u_L[6..0] u_L[6..0]

CONTROL SYSTEM speed (RPM)

Fig. 4 3-digit speed meter inputs and outputs b) Block diagram for the instrument . Which blocks do you think have to be considered in the design, in order to measure a variable-frequency pulse- train waveform? To get some ideas, you can analyse the same design built in conventional logic shown in: http://epsc.upc.edu/projectes/ed/grups_classe/06-07-q2/1BT5/06-07- Q2-1BT5.htm

Fig. 5 Block diagram for the speed meter c) Design equations . Considering the number of tooth, the speed range and the measurement resolution, find the equations that relate the rotation speed with the displayed measurement. d) Microcontroller hardware . Propose the hardware implementation using a PIC microcontroller and multiplexed 7-segment displays. Explain which peripherals will be used to perform the signal processing to obtain the RPM measurement.

Fig. 6 PIC 16F84 based design e) Software. Draw a flux diagram to show the way the microcontroller will be programmed to perform the measurements. Determine whether it is possible to design the application using or without using interruptions to the main program.

Fig. 7 Flux diagram for the problem f) Simulating and verifying the design. Use the MPLAB integrated development environment and the virtual laboratory Proteus-Virtual System Modelling to simulate and verify your designs.

------g) Improvements to the basic instrument.

a. Add a LED indicator to show the overflow condition which at the same time blank the digits when a rotation speed higher than 999 min -1 is measured.

b. Explain which changes have to be implemented in the design to increase the measurement resolution up to the hundredths of revolution, for example: 123.34 min-1.

c. Replace the 7-segment multiplexed display by a liquid crystal display (LCD) and write the necessary code

d. How can be improved the instrument’s number of reading per second

e. Add additional code to send the measurements through the RS232 interface to a remote computer.