Contact-less Tachometer - Engineering Seminar

ABSTRACT

A tachometer is a device that measures the rotation speed of a shaft or disk, as in a motor of other machine. In automotive use, it is used as a gauge showing the speed (RPM) of the engine shaft that is driving the transmission, usually in thousands of rotations per minute. What makes this device special is that it can very accurately measure the rotational speed of a shaft without even touching it. This is very interesting when making direct contact with the rotating shaft is not an option or will reduce the velocity of the shaft, giving faulty readings. This device is built on a microcontroller, an alpha-numeric LCD module, a battery and a proximity sensor or an infrared to detect the rotation of the shaft whose speed is being measured. If we were using proximity sensor, the counted pulses will detect any reflective element passing in front of it, and thus, will give an output pulse for each and every rotation of the shaft. But if we were using infrared, we will put the infrared on both shaft and the tachometer. Those pulses which we get from every rotation of the shaft will be fed to the microcontroller and counted.

INTRODUCTION

A contact-less tachometer will let you know how quickly something spins and is frequently used for buses, trains, tractors, trucks, cars and planes. This non contact tachometer version uses a sensor that will sense revolutions through pulses. A contact-less tachometer consists of a shaft encoder and electronic circuits. The output of the shaft encoder provides electric pulses. The frequency of these pulses is proportional to the rotational speed. A speed signal is obtained by processing the pulses from the encoder using an additional electronic circuit. When the wheel or shaft rotates, it has a mirror or a tab that obstructs the path of the light every time it revolves. Then, there is simply a chip to count the number of obstructions per minute. This one is extremely accurate and can handle some of the highest speeds.

This paper proposes a new solution for the processing of the pulses from the encoder to derive the speed signal. The solution takes advantages of new single chip low cost programmable microcontrollers. It is shown how to use hardware and software combined with a suitable method for the speed evaluation to design a high performance tachometer. These can work like an optical sensory as you point it like a laser at what you want to measure.

A tachometer typically use a rotating target attached to a wheel, gearbox or motor. This target may contain magnets, or it may be a toothed wheel. The teeth on the wheel vary the flux density of a magnet inside the sensor head. The probe is mounted with its head a precise distance from the target wheel and detects the teeth or magnets passing its face. One problem with this system is that the necessary air gap between the target wheel and the sensor allows ferrous dust from the vehicle's under frame to build up on the probe or target, inhibiting its function.

In the other words, the normal tachometer requires physical contact between the instrument and the device being measured. In applications where this is not feasible for technical or safety reasons, it may be possible to use a contactless tachometer to take measurements from a distance. This contactless tachometer is not only useful in terms of safety, but it is also very efficient. The efficiency depends on both the proximity sensor and the reflective element passing in front of it. Thus, it is clear that the method of contactless tachometer is a technique that worthy of being developed.

OBJECTIVE 

i. To prove that this contactless tachometer is more efficient than normal tachometer.

ii. To develop and create a new reliable contactless tachometer.

SCOPE OF PROJECT

In order to achieve the objective of the project, there are several scope had been outlined. The scope of this project includes using MicroCode Studio to program microcontroller PIC 16F877A, design the circuit by using the Proteus software and build hardware for the system. The main goal of this project is to determine the revolutions per minute of the motor speed by using this non contact tachometer. The scope of this project is:

i. To show that contactless tachometer is more efficiency when using the proximity sensor because of there is no contact between the motor and the tachometer. Besides, there will be no harm to the people who were using this contactless tachometer because of the safety that was proven.

ii. The 16F877A microcontroller, proximity sensor and alpha-numeric LCD module that been used on this project is to detect the rotation of the shaft whose speed which is being measured.

TACHOMETER

DIGITAL TACHOMETER

A tachometer is an essential part in the design of the feedback loop in the speed control of AC and DC drives. DC tachometers are spread used due to their good dynamic performances. However, the reasons listed below encourage the use of digital tachometers:

i. a better accuracy,

ii. in the case of a digital controller, no A/D conversion is needed.

iii. no maintenance is needed, as digital tachometers are brushless,

iv. noise immunity, which avoids filtering

The electronic circuit of a digital tachometer is composed of two parts: the encoder interface and the speed measure block. The encoder interface can be programmed in conventional logic. The speed is measured from the pulse train coming from the encoder, which has m marks equally spaced at the circumference. Two methods are normally employed to measure the speed: counting the pulses in a fixed period of time, and measuring the time elapsed between successive pulses.

The analysis of these two methods leads to the following conclusions. The pulse counting method is suitable for medium and high speeds, but the relative error dramatically increases with lower speeds. On the other hand, measuring the elapsed time between two (or more) pulses exhibits a high accuracy in the low speed range, at the cost of a poor response in higher speeds, or a poor dynamic response in lower speeds. In reference , the author proposed the so called constant elapsed time method. In essential, it measures the elapsed time between k successive pulses, and dynamically adjusts the value of k to obtain a near constant response time. This method was implemented on a microprocessor. A new method is proposed that provides high accuracy in a wide speed range with good dynamic performances. The circuit is implemented in hardware using only one low-cost FPGA and one EPROM device. Human beings are faced with oil and coal depletion of fossil fuels such as a serious threat that these fossil fuels is a one-time non-renewable resources, limited reserves and a large amount of combustion of carbon dioxide, causing the Earth’s warming, deterioration of the ecological environment. With the development of society, energy saving and environmental protection has become a topical issue.

CONTACTLESS ANALOGUE TACHOMETER

The tachometer employs a standard cathode-ray tube as a voltage to light-spot position actuator, a black white contrast edge on a diameter of the shaft end, and a photomultiplier light detector. The light spot is projected onto the shaft end and switched rapidly along the contrast edge about the shaft centre. Any detector output (at the switching frequency) is amplified, and by using a sampling function circle generator, the system is able to position the light-spot so as to minimize the detector output. Thus, this closed-loop system operates to maintain the light-spot on the contrast edge and the input to the circle generator is the tachometer output.

The instrument must be focused on the shaft end; no special calibration is required and the light spot readily locks onto a rotating contrast edge. An experimental instrument has been used on shafts down to 3 mm in diameter, which has a range of at least 20000rev/min in either direction, a bandwidth of about 1 kHz with a resolution of about 400rev/min limited by noise, owing to the effect of background light on the photomultiplier. Greater resolution may be obtained at the expense of bandwidth .

The light spot is controlled to sit on the contrast edge along the radius away from the optical target centre of rotation. The c.r.t.(cathode-ray-tube) is driven by a circle generator so that the two in series act as a voltage to light-spot angular displacement actuator (an 'optoelectronic shaft'). Movement of the real shaft in either direction results in a change in level of light reflected from the optical target onto the photo detector. The resultant signal is amplified to drive the circle generator and so move the light spot back onto the contrast edge. The voltage driving the circle generator gives a measure of the angular displacement (and direction) of the c.r.t. light spot and so that of the shaft. The system will operate with the light spot anywhere on the contrast edge except the optical target centre of rotation, so that special calibration is unnecessary. The c.r.t. light spot is a very low inertia moveable light source and so the system has the possibility of high bandwidth.

SENSOR

A sensor; is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. A sensor is a device which receives and responds to a signal. A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes. Sensors that measure very small changes must have very high sensitivities. Sensors also have an impact on what they measure. Sensors need to be designed to have a small effect on what is measured; making the sensor smaller often improves this and may introduce other advantages. A good sensor obeys the following rules:

i. Is sensitive to the measured property

ii. Is insensitive to any other property likely to be encountered in its application

iii. Does not influence the measured property

Ideal sensors are designed to be  linear or linear to some simple mathematical faction of the measurement, typically  logarithmic. The output signal of such a sensor is  linearly proportional to the value or simple function of the measured property. The  sensitivity is then defined as the ratio between output signal and measured property

INFRARED SENSOR

An  infrared (IR) sensor is an electronic device that emits and/or detects  infrared radiation in order to sense some aspect of its surroundings. Infrared sensors can measure the heat of an object, as well as detect motion. Infrared sensor is  electromagnetic radiation with a  wavelength between 0.7 and 300  micrometres, which equates to a  frequency range between approximately 1 and 430  THz. IR wavelengths are longer than that of  visible light, but shorter than that of  terahertz  radiation microwaves. Bright sunlight provides an  irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is  visible light, and 32 watts is  ultraviolet radiation. But infrared sensors are usually designed only to collect radiation within a specific bandwidth. As a result, the infrared band is often subdivided into smaller sections.

MICROCONTROLLER

Microcontrollers must contain at least two primary components – random access memory (RAM), and an instruction set. RAM is a type of internal logic unit that stores information temporarily. RAM contents disappear when the power is turned off. While RAM is used to hold any kind of data, some RAM is specialized, referred to as registers. The instruction set is a list of all commands and their corresponding functions. During operation, the microcontroller will step through a program (the firmware). Each valid instruction set and the matching internal hardware that differentiate one microcontroller from another [4].

Most microcontrollers also contain read-only memory (ROM), programmable read-only memory (PROM), or erasable programmable read-only memory (EPROM). Al1 of these memories are permanent: they retain what is programmed into them even during loss of power. They are used to store the firmware that tells the microcontroller how to operate. They are also used to store permanent lookup tables. Often these memories do not reside in the microcontroller; instead, they are contained in external ICs, and the instructions are fetched as the microcontroller runs. This enables quick and low-cost updates to the firmware by replacing the ROM.

Where would a microcontroller be without some way of communicating with the outside world? This job is left to input/output (I/O) port pins. The number of I/O pins per controllers varies greatly, plus each I/O pin can be programmed as an input or output (or even switch during the running of a program). The load (current draw) that each pin can drive is usually low. If the output is expected to be a heavy load, then it is essential to use a driver chip or transistor buffer.

Most microcontrollers contain circuitry to generate the system clock. This square wave is the heartbeat of the microcontroller and all operations are synchronized to it. Obviously, it controls the speed at which the microcontroller functions. All that needed to complete the clock circuit would be the crystal or RCcomponents. We can, therefore precisely select the operating speed critical to many applications.

To summarize, a microcontroller contains (in one chip) two or more of the following elements in order of importance:

i. Instruction set

ii. RAM

iii. ROM, PROM or EPROM

iv. I/O ports

v. Clock generator

vi. Reset function

vii. Watchdog timer

viii. Serial port

ix. Interrupts

x. Timers

xi. Analog-to-digital converters

xii. Digital-to-analog converters

MICROCODE STUDIO 

MicroCode  Studio  is  a  visual  Integrated  Development  Environment  (IDE)

With In Circuit Debugging (ICD) capability designed specifically for  micro Engineering Labs PICBASIC™ and PICBASIC PRO™ compiler. The main editor provides full syntax highlighting of the code with context sensitive keyword help and syntax hints. The code explorer allows us to automatically jump to include files, defines, constants, variables, aliases and modifiers, symbols and labels that are contained within our source code. We just full cut, copy, paste and undo is provided, together with search and replace features. In the MicroCode Studio, we can;

i. Full syntax highlighting of the source code

ii. Quickly jump to include files, symbols, defines, variables and labels using the code explorer window

iii. Identify and correct compilation and assembler errors

iv. View serial output from our microcontroller

v. Keyword based context sensitive help

vi. Support for MPASM

PROTEUS 7 PROFESSIONAL

Proteus PCB design combines the  ISIS schematic capture and  ARES PCB  layout programs to provide a powerful, integrated and easy to use suite of tools for professional PCB Design.

All Proteus PCB design products include an  integrated shape based auto  router and a  basic SPICE simulation capability as standard. More advanced routing modes are included in Proteus PCB Design Level 2 and higher whilst simulation capabilities can be enhanced by purchasing the  Advanced Simulation option and/or  micro-controller simulation capabilities [7]. In the Proteus 7 Professional, we can;

i. Professional Schematic Capture module

ii. Professional PCB Layout module

iii. Hardware Accelerated Display Technology

iv. Basic Simulation

v. Max. Number of Pins In Netlist

vi. Shape-based Power Planes

vii. Global Shape Based Autorouting

viii. External Autorouter Interface

ix. Custom Scripted Autorouting

x. Command Driven Interactive Autorouting

xi. 3D Board Visualisation

xii. ODB++ Manufacturing Output

xiii. Gate-Swap Optimizer

xiv. Board Autoplacement

METHODOLOGY

This chapter will cover the process involved in the development of the contactless tachometer study project. The processes involved are under constant changes due to unexpected changes or complications. The processes of the development of this project will be divided into several parts: block diagram of the process flow, flow chart of the project progress and the flow chart of contactless tachometer.It  is  a  closed-loop  with  real  time  control system.

The actual speed of the motor will be measured by using the infrared sensor. In microcontroller, it will calculate the pulses and deduce the frequency of those pulses. Then, the microprocessor will process them and display the result on the LCD display. This process is happen again and again because there is a feedback after the microcontroller and it go back to the sensor. The process will only stop when we were not pushing the ON button again.