week 6

23  February 2012

This is already week 6, and I haven't chosen motor and suitable pump for my project yet. I really have to speed up, since time is running out..! i did do some research regarding the motor... and I found that stepper is the best choice for my project. I need to use motor rotating with constant step angle; which is connected to the valve. The valve will rotate and alternately open one chamber at once to deliver air in the mattress. It will then produce alternating flow of air in the mattress.

WHAT IS STEPPER MOTOR??

             Stepper motor (or step motor) is a brushless DC electric motor that can divide a full rotation into a large number of steps. Stepper motors consists of permanent magnet rotating shaft; called rotor and electromagnets on the stationary portion that surrounds the motor, called staror. Unlike the permanent magnet DC motor, stepper motors move in discrete steps as commanded by the stepper motor controller. Because of their discrete step operation, stepper motors can easily be rotated a finite fraction of a rotation. Another key feature of stepper motors is their ability to hold their load steady once the require position is achieved. 

                A stepper motor often has an internal rotor with a large number of permanent magnet “teeth”. A large number of electromagnet "teeth" are mounted on an external stator. Electromagnets are polarized and depolarized sequentially, causing the rotor to spin one "step". Full step motors spin 360o/(# of teeth) in each step while half step motors spin 180o/(# of teeth) in each step. Micro-step motors further decrease the rotation in each step. A stepper motor controller can move the electric motor one step (in either direction) by applying a voltage pulse and rotational speed is controlled by changing the frequency of the voltage pulses.  Diagram below shows one full rotation for stepper motor.

          There are several types of stepper motors. 4-wire stepper motors contain only two electromagnets, however the operation is more complicated than those with three or four magnets, because the driving circuit must be able to reverse the current after each step. Usually, people use  6-wire motor.

On 23 February, I attend a short and brief workshop conducted by Sir Zul regarding Arduino. In this two hours workshop, Sir Zul give introduction of Arduino, highlight few importance command in Arduino program, introduce several type of Arduino board; each of it has its own function and application, did some demo, and open Q&A session to ask regarding FYP.
      From this workshop, i knew that, to drive a motor using Arduino, we have to choose Arduino Romeo. it can be done by normal Arduino, but have to add H-bridge. For the Arduino to stand alone, (since student will face trouble if attached Arduino board in FYP circuit) it requires two capacitors and one oscillator, plus 5V supply, connected to Atmega 328.