S'2 WEEK 6 => 18 FEB - 24 FEB

Direct current (DC) motor has already become an important drive configuration for many applications across a wide range of powers and speeds. The ease of control and excellent performance of the DC motors will ensure that the number of applications using them will continue grow for the foreseeable future. This project is mainly concerned on DC motor speed control system by using microcontroller PIC 16F877A. Pulse Width Modulation (PWM) technique is used where its signal is generated in microcontroller. The PWM signal will send to motor driver to vary the voltage supply to motor to change the speed when the distance changes.

Direct current (DC) motors have variable characteristics and are used extensively in variable-speed drives. DC motor can provide a high starting torque and it is also possible to obtain speed control over wide range. We need a speed motor controller in the most of the robots. For example, if we have a DC motor in a robot, if we just apply a constant power to each motor on a robot, then the robot will never be able to maintain a steady speed. It will go slower over carpet, faster over smooth flooring, slower up hill, faster downhill, etc. So, it is important to make a controller to control the speed of DC motor in desired speed.

DC motor plays a significant role in modern industrial. These are several types of applications where the load on the DC motor varies over a speed range. These applications may demand high-speed control accuracy and good dynamic responses.

In home appliances, washers, dryers and compressors are good examples. In automotive, fuel pump control, electronic steering control, engine control and electric vehicle control are good examples of these. In aerospace, there are a number of applications, like centrifuges, pumps, robotic arm controls, gyroscope controls and so on.

 Most DC motors are normally very easy to reverse, simply changing the polarity of the DC input will reverse the direction of the drive shaft. This changeover process can be achieved via a simple changeover switch or for remote or electronic control, via a suitable relay. When using a switch or relay always check the contact current ratings and allow for larger currents to be switched, as different mechanical loads and instant reverse can draw much higher currents than when the motor is being run unloaded.

Another big advantage of DC motors is that variable speed control is easy and can be achieved with just a suitable variable resistor / rheostat or variable DC power supply. For more precise control and maximum efficiency there are many other electronic PWM (pulse width modulation) solutions, although these tend to have added complexity. Most DC motors are designed to exhibit the same speed and output torque in either the forward or reverse direction. Drive shaft speeds rpm (Revs Per Minute) are quoted with motor unloaded. The specifications considered for DC geared motor SPG30-300K are stated below:

• ·         DC12V
• ·         Output Power: 1.1 Watt
• ·         Rated Speed: 12RPM
• ·         Rated Current: 410mA
• ·         Rated Torque: 1176mN.m

 DC geared motor SPG30-300K

DC motor Speed Controller

For precise speed control of system, closed-loop control is normally used. Basically, the block diagram and the flow chart of the speed control are shown below. The speed is compared with the reference speed to generate the error signal and to vary the armature voltage of the motor.

There are several types of DC motors that are available. Their advantages, disadvantages, and other basic information are listed below

Type	Advantages	disadvantages
Stepper motor	Very precise speed and position control. High Torque at low speed.	Expensive and hard to find. Require a switching control circuit
Dc motor w/field coil	Wide range of speeds and torques. More powerful than permanent magnet motors	Require more current than permanent magnet motors, since field coil must be energized. Generally heavier than permanent magnet motors. More difficult to obtain.
DC permanent magnet motor	Small, compact, and easy to find. Very inexpensive	Generally small. Cannot vary magnetic field strength
Gasoline (small two stroke)	Very high power/weight ratio. Provide Extremely high torque. No batteries required.	Expensive, loud, difficult to mount, very high vibration.

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