Roboticsin Schools

Speed Control

Had a tough time at the arena of Ro-terrain while taking turns or crossing obstacles ?? Wished that if there had been a speed controller in the bot it would have been nice ?? Then this article is just for you.

Being able to control the speed of the bot is a life saviour at many nooks and crooks at various events during techfests. Bots without speed control require more intensive controlling. However, there are solutions. You can try achieving Speed Control by these two methods :

i) Armature Voltage control method : The motor RPM is directly proportional to the voltage applied across its terminals. So if we can achieve a drop in the terminal voltage across the motor, a slower RPM can be achieved. This is accomplished by putting a resistor in series with the motor. If a variable resistor is used, speed can be varied with greater flexibility according to our will. Fixed resistors will also serve the purpose but be carefull of the wattage of the resistors you are going to use, else you will end up burning them. Resistor Wattage has to be greater than the product of Voltage Supplied and Current flowing through the supply wires.

speed control


ii) Pulse Width Modulation : This is the best method of speed control. The supply across the motor terminals is sent in form of voltage pulses across the motor terminals. Pulses are nothing but swtiching ON and OFF of signals at a specific frequency. If in a particular time period T, the voltage signal is ON for T/2 time then the duty cycle is 50% hence the motor runs at 50% of its full speed. The generated pulse looks like this :


The circuitry for designing a speed controller ( low current ) is shown below :


Of course, for better circuits, Google is your friend !!

So as you can see there is a variable resistor present in each circuit, that will serve the purpose of your speed controller knob. At full resistance the Duty cycle is 0%, at half resistance its 50%, and at 0 resistance its 100%. The relationship of resistance and PWM Duty cycle is a linear one.

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