If you look at the chassis of the zBot vehicle, you’ll find two parts requiring intelligent control: the steering servo and the DC motor. The so-called H-bridge is the normal circuit for electronic control of revolution speed and direction. The DC motor of a Tamiya car is powerful enough to propel zBot at up to 20 miles per hour. The motor then consumes more than 10 A, so we choose high-current power MOSFETs for the driver stage. There are lots of different devices to choose from. The MOSFET we require has to supply the maximum motor current and, importantly, it has to be switched with gate voltages of about 5 V. In this case, the microcontroller switches the power stage (‘low side’) directly. For high-side driving levels shifters are necessary. The schematic of the H-bridge power stage shows a few inverters, NAND gates, and two tri-stateable drivers. These logic functions are very important as the easier way, i.e.., directly controlling all four MOSFET has a fatal disadvantage.
In case of a software crash, it could happen that two or more MOSFETs are switched on incorrectly, for example, T4 and T7. In that case, the current through the transistors is limited by the internal resistors of the MOSFETs (about 10 miles) only. Such a fatal error would destroy the MOSFETs. The logic functions configured here effectively avoid illegal states. To control the DC motor, three signals are needed: DIR, PWM, and STOP. DIR controls the direction of the motor revolution, PWM the speed, and STOP brakes the motor. The software module for the DC motor is called DCM.c.
(070172-1)
(1) The complete document called Zbot — the Robot Experimental Platform Is available for free downloading from the Elektor Electronics website. The file number Is 070172-11.zip (July/August 2007).
The MOSFET stands for METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR(Fig 1). In MOSFET, the MOS part is related to the structure of the transistor, while the FET part is related to how it works. It is also known as IGFET (Insulated Gate Field Effect Transistor). The following image we have shown is a practical MOSFET. But in the digital world, the size of MOSFET is too small (in nm) that billions of them can be fabricated on a single chip.
