Intelligent Control Components in zBot Chassis
Upon inspecting the zBot vehicle’s chassis, two components stand out that require intelligent control: the steering servo and the DC motor. The standard circuit for electronic control of revolution speed and direction is the H-bridge, employed for this purpose. The DC motor used in Tamiya cars possesses enough power to propel the zBot at speeds of up to 20 miles per hour. However, this motor draws substantial current, exceeding 10 A, necessitating the selection of high-current power MOSFETs for the driver stage. The availability of various MOSFET options adds complexity to the choice.
Critical Criteria for MOSFET Selection
The ideal MOSFET for this application must be capable of supplying the maximum motor current and, crucially, be switchable with gate voltages around 5 V. In this scenario, the microcontroller directly controls the power stage (‘low side’) by switching it. However, when it comes to high-side driving, level shifters become essential. The schematic of the H-bridge power stage incorporates inverters, NAND gates, and two tri-stateable drivers. These logic functions play a pivotal role because the seemingly simpler approach of directly controlling all four MOSFETs carries a significant drawback.
Avoiding Fatal Errors with Logic Functions
In the event of a software crash, there is a risk of incorrect switching of multiple MOSFETs, such as T4 and T7. This situation could lead to a substantial current passing through the transistors, limited only by the internal resistors of the MOSFETs (approximately 10 miles). Such a critical error could result in the destruction of the MOSFETs. However, the implemented logic functions effectively prevent these illegal states, ensuring the system’s integrity and preventing fatal errors. To regulate the DC motor, three signals play a crucial role: DIR determines the motor’s rotation direction, PWM controls the speed, and STOP applies brakes. The corresponding software module for the DC motor is referred to as DCM.c.
(1) Accessing Zbot Documentation
The comprehensive document titled “Zbot — the Robot Experimental Platform” is accessible for free download on the Elektor Electronics website. Interested readers can find the file under the number 070172-11.zip, providing valuable insights into the intricate workings of the Zbot project.
Understanding MOSFET: Miniaturization and Digital Applications
MOSFET, an acronym for METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR, as depicted in Figure 1, operates based on its unique structure and functioning principles. In this context, ‘MOS’ refers to the transistor’s structural components. While ‘FET’ pertains to its operational mechanism, also known as IGFET (Insulated Gate Field Effect Transistor). While the image presented illustrates a practical MOSFET, it’s essential to note that in the digital realm, MOSFETs are incredibly small in size (measured in nanometers), enabling the fabrication of billions of them on a single chip. This miniaturization has revolutionized digital technology, paving the way for highly sophisticated electronic devices and integrated circuits.