Microprocessor Control PID Control:
A closed loop control system is one that determines a difference in the desired and actual condition (the error) and creates a correction control command to remove this error.
PID control demonstrates three ways of looking at this error and correcting it. The first way is the P of PID, the proportional term.
This term represents the control action made by the microcontroller in proportion to the error. In other words, the bigger the error, the bigger the correction.
The I in PID is for the integral of the error over time. The integral term produces a correction that considers the time the error has been present. Stated in other words, the longer the error continues, the bigger the correction.
Lastly, the D in PID stands for derivative. In the derivative term, the corrective action is related to the derivative or change of the error with respect to time.
Stated in other words, the faster the error is changing, the bigger the correction. Control systems can use P, PI, PD, or PID in creating corrective actions.
The problem generally is “tuning” the system by selecting the proper values in the terms.
Programmable Logic Controllers
Any discussion of control systems and microprocessor control should start with the first type of “mechatronic” control, the programmable logic controller or PLC.
A PLC is a simpler, more rugged microcontroller designed for environments like a factory floor. Input is usually from switches such as push buttons controlled by machine operators or position sensors. Timers can also be programmed in the PLC to run a particular process for a set amount of time.
Outputs include lamps, solenoid valves, and motors, with the input–output interfacing done within the controller.
A simple programming language used with a PLC is called ladder logic or ladder programming.
Ladder logic is a graphical language showing logic as a combination of series (and’s) and parallel (or’s) blocks. Additional information can be found in Chapter 43 and in the book Programmable Logic Controllers by W. Bolton.
RAM, or random access memory, is the set of memory locations the computer uses for fast temporary storage. The radio station presets selected by the driver (or passenger) in the car radio are stored in RAM.
A small electrical current maintains these stored frequencies, so disconnection of the radio from the battery will result in their loss. ROM, or read only memory, is the static memory that contains the program to run the microcontroller. Thus the radio’s embedded program will not be lost when the battery is disconnected.
There are several types of ROM, including erasable programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), and flash memory (a newer type of EEPROM).
These types will be explained later in this handbook. There are also special memory areas in a microprocessor called registers. Registers are very fast memory locations that temporarily store the address of the program instruction being executed, intermediate values needed to complete a calculation, data needed for comparison, and data that need to be input or output. Addresses and data are moved from one point to another