PID is an acronym that gets used more and more as factory automation requires ever better performance
from processes, machines and motion control. There exists a great deal of misunderstanding about it. This
article will talk about PID conceptually and practically so that a clearer understanding may prevail.
To begin with, PID means Proportional, Integral and Differential. It probably should have been called IPD
rather than PID because the Integral term is most effective at low frequencies, the Proportional term at
moderate frequencies, and the Differential term at higher frequencies. These frequencies are relative to the
bandwidth of the servo or process.

Open Loop Operation
One of the most significant advantages of a stepper motor is its ability to be accurately controlled in an open loop system. Open loop control means no feedback information about position is needed. This type of control eliminates the need for expensive sensing and feedback devices such as optical encoders. Your position is known simply by keeping track of the input step pulses.
Stepper Motor Types
There are three basic stepper motor types. They are :
• Variable-reluctance
• Permanent-magnet
• Hybrid

Variable-reluctance (VR)
This type of stepper motor has been around for a long time. It is probably the easiest to understand from a
structural point of view. Figure 1 shows a cross section of a typical V.R. stepper motor. This type of motor
consists of a soft iron multi-toothed rotor and a wound stator. When the stator windings are energized with DC current the poles become magnetized.

Rotation occurs when the rotor teeth are attracted to the energized stator poles.
Permanent Magnet (PM) Often referred to as a “tin can” or “canstock” motor the permanent magnet step motor is a low cost and low resolution type motor with typical step angles of 7.5° to 15°. (48 – 24 steps/revolution) PM motors as the name implies have permanent magnets added to the motor structure.
The rotor no longer has teeth as with the VR motor. Instead the rotor is magnetized with alternating north and south poles situated in a straight line parallel to the rotor shaft. These magnetized rotor poles provide an increased magnetic flux intensity and because of this the PM motor exhibits improved torque characteristics when compared with the VR type.