CNC Control System

CNC Control System

There are essentially two types of motor control systems that can be used for accurate positioning of mechanical elements:

  • Stepper Motor or Indexer Control (Open loop control)
  • Servo Motor Control (Closed loop control)

1) Open Loop Control:-The stepper motor system is based upon a special type of motor that rotates (indexes) by a fraction of a revolution each time a voltage pulse is applied to one of its winding’s. Unlike traditional motors, the stepper motor does not rotate smoothly but rather, steps from one position to another and hence its name. The overall stepper drive system is shown schematically in Figure 2.6. Stepper motors were originally used for small scale applications such as in printers, plotters, etc. However, they are now also used in industrial applications where the mechanical load on the motor is known (and stable). Although the control principles are similar, the industrial systems tend to be referred to as “indexers” in commercial literature.

The nature of the actual stepper motor drive can vary considerably. For very

small motors, the drive can be implemented on a single silicon chip. On large systems, the stepper motor drive has to include power electronics and cooling fins to dissipate heat and hence has to be implemented on a circuit board, with discrete transistors and digital circuits.

The fundamental limitation of stepper motors arises when they are used as shown in Figure 2.6 – that is, as “open-loop” devices. If the load on the motor shaft is larger than the torque generated by the electrical energy input to the motor windings, then the motor will not index from one position to another in a predictable manner. The absolute positioning characteristic of the motor is therefore lost. As a result, open-loop stepper motors are only used in situations where the load is always well defined – for example, in parts transfer (shuttle or indexing) systems where the maximum load on the

motor can be calculated during the system design phase. A stepper motor, running in “open-loop” mode would be inappropriate for positioning a cutting tool, since the load caused by tool could vary substantially depending on the work-piece properties and amount of material being removed.

Some stepper motor controllers can function in a closed-loop, where the position of the motor shaft is fed back to the controller from a resolver or position encoder. However, once these devices are converted into closed-loop systems, then they lose their cost advantage over traditional servo drives that can provide a “smoother” rotation.

2) Closed Loop Control:- A number of servo motor systems have servo drives that do not provide closed position loop control. Instead, their purpose is to provide closed-velocity-loop control. In these systems, the servo drive provides an output proportional to the difference between the actual velocity of the motor and a specified voltage signal (the set-point or reference velocity). In servo drives such as this, the velocity feedback can either be obtained by differentiating the position feedback signal (readily achieved in both analog and digital servo drives) or from an additional element known as a tacho-generator. A tacho-generator is a d.c. machine that is mounted onto the same shaft as the main motor and provides an output voltage proportional to the speed of rotation of the shaft. This closed-velocity loop form of servo drive is shown in Figure.

The “velocity” terminology in regard to servo drives will cause some annoyance to those concerned with engineering etiquette. Strictly, of course we are referring to the speed of rotation (not velocity). However, this speed is normally directly related to the linear velocity of some end-effector and so the terms tend to be used interchangeably.

The closed-position and closed-velocity loop servo drives both have roles to fulfil in industrial applications. The closed-position loop system is most useful where only one independent axis of movement is required in order to move some element or “endeffector” to a given position. However, where two or more motors are used to drive an end-effector to a given position, the axes are often interdependent. The path taken to reach that position also needs to be controlled through the velocities of the servo motors.

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