3-CNC Control - Open and Closed Loop
CNC systems require motor drives to control both the position and the velocity of the machine axes. Each axis must be driven separately and follow the command signal generated by the NC control. There are two ways to activate the servo drives: the open-loop system and the closed-loop system.

Open Loop - Programmed instructions are fed into the controller through an input device. These instructions are then converted to electrical pulses (signals) by the controller and sent to the servo amplifier to energize the servo motors. The cumulative number of electrical pulses determines the distance each servo drive will move, and the pulse frequency determines the velocity.

The primary drawback of the open-loop system is that there is no feedback system to check whether the program position and velocity has been achieved. If the system performance is affected by load, temperature, humidity, or lubrication then the actual output could deviate from the desired output.

For these reasons, the open-loop system is generally used in point-to-point systems where the accuracy requirements are not critical. Very few, if any, continuous-path systems utilize open-loop control.

Closed Loop - The closed-loop system has a feedback subsystem to monitor the actual output and correct any discrepancy from the programmed input. The feedback system could be either analog or digital. The analog systems measure the variation of physical variables such as position and velocity in terms of voltage levels. Digital systems monitor output variations by means of electrical pulses

Closed-loop systems are very powerful and accurate because they are capable of monitoring operating conditions through feedback subsystems and automatically compensating for any variations in real-time.

Most modern closed-loop CNC systems are able to provide very close resolution of 0.0001 of an inch. Closed-looped systems would, naturally, require more control devices and circuitry in order for them to implement both position and velocity control. This, obviously, makes them more complex and more expensive than the open-loop system. A closed/open comparison is shown below.

The feedback in a closed loop system is the information delivered from the CNC machine to the controller. This is not the same as the data that the controller sends to the machine. Feedback data is data that either confirms or denies that the motors have moved the machining table to the correct position and at the correct speed.

Feedback can be accomplished in one of two ways.

  • Linear Scales - Linear scales are high precision glass scales that can be "seen" by an optical encoder that follows along with the axis movement. This is the type of scale used on most DRO systems for manual mills. The accuracy of these scales are absolute in that position of the optical encoder relative to the scale is exact from one end of the scale to the other baring thermal expansion.
  • Resolvers - Resolvers work differently. See below.


The resolver mentioned above is a complicated and expensive analog device that feeds back via calibrated sine signals relative to its angle of rotation. Prior to the extensive use of digital technology all NC machines used resolvers as it was the only means to close a loop. Today the vast majority of machines use optical shaft encoders which are far less expensive and more suitable to the digital technology used with CNC machinery. However, resolvers are still used on some newer CNC machines where high shock or vibrations are present.

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