Answer: A. Feedback device
Economical CNC machines with open-loop control do not have the following feedback device functions.
Open-loop control means that in the control system, there is only a positive control relationship between the controller and the controlled object, that is, the controller gives a command signal without a feedback signal. In this case, the CNC machine tool can only execute instructions in one direction, and cannot feedback and adjust the execution results.
Therefore, economical CNC machine tools with open-loop control do not have the function of feedback device to feedback and adjust the execution results. Although it may have an operator panel and a numerical control system, these features do not solve the problem that it does not have a feedback device.
Therefore, according to the information given by the title, an economical CNC machine tool with open-loop control does not have the following functions as a "feedback device".