Automation of 1d measurement instruments calibration
Machine-building enterprises are the main consumers of a large number of hand tools such as calipers, micrometers, alesometers, snap meters and dial indicators. Some tools are kept directly at the workstations, another – in the measuring laboratories.
Depending on the intensity of operation, calibration (verification) of hand-held instruments is carried out at different calibration intervals. To calibrate a hand-held instrumentation, the various reference instruments are used and, as a rule, these are hand-operated devices with an indicating display. In order to enhance performance of calibration, reduce fatigue of the verification inspectors and eliminate the human factor, it is highly recommended to implement the ACS.
The purpose of this research is to develop two types of the automated reference devices.
The first M-1000 device is designed to calibrate and adjust the common two-point instruments such as calipers, micrometers, alesometers and snap meters with a measurement range of 0–1,000 mm. The device is designed as an automated workstation of the verification inspector and is intended to replace a wide range of the end gauge dimensions, reference rings, devices and various accessories.
The second one (API-50) is designed for automated calibration of the whole range of dial gauges from ICH-2 to ICH-50 and two-point alesometers.
Both devices make use of the automated drive with precise positioning of the working body up to 0.1 μm. The problem is solved for the first time in our country for the 1D measuring devices.
To achieve this goal it is necessary to:
• choose the control system type;
• select a drive type;
• select the type of feedback system;
• develop a schematic diagram of the drive control system;
• develop a method for compensating temperature errors;
• develop a system of "technical vision" for reading indicators;
• develop a methodology for automated calibration of the instruments themselves;
• develop a reference wide-range converter for automated calibration of instruments;
• to conduct preliminary studies in order to determine errors of the instruments based on calculation of measurement uncertainties.
As a result of the analysis of technical literature and preliminary calculations, a feedback control system based on the measuring device working body position has been selected. This system most fully meets the requirements for the accuracy of the measuring tips positioning of the device, and, also, allows of controlling the position in the real time mode.
Movement of the working body of the device is carried out using a servo drive consisting of a servo motor, servo amplifier and a controller. The ball screw pair, due to preloading, ensures high transmission rigidity and a small dead space zone during reverse.
The ACS of the devices is shown in Fig.1. To ensure accurate positioning of the measuring tips carriage, the system is equipped with a linear feedback sensor. The use of a linear sensor, as compared with a circular one, enables to make direct measurements of the carriage position which improves the positioning accuracy.
When using an ACS, a carriage with a measuring tip is driven and automatically reproduces the movement specified by the control device (in this case a personal computer). A control signal is generated by the controller that receives information on the desired position of the measuring tip from the PC, and its actual position from the feedback sensor. The control signal is a differential signal characterizing, respectively, the specified and actual movements of the measuring tip carriage.
The linear feedback sensor used in this system provides direct measurement of the measuring carriage movement. This feature enables to include in feedback all transmission mechanisms of the drive, which ensures a high accuracy of movements.
These devices are computer-controlled complex technical devices complete with the corresponding software. Accuracy of the devices depends on many factors: the error of the device proper, the error of the environment impact (for example, temperature and vibration). Accuracy of the linear feedback sensor may be influenced by errors of the instrument guides, temperature deformations and elastic deformations of the measuring carriage components subjected to the measuring force. These errors require additional research and certain steps to eliminate or minimize them.
Preliminary studies have shown that this control system can be used to automate reference instruments for 1D measurements.
Automatic control system with a feedback sensor can provide for the required accuracy when controlling reference instruments. ■