This invention relates to a position detecting device employed in an automatic positioning system.
FIG. 1 shows a position detecting system using limit switches which is the most typical of the conventional position detecting systems. For simplification in description, the drive system is not shown in FIG. 1.
As shown in FIG. 1, a moving table 30 has a protrusion 30a which is used to detect the positions of limit switches 31a, 31b and 31c. As the moving table 30 is moved, the protrusion 30a operates the actuators 310 of the limit switches 31a, 31b and 31c, so that switch signals q, r and s are applied to a controller 32.
The position detecting limit switches 31c, 31b and 31a are set at distances L.sub.1, L.sub.2 and L.sub.3 from the original point O, respectively.
The conventional device thus constructed operates as follows. When the moving table 30 is moved in the direction of the arrow to the limit switch 31c at the distance L.sub.1 from the original point O, the actuator 310 of the limit switch 31c is operated, and the switch signal s is applied to the controller 32. Similarly when the table 30 reaches the limit switch 31b at the distance L.sub.2, the limit switch 31b is operated and the switch signal q is applied to the controller 32. When the table 30 reaches the limit switch 31a at the distance L.sub.3, the limit switch 31a is operated and the switch signal r is applied to the controller 32. In response to the switch signals q, r and s, the controller 32 operates to control the driving, stopping or moving speed of the moving table 30.
Even when the controller 32 provides an instruction signal to stop the table 30, the table 30 will overrun a relatively long distance before it is actually stopped, because of the moving table's inertia and the delayed braking operation of the brake member. Accordingly, in this system, the limit switches 31a, 31b and 31c are located somewhat before the desired stopping points.
The overrun distance L can be approximately represented by the following expression: ##EQU1## where M is the mass of the moving part of the system, K is the rigidity of the mass in the feed direction, and V is the feed speed. Thus, it is difficult to eliminate the overrun distance L. Therefore, in the conventional system, in order to minimize the overrun distance it is necessary not only to set the limit switches 31a, 31b and 31c before the stop points but also to decrease the feed speed of the moving table 30 at the detection points.
However, the overrun distance varies with the speed of the moving table and it is therefore difficult to determine the proper lead time. In addition, when the table 30 is moved in the opposite direction (or returned), the positions of the limit switches 31a, 31b and 31c will now be somewhat behind the desired stopping points and must be adjusted again.
Position detecting sensors employed in the conventional positioning system can be classified into mechanical types such as a jib, a cam, a link or a gear device; electrical types such as a microswitch or a contactless switch; pneumatic types such as a pneumatic microswitch; or optical types such as a photoelectric switch, a photoelectric linear scale or an optical wave interferometer.
In the case of the mechanical type position detecting sensor, the drive sources (such as a hydraulic drive source and/or an electric motor) are, in general, stopped each time a position is detected. Therefore, the mechanical type position detecting sensor is not suitable for automatic positioning. Even if it were designed so as to be suitable for automatic positioning, it would need considerably intricate mechanical elements. In addition, because of the inertial forces and drive forces of the mechanical elements, other mechanical elements may be deformed or worn. Therefore, with the mechanical type sensor, it is difficult to improve the positioning accuracy, and it is considerably difficult to change the design or to move the detection points as discussed above in the case of reverse direction movement.
A microswitch or a contactless switch has been extensively employed as the electrical type position detecting sensor. The electrical type position detecting sensor is usually a simple on-off type digital detector. With such a position detecting sensor, however, a detection accuracy higher than that defined by the switch configuration cannot be obtained; that is, the detection accuracy is limited by the number and location of switches and cannot be improved. Furthermore, the number of sensors which can be installed over the range of movement of the moving table is limited, and it takes a great deal of time and labor to change the design and to adjust the detection points as discussed above.
In the case of the pneumatic type position detecting sensor, once a positioning point is determined it is considerably difficult to change the positioning point. Also, since an external force is repeatedly imparted to the position detecting element, it is rather difficult to maintain the accuracy unchanged.
In general, the optical type position detecting sensor is low in accuracy. Some high accuracy optical type position detecting sensors are available, but they are considerably expensive. Since the detection accuracy of the sensor is greatly dependent on the selected light source, the adjustment takes time and labor, and it is also difficult to change the positioning point.
Thus, the following characteristics are common in conventional devices:
(1) It takes a great deal of time and labor to set the detection positions;
(2) The number of detection positions is limited by the physical dimensions of the position detecting sensor used, and cannot be increased. The detection accuracy is also limited; and
(3) After the detection positions are set, it takes a lot of time and labor to change the detection positions.
A conventional positioning device comprises: a position detecting sensor; and a control unit for driving and controlling the device in response to the output signal of the sensor. The position detecting sensor suffers from the various drawbacks as described above.