JPH04268816A - Photoelectric switch for object detection - Google Patents

Abstract

PURPOSE:To offer a photoelectric switch for detecting an object which is made capable of detecting highly precisely the object to be detected without being affected by ambient temperature. CONSTITUTION:A temperature compensation thermistor 13 is connected inseries to the feedback resistor 12 of the amplifier 9 of a light receiving device in the photoelectric switch for detecting the object in which a projector and the light receiving device are arranged opposite to each other across the moving path of the objetc to be detected, and the passing of the object from the change of the quantity of received light at the time when light beam emitted from the projector toward the light receiving device is shielded by the object to be detcted, and here, providing that the temperature coefficient of the thermistor 13 is A, the feedback resistance value of the amplifier is R1, the change of the ambient temeprature is (t), and the temperature coefficient intrinsic to the optical system and the circuit system of the photoelectric switch is alpha, they are selected so that the resistance value of the thermistor is Rth=(1+At)R1, and the temperature coefficient of the thermistor is A= -2alpha. Thus, the temperature coefficient of the thermistor 13 acts so as to cancel the temperature coefficient intrinsic to the photoelectric switch, and the object to be detected can be highly precisely detected irrespective of the change of the ambient temperature.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric switch for object detection, which is well known as an optical sensor.

0002

2. Description of the Related Art It is well known to use a photoelectric switch as means for detecting and positioning an object to be detected. FIG. 2 shows the configuration of such a photoelectric switch for detecting an object. In the figure, 1 is a light emitter and 2 is a light receiver, and the light emitter 1 and the light receiver 2 are arranged opposite to each other across the moving path of the detected object 3. ing. Here, the light projector 1 includes a light emitting element 4 as a light projecting source,
It is comprised of a drive circuit 5 for the light emitting element 4, a light projecting lens 6, and the like. Further, the light receiver 2 includes a light receiving lens 7, a light receiving element 8, an amplifier 9, a comparator 10, and the like. In such a configuration, when the detected object 3 crosses the optical path while the light beam 11 is emitted from the projector 1 toward the light receiver 2, the light beam 11 is blocked by the detected object 3, so that the light beam 11 is not received by the light receiver 2. The amount changes and an object detection signal is output. In recent photoelectric switches, a small semiconductor laser element with high luminous efficiency is generally used as the light emitting element 4, and a semiconductor photodiode is generally used as the light receiving element 8.

0003

[Problems to be Solved by the Invention] By the way, if the ambient temperature changes while object detection is being performed using the photoelectric switch, the amount of light beam emitted from the projector may change, or the output sensitivity of the receiver may change. This causes problems such as causing malfunctions during object detection and decreasing positioning accuracy and detection position repeatability. Therefore, in the past, as a measure to prevent a decline in measurement accuracy, the lens and case that make up the photoelectric switch were made of special materials that were less affected by changes in ambient temperature, and the drive circuit of the floodlight was We have taken measures such as incorporating a light intensity adjustment circuit to maintain the output light intensity constant regardless of changes in ambient temperature.

However, as mentioned above, photoelectric switches that use special materials for the lens, case, etc. not only have high production costs, but also have limits on the improvement of detection accuracy, especially when the receiver amplifier is affected by changes in ambient temperature. Since there are also changes in amplification sensitivity, detection errors are inevitable.

The present invention has been made in consideration of the above points, and by adding a simple temperature compensation means to the amplifier circuit of the photoreceiver, high detection accuracy can be achieved even when used in an environment where the ambient temperature changes. An object of the present invention is to provide a photoelectric switch for detecting an object.

[0006]

[Means for Solving the Problems] In order to solve the above problems, in the photoelectric switch of the present invention, a thermistor for temperature compensation is connected to the amplifier of the light receiving circuit in the light receiver,
And the temperature coefficient of the thermistor is determined by the optical system of the photoelectric switch,
The value shall be selected to cancel out the temperature coefficient specific to the circuit system.

Here, the thermistor in the above configuration is connected in series with the feedback resistance of the amplifier, and the resistance value of the thermistor is expressed as A, the feedback resistance value of the thermistor as A, the feedback resistance value of the amplifier as Rf, and the amount of change in ambient temperature as t. Rth is Rth
= (1+At)Rf, and furthermore, assuming the temperature coefficient specific to the optical system and circuit system of the photoelectric switch to be α,
The temperature coefficient A of the thermistor is selected so that A=-2α.

[0008]

[Operation] With the above configuration, by appropriately selecting the resistance value and temperature coefficient of the thermistor and configuring the temperature compensation circuit,
The output of the light receiver, which increases or decreases with changes in ambient temperature, is corrected, allowing highly accurate object detection regardless of changes in ambient temperature, and improving detection position repeatability.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a temperature compensation circuit according to an embodiment of the present invention for an amplifier of a photodetector, in which a thermistor 13 for temperature compensation is connected in series with a feedback resistor 12 of an amplifier 9. Note that 14 and 15 are gain adjustment resistors of the amplifier, and the respective resistance values of the resistors 14 and 15 are represented by R1 and R2. Here, the resistance value of the thermistor 13 is selected so that Rth=(1+At)Rf, where the resistance value of the thermistor 13 is Rth and the temperature coefficient is A. Furthermore, the temperature coefficient A of the thermistor is The value is selected to cancel out the temperature coefficient α specific to the system and circuit system.

Next, how to determine the temperature coefficient A of the thermistor will be explained using an arithmetic expression. First, the output voltage V0 of the amplifier 9 in the circuit shown in FIG. 1 is expressed by the following equation. That is, V0 =-(Rf +Rth)(1+R1 /R2
)(1+αt)I =-(Rf +Rf
(1+At)) (1+R1 /R2)(1+αt
)I...(1) Note that t is the amount of change (variable) in the ambient temperature, I
is the output current of the light receiving element 11.

Next, when the above equation (1) is expanded, the following equation is obtained. V0 =-Rf ((1+αt)+(1+At)
(1+αt)) (1+R1 /R2) I
=-Rf (2 + (A+2α)t+Aαt2
)(1+R1/R2) I...(2) Here, the temperature coefficient α, A is a small value, and the variable t in equation (2)
Since the coefficient of the term 2 can be regarded as A×α≈0, equation (2) can be expressed as follows. V0 = -Rf (2 + (A+2α)t) (
1+R1/R2) I...(3
)

In order to prevent the output voltage V0 from being affected by the ambient temperature, the value of the temperature coefficient A of the thermistor 13 must be adjusted so that the coefficient of the variable t in equation (3) becomes 0. All you have to do is choose. Therefore, if we set the coefficient of the variable t term in equation (3) to 0, A+2α=
0, and if we solve this equation for the temperature coefficient A, we get A=-2
becomes α. Here, by substituting A=-2α into the above equation (3), V0 =-2Rf (1+R1 /R2) I

...(4). In other words, the temperature coefficient A of the thermistor 13 connected in series with the feedback resistor Rf of the amplifier 9 in FIG.
If is chosen to have the above value, the output voltage V0 of the amplifier 9 of the photoreceiver
Since the expression representing the term t, which is a variable related to the ambient temperature, is eliminated, the photoelectric switch can detect the object to be detected with high accuracy without being affected by changes in the ambient temperature.

[0013]

Effects of the Invention Since the photoelectric switch for object detection of the present invention is constructed as described above, it is possible to detect the object to be detected with high precision without being affected by the ambient temperature. It is also possible to improve repeatability.

[Brief explanation of the drawing]

FIG. 1 is a temperature compensation circuit diagram of a light receiver according to an embodiment of the present invention.

[Figure 2] Overall configuration diagram of photoelectric switch for object detection

[Explanation of symbols]

1 Floodlight 2 Photo receiver 3 Object to be detected 8 Photo receiving element 9 Amplifier 11 Light beam 12 Feedback resistance 13 Thermistor

Claims (3)

[Claims] Claim 1: Consisting of a combination of a light emitter and a light receiver arranged oppositely across a path of movement of the object to be detected, the light is received when a beam of light emitted from the light emitter toward the receiver is blocked by the object to be detected. In a photoelectric switch for object detection that detects passage of an object from a change in quantity, a thermistor for temperature compensation is connected to the amplifier of the light receiving circuit in the light receiver, and the temperature coefficient of the thermistor is determined by the optical system and circuit system of the photoelectric switch. A photoelectric switch for detecting an object, characterized in that the value is selected to cancel out the inherent temperature coefficient. 2. The photoelectric switch according to claim 1, comprising:
A thermistor is connected in series with a feedback resistor attached to an amplifier, and the temperature coefficient of the thermistor is A, the feedback resistance value of the amplifier is Rf, and the amount of change in ambient temperature is t. ) Rf. 3. The photoelectric switch according to claim 1, comprising:
A photoelectric switch for detecting an object, characterized in that the temperature coefficient A of the thermistor is selected so that A=-2α, where α is the temperature coefficient specific to the optical system and circuit system of the photoelectric switch.