GB2057691A - Electronic guard strip proximity switch - Google Patents

Abstract

An electronic guard strip proximity switch for prevention of collision with a moving object, controls a control means such as a relay (7), and comprises at least one antenna (1) forming a capacitor together with a base (2) in its ambience, a voltage source such as a generator (3) from which an alternating voltage is conducted to said antenna, and an amplifier (4) to which the control means and the antenna are connected so that the change of voltage arising from any capacitive perturbation in the antenna is conducted to the control means (7). The voltage from the amplifier (4) is conducted over a rectifier (11) to an amplifier (5) which is so connected that its direct current gain is +1, but which amplifies any change of voltage occurring in the d.c. voltage input to a multiple and which further controls through a comparator (6) the control means (7). The guard strip may be mounted on the edge of lift doors. <IMAGE>

Description

SPECIFICATION Electronic guard strip The present invention concerns an electronic guard strip for prevention of collision with a moving object, controlling a control means such as a relay, and comprising at least one antenna which together with a base in its ambience constitutes a capacitor, and a voltage source such as a generator, from which an alternating voltage has been conducted to said antenna, and an amplifier, with which the control means and antenna have been so connected that the change of voltage resulting from a capacitive disturbance in the antenna has been conducted to the control means. A moving object is here understood to be for instance: the automatic doors of a lift, or controllable platforms for installation work, or moving and transporting apparatus.

It is common practice for instance in connection with automatic doors to install protective devices to prevent persons using the doors from being caught in the doors, and also to prevent the doors from colliding with people.

Similarly, such protective means may be built to serve other purposes, such as the prevention of collisions between controllable platforms for installation work or moving and transporting apparatus or their collisions with other obstacles.

Devices used to this end include mechanical sensor strips, photocells, ultrasonic equipment, and sensors based on capacitive measurement.

Among the kinds of apparatus mentioned, those operating according to a capacitive principle are the most favourable. Among their advantages may be mentioned: attractive architectural appearance, because all components may be concealed.

Another advantage of capacitive sensors is there good reliability owing to the fact that the apparatus can be made operative without any mechanical parts which would move and be subject to wear. Capacitive sensors of this type are known in the art e.g. through the following references: U.S. Patent No. 3,370,677; U.S.

Patent No. 3,743,058; British Patent No.

1,108,884; and Finnish publicizing print No. 53234.

The weak point of the apparatus according to the U.S. Patent No. 3,370,677 are the problems incurred through the changing of the apparatus characteristics. This is evident, for instance, in that the ground capacitances arising from lift doors are different on each floor owing to mechanical tolerances. It is then impossible to set the sensitivity of the apparatus very high because of false door control signal could be obtained. The capacitance conditions are altered, and the sensitivity is restricted, also by humidity/moisture and by dents inflicted on the antennas mounted on the door edges. Another weak point of the apparatus is its sensitivity to the capacitance between the guard plate and the sensor plate.This capacitance should be minimized, compared with the capacitance against ground that is being measured, in order that the measured signal might be adequate. The low capcitance requirement implies a mechanically expensive, and therefore unfavourable, antenna design.

The objectionable feature of the capacitive sensor disclosed by the U.S. Patent No.

3,743,058, mentioned second, is that the tuning takes place at the beginning of the door's opening movement. In that case, any perturbation in the capacitive conditions occurring after the beginning of the opening movement in the antennas disposed in the door edge will derange the operation of the control means and thereby of the doors.

The sensor of the British Patent No. 1,108,884 presents the same drawbacks, mentioned above.

In other words, the apparatus is sensitive to disturbances arising from changes and from the capacitance conditions in the mechanical equipment itself. Moreover, a weak point of the said British apparatus is the potential difference between the screening voltage and the measuring antenna. Although it is possible by means of the screening voltage to prevent the effect of objectionable ground capacitances, the capacitance of the parts carrying screening voltage must not be large with reference to the measuring antennas because this capacitance attenuates the measuring signal that is obtained.

As a consequence the use of coaxial leads with screening voltage for measurement signal transfer from the antenna to the amplifier is limited. In order to increase the sensitivity, the amplifier frequently has to be installed at a mechanically awkward point, such as immediately adjacent to the sensor strip, affixed to the moving door. If two separate door halves are used, two separate sets of amplifying apparatus are required.

In the apparatus of the Finnish publicizing print No. 53294, part of these drawbacks have been eliminated. In this apparatus, the circuit of the tuning unit is such that the set/hold circuit becomes tuned at the beginning of the door's closing movement, whereby only the differential voltage due to any capacitive perturbation changing in its magnitude during the closing movement is enabled to pass through the set/hold circuit and amplifier to the control means. In the said apparatus the doors cannot be closed fully without a special override switching by which the measuring of capacitance is terminated. The weak point is exactly this switch, which renders the apparatus more prone to faults and more expensive.

The object of the present invention is to eliminate the drawbacks mentioned and to provide a simple and reliable operating guard strip of a new type.

The guard strip of the invention is characterized in that the voltage from the amplifier (4) has been conducted over a rectifier (11) to an amplifier (5) so connected that its d.c. gain is +1, but which amplifies to a multiple any change of voltage in the incoming d.c. voltage, and which further over a comparator (6) controls the control means (7). By the aid of the invention, the variation of capacitance can be continuously measured within a very wide span, whereby obstacles of greatly different sizes can act upon the apparatus when entering the field of observation with a velocity exceeding the velocity that has been set. When an obstacle enters the field of observation of the apparatus, the capacitance measured by the apparatus increases and the voltage rise, proportional to the increase of capacitance, is admitted through the amplifier to the control means.

One possible application consists of the automatic doors of a lift, where the antennas mounted on the edges of opposed doors are mutually at the same potential. This enables the doors to be run to their fully closed position without any separate override circuit, because as the doors approach each other the measured capactitance decreases since the influence of the opposite door increases.

Another application are controllable platforms for installation work and moving and transporting appratus, where antennas have been mounted on the edge of the equipment. When the apparatus approaches any object, the capacitance between the antenna and this object will increase, whereby the control means reacts and collision is avoided.

The invention is described in the following in detail with the aid of an example, with reference to the attached drawings: Fig. 1 displays the overall placement of the guard strip, connected to the automatic doors of a lift; Fig. 2 presents, in terms of commonly understood components, the electrical embodiment of one operational block of the guard strip of Fig. 1; and Fig. 3 shows the cross section of a guard strip of the invention, affixed to the automatic doors of a lift.

The operation of the guard strip is based on measurement of the capacitance between an antenna 11, disposed in the end face of the cage door, and its ambience 2. Fig. 2 illustrates the measuring principle. The capacitance to be measured is Co. When Co is connected by means of the resistor R to the alternating voltage produced by the generator 3, a voltage Uc is established across Co, having a value proportional to the magnitude of Co. Because of the high impedance levels of R and Co, the voltage to be measured, Uc, has to be connected to the measuring circuit with the aid of an amplifier 4.

After the amplifier 4, the voltage is rectified and filetered to be available at the point P.

The amplifier 5 measures the voltage at P. If the voltage is constant (Co = constant), the amplifier 5 will operate in likeness with the amplifier 4. Its output voltage will be the same as the voltage at P. In that case the output voltage of the comparator 6 is positive and the relay 7 is deenergized. When the door edge approaches a person, or an object, the value of Co will increase, whereby the voltages Uc and P are reduced. The amplifier 5 amplifies the change of voltage at P to a multiple. Since the absolute value of the voltage P was reduced, the output voltage of amplifier 5 will become more negative. The change in the negative direction will be greater for greater change of voltage at P. If the output voltage of amplifier 5 fails short of the action threshold of the comparator 6, the comparator output voltage will change to be negative.The transistor T is supplied with base current, and the relay 7 will attract its armature.

Since the amplifier 5 has been connected to operate as an a.c. amplifier, its output voltage will begin to rise towards positive immediately after the value of Co no longer increases. When the output voltage of amplifier 5 surpasses the action threshold of the comparator 6, the relay 7 will drop its armature. In other words, the relay 7 may attract is armature only if the value of Co increases, and it remains energised a brief instant only if the value of Co remains constant and returns to be as before. The sensitivity of operation of the measuring circuit is controlled by the trirnmer 8, which determines the action threshold of the comparator.

In actual practice, the end faces of the doors are covered with an electrically conductive material 9. With a view to improving the measuring sensitivity, the end face of one door half has been divided into two sections of equal size. One pole of the capacitance Co consists of the electrically conductive surface 1; the other pole is Ground 2. Each section's capacity is measured on a channel of its own. The channels control independently with the aid of the comparator 6, the relay 7.

In the door-closing phase, the value of Co is strongly reduced, because the area between antenna 1 and ground 2 becomes less and the influence of the opposite door surface at the same potential increases. Since nothing but an increase of the value of Co can cause the relay 7 to attract its armature, the doors may close, in spite of the approach of the opposite door leaf.

Substantial improvement of the measuring sensitivity is achieved if the antenna section 1 is constructed and connected as shown in Fig. 3. The antenna plate 1,9 consists of two electrically conductive surfaces with an insulation 10 between them. When the output voltage of the amplifier 4 is connected to the electrically conductive surface 9, the capacitance arising from the door leaf will not be connected in parallel with Co because the voltage of the surface 9 is always at the same potential as the surface 1. Hence follows that the value of Co is smail, and even a minor perturbation (= change of capacitance) will cause a major change in the value at rest of Co.

Between the amplifier 5 and the comparator 6, separately in each channel, has been connected a diode 1 2, in order to render possible the parallel operation of a plurality of antennas. It is advantageous, owing to practical reasons, to connect several antennas in parallel, because hereby the sensitivity of measurement may be improved even further.

Claims (4)

1. Electronic guard strip for prevention of collision with a moving object, controlling a control means such as a relay (7), and comprising at least one antenna (1) forming a capacitor together with a base (2) in its ambience, a voltage source such as a generator (3) from which an alternating voltage has been conducted to said antenna, and amplifier (4) to which the control means and the antenna have been connected so that the change of voltage arising from any capacitance perturbation in the antenna has been conducted to the control means (7), characterized in that the voltage from the amplifier (4) has been conducted over a rectifier (11) to an amplifier (5) which is so connected that its direct current gain is + 1, but which amplifies any change of voltage occurring in the d.c. voltage input to a multiple and which further controls through a comparator (6) the control means (7).

2. Electronic guard strip according to claim 1, characterized in that in the comparator (6) has been incorporated a trimmer potentiometer (8) for adjustment of the sensitivity of operation.

3. Electronic guard strip according to claim 1, characterized in that between the amplifier (5) and the comparator (6) has been connected a diode (12).

4. Electronic guard strip substantially as described herein with reference to the accompanying drawings.