WO1991010092A1

WO1991010092A1 - A method and an arrangement for protecting a conductive object from damage by another moving object

Info

Publication number: WO1991010092A1
Application number: PCT/SE1990/000877
Authority: WO
Inventors: Roy Augustsson
Original assignee: Roy Augustsson
Priority date: 1989-12-29
Filing date: 1990-12-27
Publication date: 1991-07-11
Also published as: SE8904409D0

Abstract

The present invention is providing a method and an operating arrangement to effectively obtain protection detection of an object, especially a person, positioned close to a machine in motion or a moving part of a machine in the case of accidental collision or contact by the machine with the person without the use of any electric cabling arrangement between the person and the machine. This is accomplished by detecting an induced AC signal in the body of the person transferred to the machine at the moment of ohmic contact by utilization of an insulated conducting sensor surface covering the total surface of the machine. This sensor surface is connected to an electronic circuitry outputting signals to the power and controlling unit of the machine for stopping or reversing its motion in a case of dangerous contact between the machine and the object, i.e. a person.

Description

A iETHOD AND AN ARRANGEMENT FOR PROTECTING A Cα©UCTIVE OBJECT FROM DAMAGE BY ANOTHER MOVING OBJECT

The present invention relates to a method and an arrangement for achieving a safety control system to protect a conductive object from damage by a moving portion of a machine, and specifically to a method and an arrangement for protection of an electrically conducting object like a human body, within the operating radius of a moving part of a machine, i. e. a moving arm of an in¬ dustrial robot.

BACKGROUND OF THE INVENTION

It is well known that a lot of accidents at factories, workshops and the like are caused by moving parts of machines when such a part unexpectedly did come into contact with a nearby positioned object like a person. Such moments of risk are at present continuously increasing with the increasing use of automatic machines like industrial robots, trucks automatically run along a circuit by computer control etc.

To prevent incidents when objects or persons could be hit by different moving machines or moving parts of a machine, limits will normally be defined as well as areas are marked wherein there is an element of risk that an object may be damaged or a person may be hurt by a certain machine. These limits can be represented by a fence, by a blocking barrier, or in a common case by just a painted line on a floor. One drawback of this is that a specific machine due to such actions may need to occupy a far too large amount of surface area where it is supposed to be operating. Another metod is to prepare the moving machine or moving arm of the machine with sensors which react upon objects which by these sensors are defined as obstructing objects, and the machine will immediately be stopped upon detection of such an object to prevent a hard collision between the machine and the intruding object. These sensors can be of various kinds from a simple mechanical contact switch to electronic and optical, infrared or acoustic sophisticated sensing devices.

The drawback is still that it has proven difficult to design such a protection system which is general. Using contacts will result in a plurality of sensors which may increase the size of the machine considerably at the same time as there will still be unprotected spots where a person unintentionally may get in touch with the machine. Electronic, optical, infrared and acoustic systems have the disadvantage that they will as a rule be sensitive also at some distance from the machine which again implies a restricted area for surrounding objects not to trigger an alarm. This implies that a person may come within the sensing region of the protection system which then stops the operation of the machine when in reality there still were no elements of risk that the machine could reach the person. Any such breaking of an operation cycle of a machine is undesirable as it may even affect other processes within the present production and will always result in inefficiency, besides any such unnecessary action will indeed cause moments of disturbance to the workers.

To avoid such unneccessary incidents there is a general need for a smart system which detects only when there is a hit, in other words, when the object or person really comes into contact with the moving part of the machine and there is an explicit risk of a damage of the object or even an injury to a person. If for example a pivoting arm of a machine just is reaching its limit position when it comes into contact with the person it should normally not be necessary to take any specific actions, but in any other situation the motion of the part must immediately be stopped by actions such as locking or reversing the driving source of this part. Accordingly there is also a general need for a system detecting such direct contact with the object where any reachable surface spot of the protected machine must be sensitive for such a contact.

PRIOR ART

Several system are disclosed for detecting for example that a person is getting in touch with an object. U.S. Patent 4,011,554 and U.S Patent 4,091,371 describe monitoring systems sensing for example when a doorknob is touched by a person. These systems work capacitively and may react even before the doorknob really is touched. This is expected to be the case with the systems described in U.S Patents 4,037,221; 4,349,858 and 3,697,971 which all function by sensing a change of a capacitance comprising a part of an electronic circuitry. These systems are not simply applicable for a moving machine or a moving part of a machine.

In FR-2,305,681 is disclosed a system for protecting a person working at a cutting machine. In this system the person to be protected does not have to be connected to the protection system. This system works in such a way that a conducting object at the location to be protected will make a short circuit between two adjacent surfaces each connected to separate inputs of the monitoring circuit. This kind of system can only be used for protection of a well defined very limited position and cannot be applied to a moving machine or a general moving part of a machine.

The Finnish publication 57,015 discloses a more general system to protect a person from a dangerous machine. This method implies that the person is wearing an electrically conductive cover, for example a special glove, which is connected to the particular machine by electric wiring, whereby a touching of the elec¬ trically conducting machine will cause a shorting of a monitoring circuit which will stop the machine in such a dangerous situa¬ tion.

In EP-A-0,362,937 is disclosed a similar application using a metal protective glove worn by the operator contacting the machine metallic component. This is particularly directed towards machinery having a rotating tool. Consequently also this system like the previous Finnish disclosed system suffer from the constraint that it is necessary for a person to be electrically conneted by a wire to the machinery to be protected.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method and an operating arrangement to effectively obtain protection detection of especially a person positioned close to a machine in motion or a moving part of a machine which by accident might collide or come into contact with this person without the necessity that this person has to be conneted to any monitoring circuitry by any electric cabling arrangement.

Another object of the present invention is to provide a method and an operating arrangement with the capability to make the total body of the machine sensitive to a collision introducing ohmic contact with an electrically conductive body like a person which is subject to an AC electrically induced magnetic field.

Still a further object of the invention is to provide a method and an operating arrangement which does not necessitate any fence, blocking barrier, or painted limiting lines on a floor as the system according to the present invention will protect any electrically conducting object or a person carrying conductive ohmic clothing when working within the reach of a machine in motion or a moving part of a machine.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

Fig. 1 is a simplified block diagram visualizing a preferred embodiment of the present invention utilizing AC power line induction;

Fig. 2 is a simplified block diagram of an embodiment utilizing the present invention for detection of induced AC signals in an electrically conducting object subject to an AC electrically induced magnetic field.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS It is well known that if a metal conductor is subject to changes in an existing surrounding magnetic field an electric current will be induced in the conductor. This implies that within any object which is electrically conducting there will be induced an electric signal when the object is subject to an alternating magnetic field. The human body is typically to be considered as a conducting object and will be subject to induced potentials and resulting currents where a closed loop is present and will accordingly be considered as an inductive body.

In Fig. 1 is demonstrated a simplified block diagram illustrating the basic principle of the present invention. A power line 1 is present in the vicinity of an inductive object 3, especially a human body. The human body is then subject to an induction field 2 from the power line 1 carrying for example a 50 or 60 Hz alternating current (AC). The inductive object 3 by means of stray capacitance 4 also has a capacitive coupling to ground which acts as reference potential. A conductor or conducting surface 5 connected to an electronic sensor circuitry 7 will receive a small AC current or voltage signal when an ohmic contact at 6 is established between the inductive object 3, (i.e. a naked portion of a human body), and the conducting surface 5. A small AC signal will then be transferred from the inductive object 3 to the electronic sensor 7 through the ohmic contact 6 and conducting surface 7. The electronic sensor, which can be of any well known type of current or voltage sensing electronic circuitry, produces an output signal at 8 as an indication of the transferred AC signal from the inductive object 3 at the established ohmic contact 6. This can simply be demonstrated by touching with a finger the input terminal of a regular audio amplifier which will react by producing a humming sound in a speaker connected to the amplifier. This works everywhere in the neighbourhood of electric power lines carrying 50 or 60 Hz AC. The audio amplifier then represents the electronic sensor 7 and the hum signal to the loudspeaker corresponds to the outputted signal 8 in Fig. 1. This works particularly well with the European 50 Hz power line network as this is by standard a three phase system referenced to ground. But even if the electric AC current being the source of the induction field is not directly referenced to ground the principle in Fig. 1 is still working, because the important point is that the conducting object in which the AC signal is induced and the electronic sensoring system do have a common signal reference. From this should be realized that the stray capacitance to ground plays the important role. Humans always show a capacitive coupling to the environment and especially to ground through their feet. The amount of capacitance will be dependent of the insulating layer represented by the shoes and the amount of surface taking part in the creation of this capacitor 4, which in reality is just a standard plate condenser. The larger the capacitance of that condenser is the larger the coupling factor to ground will be. The extreme will of course be if the person is standing with naked feet on a conducting grounded surface, then there will no longer be a capacitive coupling but an ohmic coupling to ground. The human environment is today such that in any place, except specially extremely magnetically shielded spaces, there will be a numerous amount of different electromagnetic waves of different frequen¬ cies present which will induce di ferent AC signals in the human body.

In Fig. 2 is demonstrated in a simplified block diagram of an embodiment of the present invention for investigating such elec¬ tromagnetic AC fields for example in front of a Cathode Ray Tube (CRT) commonly used as a computer display screen. The present AC field 2 will induce a signal within the body 3 which from the discussion above has a capacitive coupling 4 to a reference ground. At 6 is attached an ohmic transition to the body 3, which connects the body 3 to the high impedance input terminal 20 of a standard operational amplifier 12, in the preferred embodiment a CA 081, produced by several semiconductor manufacturers. The other input terminal 21 of the operational amplifier 12 is, by a feedback resistor 14 connected to the output 22 of the amplifi¬ er for adjustment of the sensitivity of the circuitry. Instead of a fixed resistor R2 a potentiometer may be inserted. A resistor 13 having a high resistance of 20 Mohm or more is connected between the input terminal and ground for internally referencing the high impedance input 20 of the operational amplifier 12 to ground. The low impedance output terminal 22 is driving a standard metering device 30, which will indicate an appearing signal at the inputting point 6 and 20. A person sitting in front of a magnetically non-screened CRT display while connected to the device 7 will observe at the metering device 30 the variations in the magnetically inducing AC field surroun¬ ding the display by just pointing for example an arm in different directions.

The basic principle discussed here is utilized in a second preferred embodiment for protection of a person working close to and within the reach of a machine in motion or a moving part of a machine. In a preferred embodiment the machine constitutes an industrial robot with an arm which when it swings to either side could hit the person if the arm by some reason should swing to a further position to which it during controlled operation not will be able to move. The complete swinging arm is first coated with a non-conductive paint i. e. a two component plastic based paint. On top of the non-conducting layer of paint is then sprayed a conductive paint, in the preferred embodiment compri¬ sing a large amount of very small particles of nickel and a binding agent. The in this manner created conducting surface will be separated from ground and is then connected to the high impedance input terminal of an amplifier similar to amplifier 12 in the first embodiment in Fig. 2. The output of this latter amplifier is connected to the power controlling unit for the robot. This power controlling unit will stop or reverse the motion of the robot's arm upon a signal received from the amplifier output. In the preferred embodiment is further included some additional electronic circuitry to balance a possible AC signal normally induced in the arm itself. If the arm of the robot should collide with the person and an ohmic contact is achieved the electronic sensing circuitry will receive an induced AC signal from the body of the person through the ohmic contact point via the conducting layer and to the input of the circuitry, which will react by putting out a signal to the power controlling unit of the robot, and the power controlling unit will stop or reverse the dangerous motion.

To make certain that any hit between the arm of the robot and the

person who will partly be covered by clothing is sensed by the arrangement according to the present invention, the clothing is in the preferred embodiment prepared to be conductive by using interwoven conducting threads in the clothing. In another embodiment the clothing will be sprayed with an agent which makes the clothing slightly conductive. It is here important to secure that the conductive clothing is in contact with the naked skin of the person to obtain the necessary electric path from the body to the conducting layer of the robot's arm at an occational contact. Accordingly the person does not by any means have to be connected to the robot but can move freely within a normally restricted area but will still be protected from dangerous hits by the arm of the industrial robot close to his working position.

Any detailed description of the electronic circuitry has not been given as this constitutes known technique and does not define the invention but is only an element to emasculate the invention.

The preferred embodiments of this invention should be considered in an illustative and not in a limiting sense and further objects and features of the present invention, which may admit to a number of possible variations, that will be apparent to anyone now familiar with this disclosed technique, and are considered within the scope of the present invention according to the accompanying claims.

Claims

CLAIMS 1. A method for detection of an induced AC field (2) in an elec¬ trically conducting object (3), said method comprising the steps of: providing AC induction means for creating said electrically induced AC field in said electrically conductive object; transferring said electrically induced AC field in said elec¬ trically conductive object as an AC current or AC voltage signal to a second conducting object (5) at the site of an ohmic contact (6) between said second object and said electrically conductive object then using that said electrically conducting object is subject to a capacitive coupling to a common ground by stray capacitance (4); and detecting and measuring at the second object said AC signal transferred from said electrically conducting object to said second object.

2. The method according to claim 1, for protection of said elec¬ trically conductive object wherein said second object is a machine in motion or a moving part of a machine when said object is making ohmic contact with said machine or said moving part, further comprising the steps of: providing electrically conducting covering surface means for said machine or moving part wherein said surface means will be receiving said AC signal upon ohmic contact between said machine or moving part and said electrically conductive object; and providing insulation means between said electrically conducting covering surface means for said machine or moving part and said machine or moving part itself when said machine or said moving part is also made from electrically conducting material.

3. The method according to claim 2, further comprising the steps of: using said detecting and measuring of said electric AC signal as signal discriminating means connecting to the covering surface means for detection of ohmic contact between said machine or moving part and said electrically conductive object transferring said AC signal current; and using said detection of ohmic contact between said machine or moving part and said electrically conductive object for preven¬ ting said machine or said moving part to move any further towards said electrically conductive object by stopping or reversing the motion.

4. An arrangement for application of the method according to claim 1 for detection of electrically induced AC voltage or current signals in an electrically conducting object in the presence of a varying magnetic field, c h a r a c t e r i z e d in electronic circuitry means (7) connected to said electrically conducting object (3), said means comprising a DC powered high impedance operation amplifier (12) including a low impedance output terminal (22) connected to a metering device (30) for presentation of the detected induced AC signal.

5. An operating arrangement for application of the method according to claims 1 to 3 comprising: a machine in motion or a moving part of a machine, power and control means for said machine or moving part, electronic detecting and discriminating means (7), an electrically conducting object (3) subject to an AC elec¬ trically induced magnetic field (3) adjacent to said machine or moving part, c h a r a c t e r i z e d in surface covering electrically conducting means (5) including electric insulation means underneath of said conducting means covering the surface of said machine or moving part and said conducting means connected to said electronic detecting and discriminator means, whereby said electronic detecting and discriminator means having output means (8) connected to said power and control means, and said electronic detecting and discriminating means is detecting any additional AC current or AC voltage transferred to said electrically conducting means in case of ohmic contact (6) with said electrically conducting object.

6. The operating arrangement according to claim 5 further charac- terized in that when said electrically conducting object is transferring an AC signal to said surface covering electrically conducting means at an ohmic contact between said electrically conducting means and said electrically conducting object, said detecting and discriminating means detects this AC signal and outputs a signal to said power and control means, and in that said power and control means is sensitive to said output signal from said detecting and discriminating means for stopping or reversing a progressing motion.

7. The operating arrangement according to claim 6 further charac¬ terized in that said induced AC signal in said electrically conducting object is induced by the presence of power line associated magnetic fields present at the location of said operating arrangement.

8. The operating arrangement according to claim 7 further charac¬ terized in that said conducting means (5) includes an elec¬ trically conductive paint and said insulation means includes an electrically non-conductive paint.

9. The operating arrangement according to claim 8 further charac¬ terized in that in the case of protecting not naked parts of a person the total possible contact surface of said person is made electrically conductive by introducing conductive elements into the clothing of said person, said conducting elements constitu¬ ting a conducting surface and in ohmic contact with the naked skin of the person at least at one point.

10. The operating arrangement according to claim 9 characterized in that the clothing of said person is made conducting by introducing interwoven conducting threads or by spraying of the clothing with a conducting solution.