GB2169105A - Door or gate obstruction control - Google Patents

Abstract

An obstruction sensing and control arrangement for door or gate opening-closing mechanism compares at CB the rate of change of a predetermined characteristic of the motor M with a preset rate of change and if exceeded stops or reverses the motor M in the opening/closing mechanism. The characteristic preferred is motor body temperature which is monitored indirectly. The control arrangement also includes a back up static control threshold to sense a second static operating characteristic of the motor and a second comparator CA means set at a preset threshold level to stop or reverse the motor. A common motor sensor is used for both the dynamic and static control thresholds. The static threshold level may be altered automatically in response to temperature changes. <IMAGE>

Description

SPECIFICATION Door or gate obstruction control The present invention relates generally to power operated gates and doors and moreparticularly to a safety control arrangement associated with such doors and gates.

It has become reasonably common in recent times to provide garage doors and property access gates with power operated opening and closing mechanisms. Commonly such arrangements include a suitable operating mechanism (dependent upon the nature of the door or gate being opened or closed) and an electric motor (which might be an AC or DC motor) to power the operating mechanism.

The operating mechanism might be directly controlled by suitable switching associated with the equipment or it might be remotely controlled using radio signals directed to a suitable receiver arranged with the mechanism.

The more common usage of remote control arrangements with such equipment has led to the substantial danger of seriously injuring persons or equipment which might be accidently located in the path of movement of the gate or door. For example the operating mechanism could conceivably be operated either intentionally or unintentionally from a position beyond the view of the operator and in such circumstances it would be possible for a person such as a child or perhaps a less agile elderly person to be seriously hurt by the door or gate. There have been many proposals for providing safety mechanisms with such doors and gates to try to avoid this danger.One such arrangement required the positioning of a mechanical trip bar which would be hit by any obstruction in the path of travel of the door or gate, the trip bar being operably connected to a suitable mechanical linkage for actuating a switch to either stop or perhaps reverse the operating motor. A similar concept in a mechanical system where a chain drive for the door or gate was employed, used deflections in the chain indicative of an obstruction being hit, to physically actuate suitable switching to stop or reverse the electric operating motor.

These mechanical systems have been found in practice to lack sensitivity and reliability and moreover are relatively complicated and expensive to produce.

A second approach to this problem recognized that when a significant obstruction was engaged by the door or gate, the operating motor would draw significantly more current than was usual. Thus monitoring motor current, theoretically, gave some idea of whether or not the door or gate was operating freely.

In a known static safety control system, it has been known to monitor motor current (after a predetermined start-up period) during the period when motor current should be theoretically relatively stable and to compare this current to a present value such that if this preset current is exceeded indicative of an obstruction being engaged, a suitable control actuates to either stop or reverse the operating motor. While this known system is theoretically functional, it has proved to have significant problems in practice. In practice the current drawn by the motor during the opening or closing operation is never uniform. For example, on very cold days, lubricating grease will be vastly stiffer than on hot days thereby resulting in significant friction increases. Similarly wind loading varies considerably and has significant effects on loading of the door or gate during opening or closing.Furthermore, the current drawn by the motor running in one direction could vary up to 10% from that which would occur with the motor running in the reverse direction regardless of all these factors. In addition with certain types of doors or gates (typically lifting and tilting garage doors) the load during opening and closing is itself not constant and the theoretical current drawn by the motor increases substantially midway through its cycle.

To achieve a sensitive safety control operation is is desired to place the set point as close as possible above the theoretical current drawn, however, because of the above factors, it has been found in practice that the set point must be placed greatly above the theoretical current drawn to avoid the safety control operating when it is not necessary. Often this has resulted in the safety control set point being set that high that it has very little appreciable effect in controlling the door or gate when an obstruction is in fact met.

The principal objective of the present invention is to provide a safety control for door or gate operation mechanism of improved performance characteristics which will be activated by the door or gate effectively meeting an obstruction in its path of travel. A so called "dynamic set point" or dynamic control threshold is achieved effectively by sensing rate of change of operating characteristics of the operating motor of the door or gate mechanism which follows the normal motor load characteristic curve.

Accordingly the present invention provides an obstruction control arrangement for a door or gate opening/closing mechanism having an electric operating motor, said control arrangement comprising first motor sensor means adapted to sense a first operating characteristic of said motor, circuit control means arranged to determine an operating rate of change of the first operating characteristic sensed by said motor sensor means, first comparitor means arranged to compare said operating rate of change of the operating characteristic with a preset rate of change level, and activating means arranged to stop or reverse said electric motor in response to an activating signal from said comparitor means upon said operating rate of change of the operating characteristic exceeding said preset rate of change level being indicative of said door or gate being obstructed in its normal course of travel, Conveniently the control arrangement further includes filter means arranged to pass signals only in a predetermined frequency range from said motor sensor means to the comparitor means. The frequency range transmitted would normally be less than 10 Hertz and preferably in the range of 5 to 10 Hertz.

In accordance with a preferred embodiment the control arrangement further includes a back-up static control threshold of the type including a second motor sensor means adapted to sense a second static operating characteristic level of said motor and second comparitor means arranged to compare the sensed static operating characteristic level with a preset static threshold level whereby an activating output signal is generated upon said sensed static operating characteristic level exceeding said preset threshold level and is delivered to said activating means to stop or reverse said electric motor. Conveniently a common motor sensor means is used for both the dynamic control threshold and the static control threshold.The arrangement may further include temperature compensation means arranged to sense operating temperature and to move the preset static threshold level automatically in response to temperature changes.

Preferably the temperature compensation includes a thermal sensor such as a Silicon diode or a Germanium diode arranged in a voltage divider network used to establish the present rate of change level for the comparitor means.

In a preferred embodiment the operating temperature of the motor body is monitored indirectly as it is not desirable to have the thermal sensor physically attached to the motor body as this is most often remote from circuitry associated with the control of the door or gate opening/closing mechanism and would require additional wires to carry the required information back to the main circuitry.

Accordingly the present invention preferably proposes using the thermal sensor in contact with a power supply bridge rectifier for the operating motor as the entire motor current must flow through this bridge rectifier. Consequently it follows that the temperature of the bridge rectifier will rise in proportion to that of the remote operating motor. It is, however, desirable that the thermal inertia of the operating motor and the bridge rectifier circuit, be perfectly matched to achieve optimum results.

Simultaneously, the same thermal sensor will also sense ambient temperature changes and consequently compensation is achieved for operating temperatures resulting from both ambient changes and changes due to the operation of the equipment.

A possible alternative location for the thermal sensor could be against a series resistor used to monitor the operating motor current.

However, this is not quite as good a location as the power supply bridge rectifier circuit as the temperature rise of the monitoring resistor is much lower than the rectifier circuit where the power developed in heat is much higher.

In accordance with a further preferred embodiment, the first sensor means comprises at least one sensor arrangement adapted to sense the first operating characteristic of said motor at repeated predetermined time intervals, said circuit control means including a first circuit operating to determine an actual change of the operating characteristic over said predetermined time intervals and said first comparitor means including a second circuit operating to compare said change of operating characteristic over said predetermined time intervals with a preset change whereby said activating signal is generated if said actual change exceeds said present change.Conveniently the first sensor means comprises a plurality of said sensor arrangements each being arranged to sense the operating characteristic at said predetermined intervals of time, all the sensor arrangements subsequent to a first one of said sensor arrangements commencing their respective predetermined time intervals within the predetermined time interval of said first sensor arrangement whereby the sensor arrangements are staggered with respect to one another so as to reduce the response time required to activate said control arrangement.

Preferably the preset change is determined by accumulating incremental changes in the operating characteristic over a predetermined number of said time intervals immediately prior to the actual change of the operating characteristic compared at the end of any said time interval.

In some embodiments of this invention there is provided inhibitor means arranged to inhibit operation of the control arrangement during an initial time period of each operating cycle.

The present invention will now be described with reference to several preferred embodiments illustrated in the accompanying drawings, in which: Figure 1 illustrates typical theoretical and practical operating characteristics of a door or gate operating mechanisms during both opening and closing cycles of operation; Figure 2 shows, in block diagram form, a control arrangement in accordance with a first preferred embodiment of the present invention employing both a dynamic threshold control and a temperature compensated back-up static control; Figure 3 shows typical circuitry for a temperature compensated static control system; Figure 4 shows typical circuitry for a dynamic tracking control system; Figure 5 shows typical circuitry for a control system employing both temperature compen sated static control and dynamic tracking control;; Figure 6 illustrates graphically operating characteristics according to the present invention; Figure 7 shows a circuit diagram illustrating a second preferred embodiment of the present invention; Figure 8 illustrates graphically the operation of an embodiment of the type shown in Fig.

7; Figure 9 illustrates graphically the operation of a control arrangement similar to Fig. 7 but incorporating multiple sensors adapted to sense operational characteristic changes over predetermined intervals with the intervals commencing in staggered time relationship relative to one another; and Figure 10 shows graphically a typical operating motor current variation during a door opening cycle with a control arrangement according to Fig. 7.

Fig. 2 of the drawing schematically shows in block diagram format a first preferred control arrangement employing two independent control level sensing modes of operation. In the drawings M represents the operating motor for a gate or door opening/closing mechanism. Rs is a sensing resistor used to sense load or current drawn by the motor M. LPF represents a filter adapted to pass signals in a low frequency band (typically from 5 to 10 Hz) from the sensor Rs to the control circuit.

An amplifier A amplifies and transmits the signal from the low frequency pass filter LPF to a temperature compensated comparitor CA and separately to dynamic tracking controlled comparitor CB which is adapted to compare rates of change of the current drawn by the motor M with a preset level. Naturally, if desired the comparators CA and CB could be used in quite separate control arrangements.

Each of the comparitors CA and CB generate an activating signal in response to the load sensed by Rs exceeding a predetermined limit indicative of the door or gate hitting an obstruction. The activating signals are then combined by an OR gate combiner circuit C and used to either stop or reverse operation of the motor M. Finally an inhibitor circuit In is used to inhibit the control circuit for a predetermined period at start up of the operating motor for reasons which will be explained hereinafter.

Typical load operating characteristics of the motor M are shown in the drawings (Figs. 1 and 6). Fig. 1 shows both an opening and a closing characteristic which can be up to 10% different from each other. These graphs show motor current I against time but it will be appreciated that other operating characteristics might also be employed. Immediately upon start up there is a rapid rise in motor current drawn which peaks quickiy and settles back almost as quickly into a theoretical substantially uniform load current. The full line graph (a) in Fig. 1 demonstrates a typical theoretical opening and closing load curve against time t.

The critical start up period is irrelevant from a safety viewpoint and moreover the high load experienced in this period makes monitoring of the load pointless with regard to safety control. For this reason the inhibitor circuit In is included to override the control circuit for the initial start up period ts. Thereafter the control circuit becomes effective. In the conventional static control system a constant set point SP is established above the theoretical load curve such that if an obstruction is hit by the door or gate, the load curve rises rapidly as indicated at (c) and the door or gate is stopped or reversed.However as indicated in the introduction to this specification, the practical load curve (b) for many reasons will vary considerably from the theoretical curve (a) and it will be obvious from these graphs that either the set point SP must be located too high for safe operation or the control system will activate itself when there is in fact no obstruction.

According to a preferred aspect of the present invention it has been recognized that the rotary action of the armature of the motor M with the resulting commutation of the brushes against the commutator give rise to finite opening and closing of the motor circuit at a relatively high "chopping" rate. Typically the frequency of this occurs at around 300 per second but of course this high frequency chopping rate depends on the speed of the motor which is variable. In addition where the primary power source is derived from the 240V main supply there will also be high components of 50Hz and 100Hz. It is also recognized that frequencies resulting from an obstruction being hit by a door or gate will occur with a relatively well defined low frequency range width (normally within the range of 5 to 10Hz).In the proposed system the aforementioned chopping high frequency component (and in other higher frequency component) is filtered out leaving only a DC average and frequency components up to a predetermined level, typically 10Hz. This is achieved by the low frequency pass filter LPF shown in the drawings.

Fig. 6 of the drawings illustrates graphically the performance of the control system according to the preferred arrangement of Fig. 2.

Firstly there is provided a movable constant static set point Ssp. The set point is movable in response to temperature changes and in particular to both sensed ambient temperature changes AHS and sensed motor temperature changes MHS. If the temperature is generally cold then the motor is likely to draw more current and thus the set point Ssp is moved upwardly. Conversely if the temperature is hot the motor is likely to draw less current and the set point Ssp will move down. Conveniently both ambient and motor temperature changes can be sensed by single temperature sensing Silicon or Germanium diode Di. The positioning of this diode is conveniently as previously discussed.As shown in Fig. 3 the signal from the motor sensing resistor Rs is filtered and amplified by an amplified A and supplied to one input terminal of a non inverting voltage comparitor IC2. The temperature sensing diode Di directs its signal to the other terminal of the comparitor and in this manner an automatic variation of the set point Ssp is achieved in response to temperature variations. Normally when the static temperature compensated control is being used as back-up in a combined system of the type shown in Figs. 2 and 5, the basic set point will be arranged at a relatively high point such that it would only be used should the dynamic control fail for whatever reason.

Fig. 6 also illustrates the automatic tracking or dynamic threshold capability of the filter ATF to provide an automatic dynamic set point Dsp above the actual load (b) regardless of changes in this load. Thus in the monitoring zone beyond the initial inhibit period ts should an obstruction be hit by the door or gate, the low frequency increase in load is passed by the filter LPF to the control system comprising the tracking filter ATF and the comparitor CB.

The load thus follows the curve (c) and the comparitor CB will sense when the load exceeds the dynamic set point Dsp and will send a signal in response thereto to stop or reverse the motor M.

Suitable circuitry for the automatic tracking filter ATF is shown in Fig. 4. This circuit maintains its DC output at Vbb/2 for all input components up to 5Hz such that it is not responsive to these components and more particularly maintains its output at Vbb/2 regardless of the absolute magnitude of the magnitude of the DC input. Its response is basically a high pass filter and allows through only those motor "impulse" components within the range 5Hz to 10Hz.

Experiments have shown that for most applications requiring a motorized door/gate to sense an obstruction, the impulse components predominantly lie within this spectrum. If required the band of interest could be lowered by changing R3/C1 and raised by appropriately decreasing the component values of the input filter LPF. The combination R3/C1 gives this filter the required tracking characteristic in that the voltage across Cl follows (with some time delay) the input voltage being examined.

The integrated circuit IC3 arranged in the manner illustrated is in fact an Inverting Amplifier such that an impulse rising at Vin will cause the output to fall from its quiescent Vbb/2 level toward ground potential and vice versa. The gain (AC gain) is determined by the ratio of the components R2/R,. The resistors R, and R3 should preferably be equal and similarly the resistors R2 and R4 should preferably be equal to achieve good common mode operation. Should the components not be matched in this manner, the output voltage will not remain itself at Vbb/2 for all input components up to 5Hz and consequently erratic operation could result. Normally components matched within +2% would provide satisfactory operation.The magnitude of the output impulse response has been found to be adequate to trigger the following level comparitor IC4 of the comparitor circuit CB, if the gain of the integrated circuit IC3 is set within the range of 5 to 10. It is preferred for the capacitor C1 to be a low leakage type whose value could be of the order to 10 microfarad when R, and R3 are of the order of 100K ohm. The time constant R3Cg is typically 0.1 second but could be lowered to 0.5 second if required.

Once a sufficiently high impulse component at the input has been allowed through to the tracking filter ATF and consequently causes the output to swing below a predetermined value as set by the comparitor CB, then an overload signal is generated which will stop the motor M. Subsequently, depending on the existing motor control logic, either a dwell will result or the motor will be instantly reversed.

In either case the output of the tracking filter ATF will over shoot in the opposite direction, however, this is of no concern to the circuitry as an inhibit signal from the Inhibitor In holds the total response of the control circuitry in an "off" state.

Fig. 7 of the accompanying drawings shows a second preferred control arrangement in accordance with the present invention. In this embodiment, the control arrangement comprises circuitry which monitors the rate of change of current (i.e. the slope of the current versus time graph) during the operation of opening the door or gate. The measured rate of change of current is then compared with a preset rate of change, and if this preset rate of change is exceeded the safety control circuit will reverse or stop the operation of the motor. Therefore by comparison with a system which requires the motor current to reach a static threshold point, the present system will sense when the motor current is likely to exceed a threshold value thereby allowing much quicker operation of the safety circuit.

Fig. 7 of the drawings shows a circuit using a rate of current change measurement method. The current drawn by Motor M is monitored by measuring the voltage across a resistor Rsense, which is in series with the motor coil. This voltage reading will be proporitional to the current being drawn by the motor. This signal is then fed to an amplifier A via a low pass filter LPF. The low pass filter has a cut off frequency typically of 10 Hertz.

The voltage signal is then amplified and inverted and subsequently becomes an input to the comparator. The other voltage input to the comparator is generator by an R-C integrator.

Both the comparator and R-C integrator are connected to a microprocessor at points A and B respectively. Point B provides a pulsed output varying between a logic "high" voltage and a logic "low" voltage. When the output of point B goes high the transistor Q1 is switched on, thereby discharging capacitor Clef.

Upon output B going low the transistor Q1 is switched off and a counter internal to the microprocessor is initiated. When the transistor Q1 is switched off the voltage V,,, increases from a zero value. This rate of increase is controlled by the Rre,-C,e, time constant (i.e.

the R-C integrator). The output of the comparator V,,, remains at a logic "high" level until Vre, exceeds the voltage value of the other comparator input, V,,, whereupon V,,, goes to a logic "low" level. When the output voltage of the comparator, V,,, goes to a logic "low" level, the internal counter of the microprocessor is stopped and the value of this counter is thereby proportional to the motor current. This count is used as a digital number representing the motor current for that particular measurement interval.

The microprocessor is programmed to calculate the actual rate of change of current of the motor M, to compare this rate of change with a preset rate of change, and to activate the control arrangement of the motor M in the case where the actual rate of change of current of the motor M exceeds the preset value.

The value of the motor current is sampled by the microprocessor at a rate of 500 Hertz (i.e.

at a 2 msec intervals). Four such motor current samples are accumulated by the microprocessor to obtain an accumulation of current change over those four sample points thereby magnifying and noise averaging the change in current. The microprocessor uses the first sample point as the base value of motor current and to this is added the increment in current that will give the preset rate of change. The microprocessor will then compare the actual rate of change with this preset rate of change, and if the actual rate of change exceeds the preset rate, the safety control circuits will be activated.

Fig. 8 shows a graphical representation of the operating characteristics of the control arrangement in accordance with Fig. 7. The graph illustrates a portion of cycle showing motor current I against time and the slope measurement time intervals as shown on the graph represent the time intervals in which the rate of change of motor current is determined.

Each set of four samples are compared with the present rate of change. As shown in this graph when the actual rate of change equals or exceeds the preset overload value during that time interval the safety control system is activated. Also shown on this graph are two horizontal lines one representing the average current level and another representing a trip current. The trip current line is positioned at a level equivalent to a maximum current which is not to be exceeded by the motor, and is typically the current level used by a safety control system which simply monitors current levels. It can be seen from Fig. 8 that the slope comparison technique will enable the safety control circuits within a considerably shorter time that a system using only instantaneous current comparison techniques.

In order to further reduce the response time of the safety control system, a further embodiment of this invention incorporates a plurality of sensing means which have staggered measurement time intervals. Fig. 9 shows a graphical representation similar to Fig. 8 of the operation of such a system. As can be seen from Fig. 9 the sensing means circuits 1, 2, 3, and 4 are arranged such that they operate in parallel, but have their slope measurement time intervals staggered in relation to one another, i.e. the measurement time intervals of circuit 2 commence slightly after circuit 1, the measurement time intervals of circuit 3 commence slightly after circuit 2, the measurement time intervals of circuit 4 commence slightly after circuit 3 and at the end of the circuit 4 time measurement period, circuit 1 again commences measurement.As can be seen from the graph the current draw of the motor M begins to be excessive at point A. However, the measurement time intervals of circuit 1 would produce an average current slope less than the required overload slope even though the measurement time interval overlaps point A. The same occurs with circuit 2. However circuit 3 inciudes enough data points to give a measured slope which equals or exceeds the overload slope and therefore activates the safety control circuit. It can be seen from the graph that if circuit 1 was the only sensing means being used it would require a further measurement time interval so as to sense the overload slope, and would therefore result in a significant delay in the activation of the safety control circuits.It can be seen therefore, that by using multiple sensing means which have staggered measurement time intervals, the reaction time of such a system can be greatly improved.

Fig. 10 shows a curve of current I vs time t during the opening of a door, and illustrates the variation of the current level during the cycle from closed to open. It can be seen that part way through the cycle, the current decreases, and in relation to the trip current level, conventional safety circuit would have an increased response time to an overload or obstruction (and hence force), thereby resulting in poor sensitivity. Superimposed over the current level are a representative sample of preset overload slopes, which show the rate of current increase required to operate the safety control circuits. The graph demonstrates that this type of system has a constant degree of sensitivity, since the rate of change measurement system "follows" the current curve providing a dynamic tracking control similar to the embodiment of Fig. 2.

The current peak shown in Fig. 10 results during motor start up. During this period, the safety control circuit is provided with a sensing means to control the operation of the motor and the safety control circuit. This sensing means monitors the rise and faill of the start up peak, checking that it both reaches a peak, and falls again to normal operating conditions.

If the motor stalls, the current peak will not fall and therefore the safety control circuit will prevent further operation. Also, if as a result of some other failure, the motor fails to start or we have a failure in the measuring circuit, then again the safety control circuit will prevent further operation. This will be the case for both directions of operation.

The use of such a system has several advantages over previously used static current sensing means. Because the measurement of current slope is a relative measurement no complicated field adjustment is required upon installation to check and adjust set points. The overload rate of current change can be simply programmed into a microprocessor, and the use of such microprocessors will also greatly reduce the cost of such a control circuit and allow other functions such as switching, to be incorporated in its operation. The use of a safety control arrangement which employs a rate of current change sensing system means that normal variations in current drawn by the motor, which result from wind loading or variation in bearing friction, guide effects will not result in activation of the safety control circuit, and only those current variations which are excessively high will result in activation of the safety circuit. In addition the use of multiple sensing circuits will result in a very fast response time, where only 0.15 of a second is required to sense and produce action as a result of an overload. Also the overload sensitivity will be the same for both directions of travel of such a door or gate, the force required to activate the safety circuit will be constant over the full operating range, and the system will automatically adapt to any number of drive motors used.

Claims (13)

1. An obstruction control arrangement for a door or gate opening/closing mechanism having an electric operating motor, said control arrangement comprising motor sensor means adapted to sense a first operating characteristic sensed by said motor sensor means, first comparitor means arranged to compare said operating rate of change of the operating characteristic with a preset rate of change level, and activating means arranged to stop or reverse said electric motor in response to an activating signal from said comparitor means upon said operating rate of change of the operating characteristic exceeding said preset rate of change level being indicative of said door or gate being obstructed in its normal course of travel.

2. An obstruction control arrangement according to claim 1 further including filter means arranged to pass signals only in a predetermined frequency range from said motor sensor means to the comparitor means.

3. An obstruction control arrangement according to claim 2 wherein the predetermined frequency range is from 0 to 10 Hertz.

4. An obstruction control arrangement according to any one of claims 1 to 3 further including a further motor sensor means adapted to sense a second static operating characteristic level of said motor and second comparitor means arranged to compare the sensed static operating characteristic level with a preset static threshold level whereby an activating output signal is generated upon said sensed static operating characteristic level exceeding said preset threshold level and is delivered to said activating means to stop or reverse said electric motor.

5. An obstruction control arrangement according to claim 4 wherein a common motor sensor means is used for said first mentioned and second further motor sensor means such that one operating characteristic of said motor is sensed thereby.

6. An obstruction control arrangement according to claim 4 of claim 5 further including temperature compensation means arranged to sense operating temperature and to move the preset static threshold level automatically in response to temperature changes.

7. An obstruction control arrangement according to claim 6 wherein said temperature compensation means comprises a thermal sensor located in thermal exchange relation with a power supply bridge rectifier for the motor.

8. An obstruction control arrangement according to any one of claims 1 to 3 wherein said first mentioned sensor means comprising at least one sensor arrangement adapted to sense the first operating characteristic of said motor at repeated predetermined time intervals, said circuit control means including a first circuit operating to determine an actual change of the operating characteristic over said predetermined time intervals and said first comparitor means including a second circuit operating to compare said change of operating characteristic over said predetermined time intervals with a preset change whereby said activating signal is generated if said actual change exceeds said preset change.

9. An obstruction control arrangement according to claim 8 wherein said first mentioned sensor means comprises a plurality of said sensor arrangements each being arranged to sense the operating characteristic at said predetermined intervals of time, ail the sensor arrangements subsequent to a first one of said sensor arrangements commencing their respective predetermined time intervals within the predetermined time interval of said first sensor arrangement whereby the sensor arrangements are staggered with respect to one another so as to reduce the response time required to activated said control arrangement.

10. An obstruction control arrangement according to claim 8 or claim 9 wherein the present change is determined by accumulating incremental changes in the operating characteristic over a predetermined number of said time intervals immediately prior to the actual change of the operating characteristic compared at the end of any said time interval.

11. An obstruction control arrangement according to any one of claims 1 to 9 further including inhibitor means arranged to inhibit operation of the control arrangement during an initial time period of each operating cycle.

12. An obstruction control arrangement according to any one of claims 1 to 10 wherein the operating characteristic is current drawn by said motor or a voltage drop characteristic of said motor.

13. An obstruction control arrangement for a door or gate opening/closing mechanism substantially as hereinbefore described with reference to the accompanying drawings.