Electricity: motive power systems – Automatic and/or with time-delay means – Rate-of-change of a condition
Patent
1985-06-05
1987-06-23
Shoop, Jr., William M.
Electricity: motive power systems
Automatic and/or with time-delay means
Rate-of-change of a condition
318456, 318286, 318331, 318468, H02P 308
Patent
active
046755861
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to apparatus for detecting obstruction of a motor driven door.
The invention was developed for detecting obstruction of a motor driven garage door during opening and closing operation and will herein be described with reference to that use but it will be understood that the invention is equally applicable to use for detecting obstructing of other types of electric motor operated doors.
BACKGROUND OF THE INVENTION
Motor driven garage doors of the upward opening and downward closing type have been provided with obstruction detection switches operated by means of a trip bar at the lower leading edge of the door. The switch is arranged so as to stop or reverse the door if the trip bar strikes an obstruction during descent. While such means are effective they add to the complexity of the door manufacture and installation and therefore to cost.
It has been practiced to sense the absolute load of the driven system either by reaction of the drive, for example as described in Australian patent specification No. 521,735, or by change in current flow in the motor circuit. Such systems suffer from nuisance trips due to changes in ambient and local conditions or are maladjusted or desensitized so that nuisance trips do not occur but then the system tends to become a safety hazard. When the door is a rolling door then the motor load changes progressively and non-linearly between open and closed positions as a result of friction and making adjustment of trip level even more difficult. Thus, to date, for rolling doors the only satisfactory method for reversing the door when an obstruction is encountered during descent has been the trip bar previously described.
DISCLOSURE OF THE INVENTION
According to one aspect the invention consists in a monitoring circuit for a door operating D.C. electric motor, said circuit comprising first means for detecting the back EMF of the motor and producing a load signal indicative of the magnitude thereof; and second means for detecting the time rate of change of the load signal and issuing an obstruction signal if the time rate of change of the load signal exceeds a predetermined value.
For preference, the apparatus monitors the back EMF continuously and is sensitive to changes occurring during any interval of predetermined short duration.
According to a second aspect the invention consists in a monitoring circuit according to the first aspect further comprising third means for comparing said load signal with a reference signal to issue an obstruction signal if said load signal changes by a predetermined magnitude.
As is well known the speed of a DC motor reduces with increasing load and the back Electro Motive Force (EMF) developed by the motor is proportional to motor speed. Thus if the load is increased the motor speed decreases causing the back EMF to decrease. The present invention provides apparatus which produces a signal responsive to the rate of change of back EMF of the motor. This may be achieved by sampling the back EMF during one time interval and comparing it with a stored EMF detected during a previous time interval the time intervals being at predetermined spacing. In a preferred embodiment a differentiating circuit is used to generate a signal the voltage of which is proportional to the change or back EMF of the motor during a predetermined period, that is to say, the rate of change of back EMF. If the differentiated signal voltage exceeds a predetermined voltage then the circuit trips and an obstruction signal is issued which may be used to stop or reverse the motor.
By way of example only in embodiment of the invention will now be descrioed with reference the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is schematic circuit diagram of a monitoring circuit according to the present invention.
BEST MODE OF CARRYING OUT THE INVENTION
In the embodiment described a steel roller garage door is driven between open and closed positions by a low impedance 24 voltage DC motor energised from a 240 volt: 24 volt step down tran
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patent: 4585981 (1986-04-01), Zintler
Eigner Otto
Jones Richard E.
Byrne & Davidson Industries Limited
Ro Bentsu
Shoop Jr. William M.
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