Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Patent
1991-07-26
1993-09-14
Ro, Bentsu
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
318569, 318432, B23P 1906, G05B 1502, G05D 1702
Patent
active
052452654
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a system for controlling the operation of a motor, and is more particularly of application to the screwing together of tubular components, for example, adjacent sections of tubing string or sub assemblies to the top of a tubing string.
Tubing components are conventionally screwed together using equipment which basically comprises an upper tong and a lower tong. The lower tong has a set of fixed jaws which, in use, securely grip one of the tubing components (usually the top section of a tubing string), whilst the upper tong includes a set of rotatable jaws which firmly grip the other tubing component(s).
The output of a motor, which is usually a hydraulic motor, is connected to the rotatable jaws in such a way that the motor is operable to rotate the jaws, and hence, in use, the tubing component gripped by them. The motor is used to both screw one component into the other, and to tighten them one against the other. Alternatively the motor is used to loosen the components and unscrew the components.
Problems can arise if the threads do not properly inter-engage, as a result of, for example, one of the threads being cross-threaded or of the contamination of the threads with grit. In such a situation, operation of the motor to screw the two components together may not produce an adequately secure connection, and may result in the damage, by, for example, galling of the threads.
In an attempt to reduce the risk of this happening, it is known to link the motor to a monitoring system. The monitoring system operates a device, usually a hydraulic by pass valve. A load cell is used to provide a measure of the torque being exerted, in use, on the tubing component in the upper tong. When the torque exerted passes a pre-determined maximum value, the system operates to shut down the motor.
This approach has the disadvantage that there is no automatic control of the operational speed of the motor or applied torque. Thus, if, for example, the thread on one of the components failed completely, resulting in the sudden virtually complete loss of any resistance to the tightening of the components, the motor may rotate the component held in the upper tong at a dangerously high speed.
In one broad aspect, according to the invention, there is provided a system for controlling the operation of a motor being used for the rotational assembly or dis-assembly of two members, the system comprising means monitoring, and generating a digital or analogue signal representative of the state of the two members, means relaying the representative signal in a digital form to a first processor; and means relaying the signal in an analogue form to a second processor; the first processor comprising a computer, which, in use, digitally analyses the representative signal and is so programmed as to generate a first control signal in response to the representative signal; the second processor comprising means converting the first control signal into analogue form, and means comparing it with the analogue representative signal and generating a second control signal representative of the difference between the first control signal and the representative signal; the second processor being so connected as to transmit the second control signal to the motor control means.
Such a system may be used to control a motor which is operable to rotate, for example, a section of tubing string, and the monitoring means preferably comprise means monitoring the load on the tubing by measuring the torque being exerted by the motor.
In this case, there may also, with advantage, be provided means measuring and generating signal representative of, the angular position of the tubing, the representative angular signal being transmitted, in use, to the first processor. The position measuring means may, for example, include a turn counter which, in use, generates a digital representative signal. The signal representative of angular position may be processed to give an indication of the rotational speed of the tubing being
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Frank's International Ltd.
Ro Bentsu
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