This invention pertains to apparatus for controlling an irrigation system having a self-propelled distribution line including conduit sections pivotally joined in end-to-end alignment and drivers for driving the sections. More specifically, it pertains to such a control apparatus having conduit misalignment indicators connectable to a pair of joined conduit sections, sensors mounted on the indicators for generating electrical signals indicative of the relative alignment of the associated conduit sections, and a circuit responsive to the sensor signals for generating a control signal for controlling operation of the driver.
Although the present invention may be used with other types of alignment control applications, it is particularly intended for use in a self-propelled sprinkler irrigation system having a plurality of end-coupled irrigation conduits which pivot collectively about a central point. Each of the sections is supported by a tower having a motorized driver which rotates the irrigation line around the center pivot point. As is apparent, the outer towers must travel relatively further than the inner towers.
Each of the towers is normally supported on wheeled carriages which are driven by reversible electric motors. In order to provide a controlled uniform water distribution it is important that the conduit sections be maintained in substantial linear alignment.
A center-pivot system typically substantially continuously drives the outermost conduit tower. The less distant towers are then driven intermittently as required to maintain the irrigation line in alignment.
In such linear irrigation systems, a conduit tower remains in a position until the two associated joined conduit sections indicate that the tower has assumed a lagging position. The tower motor is energized to drive the tower to bring the conduit sections into either an aligned position or into a leading misaligned position. The motor is then turned off to let the tower again stand until it is in a lagging position. Following this procedure for all of the conduit sections, the overall system is maintained in general alignment.
Various control apparatus have been developed to detect the relative alignment of adjacent conduit sections and, thereby, the relative position of an associated tower. One such system uses a strain gauge mounted on a bar which extends between the ends of adjacent conduit sections and is fixedly fastened to the sections. The amount of bend sensed by the strain gauge provides an analog determination of relative alignment. This however, requires that potentiometers used in the apparatus be adjusted to compensate for changes in values caused by temperature changes.
Another control system is disclosed in a patent to Sandstrom et. al., U.S. Pat. No. 3,823,730, entitled "Alignment Control System". In this system a pair of photodiodes and photosensors are attached to one conduit section and a shield attached to the other section. As the two pipe sections move out of alignment the shield shifts away from the photosensors, allowing them to function. Alternatively, a potentiometer sensor may be used which provides an analog signal indicative of the relative alignment of the two conduit sections.
Another type of control apparatus is disclosed in the patent to Fraser et. al., U.S. Pat. No. 4,073,309, entitled "Irrigation Apparatus" which discloses the use of a magnetic reed switch and a permanent magnet which again has a movable shield interposed them which is attached to a different conduit section than the reed switch/magnet combination. This system includes one reed switch to indicate when misalignment occurs regardless of the direction and a second reed switch which indicates when there is excessive misalignment. This system activates the motors when misalignment occurs on the presumption that misalignment has occurred in a lagging direction. The driver is operated until the system returns to alignment.
The use of analog-type control apparatus, such as the strain gauge or potentiometer, typically have problems because the values of critical circuit components, particularly resistors, vary with changes in ambient temperature. Thus, they must be readjusted often. Further, these apparatus operate to varying degrees of sensitivity depending on the condition of the components. The apparatus discussed in Fraser et. al. only has a single sensor which indicates alignment or misalignment so that the direction of misalignment is not directly determined. When it is operated the associated tower is brought into alignment and then stopped.
It is therefore, desirable to have digital electronic controllers which are generally insensitive to variations in values of circuit components. Such a system is disclosed in the previously mentioned patent to Sandstrom et. al. However, this system uses photodetectors which are shielded when the conduit sections are in alignment. This system therefore produces an indication of alignment when there is a failure of the photodiodes associated with the apparatus to conduct. Further, environmental contaminants such as dust can interfere with the proper operation of such a system by blocking the light path. This system also provides for direct connection of the sensing elements to associated conduit sections. The sensitivity of such controllers is limited by the physical size of the actual sensors used.