Pivot

The invention relates to a device to maintain the alignment of two articulated elongate members, particularly adjacent spans of a mobile irrigator or pivot, there being a flexible joint between the spans, connecting them together and having freedom of rotation about the x- and y- axis and limited freedom to rotate about the z-axis, the joint including a sensor device for sensing movement and converting same to electrical energy for feeding to a computer for correcting alignment. The sensor may be a Hall Effect device or an optical sensor device.

The invention relates to a pivot that is a mobile irrigator that is a 
linear irrigator that usually rotates about a fixed "mast" at one end, or 
which is mounted for linear movement in a direction substantially at 
90.degree. to the length of the irrigator, and particularly to a mobile 
irrigator and alignment system. 
Whichever kind of irrigator is contemplated, they are usually made up of 
separate spans or parts of the irrigator mounted for movement on 
intermediate mobile means such as wheeled towers between either the pivot 
and an end tower, or towers at the opposite ends. Each span is connected 
mechanically and fluidically that is to say each span is in fluid 
communication so that water, effluent or the like for irrigation can pass 
from one end of the pivot to the other for distribution over the ground to 
be irrigated. There is usually a cam and microswitch arrangement which 
attempt to control alignment. However, each pivot can be several hundred 
or even thousands of meters in length. 
The pivot is thus a relatively massive structure and this leads to problems 
of alignment which the conventional cam and microswitch arrangement cannot 
satisfactorily control. One section or another can become out of line, 
leading to positive or negative "bowing", or positive or negative 
"dog-legging". This in turn leads to difficulties in movement over the 
ground being irrigated, uneven irrigation and sometimes loss of mobility. 
Moreover, over such a length there is often a pressure drop from one end 
of the pivot where the irrigating liquid (generally water enters, to the 
other, and this again leads or can lead to uneven irrigation. 
It is accordingly an object of the invention to seek to mititage these 
disadvantages. 
According to one aspect of the invention there is provided a device to 
maintain the alignment of two articulated elongate members, comprising a 
flexible joint adapted to be connected between the members and having 
freedom of rotation about the x- and y- axis and limited freedom to rotate 
about the z-axis. 
The device may include a means to detect movement of the joint. 
The sensor means may comprise an optical sensor device adapted to sense 
movement of the joint and provide a commensurate electrical signal. 
According to a second aspect of the invention there is provided a mobile 
irrigator comprising a plurality of spans articulated together, at 
connection points, sensing means mounted at the connection points for 
securing the relative alignment of the spans, and control means adapted to 
respond to output for the sensing means and control the alignment of the 
spans. 
Using the invention it is possible to provide a pivot or linear mobile 
irrigator in which the spans are aligned, that is to say the spans stay 
aligned in an imaginary line drawn between the opposite ends of the 
irrigator. 
The sensing means at each span may be a Hall Effect means. This provides a 
simple, temperature independent means for providing an electrical output 
which can be picked up and utilised by the control means. 
The control means may be connected with a remote computer.

Referring to the drawings, in which like parts are indicated by like 
reference numerals there is shown in FIG. 1 part of a centre pivot 
irrigator 1 which pivots about a central mast or pivot 2 and which 
comprises a plurality of separate spans 3 supported on intermediate mobile 
towers 4 driven by a motor 5 and articulated together at pivot points 6. 
Only one span 3 is shown in FIG. 1. Each span 3 comprises a pipe 7 for 
distributing irrigation fluid such as water, supported by a steel 
framework 8. 
The end span of the irrigator 1, that is the one furthest from the centre 
pivot is supported on an end wheeled tower or frame and effectively 
controls the speed of the irrigator over the ground. 
Referring to FIG. 2, there is shown schematically a system 9 for 
controlling a centre pivot 1 like that shown in FIG. 1. 
The last tower as previously stated controls the speed of the irrigator and 
is started directly by a master control unit 10. The intermediate or slave 
towers sense their alignment with the immediately adjacent one and when 
any error exceeds a preset limit (20') their motor is started and is only 
stopped when the error is eliminated. 
The master control unit 10 is located at the centre point 2. This allows an 
operator to communicate with and program the system via say a 16 key 
keyboard 11 and alphanumerical display, or by connection to a remote 
computer it also monitors the irrigator and in the event of a failure 
closes down the machine and warns the operator. 
Referring to FIG. 3 each span is connected by a flexible joint 15 which is 
an integral part of the sensing element. It will be understood that for a 
centre pivot (or linear irrigator) to operate there must be freedom of 
rotation at each joint about the x- and y- axis as well as limited freedom 
to rotate about on the z-axis to accommodate yaw. 
FIG. 3 shows the general arrangement of the sensing joint 15. Two adjacent 
spans are coupled to a steel block 16 one by a cleve arrangement 17 which 
allows freedom of rotation about the x-axis and the other by an 
arrangement 18 which allows freedom of rotation about the y-axis. 
Arrangement 18 is fixed directly to the span pipe 7 and is located to the 
block 16 by a pin 19 which passes through the block 16 and extends below 
it. Cleve 17 is held to the span pipe 7 by two bolts 20 which pass through 
two ball sockets which allows for a limited degree of yaw. 
A linear Hall effect sensor 21 is mounted in a non-magnetic body 22. A 
cylinder 23 which contains a small magnet 24 fits into the body 22. Four 
screws locate the whole arrangement to the steel block 16, a pin 19 
passing through the cylinder 23. With the two adjacent spans in a required 
straight line the cylinder 23 is rotated until the magnet 24 is directly 
opposite the sensor 21 so that maximum flux density is recorded. The 
cylinder 23 is then clamped to the pin 19 by a grub screw (not shown). 
Since pin 19 is fixed to arrangement 18 as well as cylinder 23 a rotation 
of 18 about the y axis corresponding to a movement of the span to which 18 
is fixed will mean a rotation of cylinder 23 the magnet 24 will move away 
from the sensor 21 and a different magnetic flux density will be recorded. 
In this way the alignment of one span to an immediately adjacent one is 
sensed. 
The sensor 21 is a two channel linear Hall device. As magnetic flux density 
within the range of the sensor 21 increases so the voltage output of one 
channel will increase while the other will decrease. When the magnet 24 is 
directly opposite the sensor 21 one channel will have a maximum output 
while the other will have a minimum output. The two channels are fed to 
four voltage comparator. These are set as follows: 
Comparator 1, set to give a logic one when the flux density corresponds to 
the desired error in alignment at which point the tower will seek to 
reduce the alignment error to zero. 
Comparator 2, set to logic one when maximum flux density is reached i.e. no 
error in alignment, at this the motor will switch off. 
Comparator 3, is set to give a logic one when the flux density is less than 
maximum but more than the threshold of comparator 1. Should the tower have 
received a change in direction signal from the master control the motor 
starter will be isolated until a logic 1 is received from comparator 2 
should no signal have been received from the master control unit then the 
output of this comparator will be ignored. 
Comparator 4, is the safety limit and is set to give a logic one at a flux 
density less than comparator 1, corresponding to an unacceptable error in 
alignment. A logic one from this comparator will isolate the motor starter 
and send a signal to the master control unit to shut-down the complete 
machine. The tower number from which this signal originated will appear on 
display at the master control unit. 
##STR1## 
SAFETY SYSTEM 
The safety system is linked to the main contactors and should the master 
control unit receive any signal from a safety circuit then the main power 
contactors will be released closing down the whole machine. The main 
safety components of the control system are: 
1) Alignment, should the error in alignment exceed a preset limit then the 
machine will shut-down and display the tower which activated the safety 
circuit. 
2) If the machine fails to move in a period of 10 minutes the safety 
circuit will be activated. 
3) Options for low pressure, temp, shut down, the display will register the 
cause of the shut down. 
Referring now to FIGS. 4 to 15, there is shown embodiment in which pipes of 
each span 3 are connected by a flexible joint or coupling 9 at the pivot 
points 6. There are sensing means 10 at each pivot point 6 comprising a 
support arm 11 on one pipe 7 of one span 3 and a support arm 12 on pipe 7 
of adjacent span 3'. The support arm 11 has a projection 13 overlying a 
socket 14 in the support arm 12. 
There is a ball 15 mounted in this socket, which ball 15 is fixed in one 
plane by a horizontal (in use) pin 16 extending through a diametrical slot 
17 in the ball 15. There is an input shaft or spigot 18 projecting from 
the ball 15 which is supported in a bearing 20 in the projection 13 and 
which terminates at a distal end 21 in a coupling 22 to a sensor unit 
which is an input to a gear-box 23 (FIG. 5). The gear-box mounts on an 
output shaft 25 a rotatable disc 24, the periphery of which is movable 
past three sensors 26 of the sensing means 10. The sensing means comprises 
the three sensors 26, which are optical sensors. 
The gear-box 23 is a 25:1/50:1 avoid gear-box in the embodiment shown in 
FIGS. 4 to 16. The disc 24 carries "code" means in the form of slots cut 
in its periphery and which transmit light. The code produces required or 
desired output at the sensor signals, e.g. "on", "off" and "safety" at the 
sensor 26. The slots 27 (of which only two are shown, FIG. 6) interrupt 
light from the light emitting sensors 26, the light passing between an 
emitter 28 and receiver 29 of a respective sensor. The output from each 
sensor 26 is then transmitted electrically to control means for the pivot 
1, adapted to respond to output from the sensing means 10 to maintain the 
spans 3 and 3' in the desired relative configuration. Thus in operation 
movement of one span 3 relative to the other 3' is directly sensed by the 
input of the gear-box 23 via arms 11, 12 and ball 15 and socket 14. This 
angular movement is amplified at the output 25 of the gear-box 23 which 
then rotates the disc 24 to rotate it and bring the slots 27 into register 
with the sensors 26 so generating electrical output to which the control 
means can respond. Thus a slot 27 which extends 50.degree. on either side 
of one sensor 26, say that identified further by "C" in FIG. 6, allows the 
pivot to operate on an alignment of .+-.2.degree. . Should the alignment 
exceed 2.degree. , the disc 24 would block the light path between 28 and 
29 of sensor 26C and its output drops to logic "0". 
A further embodiment of sensing means, designated 110, is shown in FIGS. 7 
and 8. This embodiment is similar to that of the embodiment described 
above, where like parts are identified by reference numerals with the 
addition of 100 as compared to the first embodiment, thus the gear-box in 
FIGS. 7 and 8 is shown as 123. In this embodiment, the sensors 126 are 
mounted horizontally, in use and as viewed. This allows the assembly to be 
smaller. The circuits of FIGS. 13, 14, 15 can be used in the second 
embodiment too. The sensors 26, 126 thus act as optical sensors or 
switches. The slots 27, 127 could be blanked off parts of a transluscent 
or transparent disc, or clear parts of an opaque disc. In all cases, they 
affect the transmission of light at the sensors 26, 126, so producing an 
electrical signal. The sensor could also be a plurality of light 
transmitting means such as optical fibres, say of 50 mm diameter, mounted 
in a block, e.g. a plastic block by moulding, to produce a robust sensor 
which is compact, light, readily installed and which is stable in 
virtually all atmospheric conditions. 
A sensing means 10 as hereinbefore described could be mounted at every 
point 6 of the pivot 1. Also, the ball 15 in every embodiment is fixed on 
one plane to provide accuracy in measurement of misalignment between the 
frames 3,3'. 
In every embodiment, on the centre pivot alignment system, the speed is set 
by the end tower which in turn is controlled by the master control device 
110 as shown in FIGS. 9 and 10. The intermediate towers (referred to 
simply as towers) sense their angular alignment with their neighbour (an 
adjacent tower) and when the error exceeds a preset limit the motor is 
started and only stopped when the error is eliminated. 
The master control device 110 would in the case of a pivot be located at 
the centre point of the apparatus. It allows the operator to communicate 
with and program the system via a 16 key keyboard and a dot matrix LCD 
display or by connecting a remote computer. The master also-monitors the 
apparatus and in the event of a failure it will close down the apparatus 
and warn the operator. 
Before the apparatus is started, it is aligned (i.e. the different spans 
extend in a straight line). This can be carried out using one of two 
methods: 
1. "AUTOMATIC ALIGNMENT" On pressing this key, the message "MACHINE 
ALIGNING" is shown on a display until the apparatus (pivot) has 
straightened itself up at which point the main menu will be displayed and 
the machine may be started. 
2. "ISOLATE" This allows manual alignment to be carried out. A tower may be 
isolated in this menu by keying in the tower number followed by the enter 
key and may be de-isolated in a similar fashion. The pivot may be rotated 
whilst in this menu by holding down the forward or reverse key. All towers 
may be de-isolated by pressing AC or by pressing STOP which also causes 
the display to return to the main menu. 
The apparatus or pivot may be operated according to PROGRAMMABLE FUNCTIONS, 
as follows: 
APPLICATION RATE (AMOUNT OF WATER APPLIED) 
For normal operation, the operator enters the depth of water application he 
would like to have applied on each revolution of the pivot in millimeters, 
and also the application he would like applying each day (i.e. average 
application for a 24 hour period) also in millimeters. If either one of 
these values or the combination of values is not possible, then the 
minimum (or maximum if applicable) value is automatically selected as a 
default. Examples of impossible settings might be: 
1. 1 mm per revolution and 10 mm per day on a machine that would take 3 
hours to make one revolution. In this case the machine would have to make 
10 revolutions per day to apply 10 mm, but would only have enough time to 
make 8 revolutions in a 24 hour period even if working non stop. 
2. 1 mm per revolution on a machine that has a minimum application rate of 
1.5 mm per revolution (i.e. with the end tower motor running continuously 
for the complete revolution). 
3. 20 mm per day on a machine that is only capable of applying 10 mm per 
day with the water running continuously. However, if the machine was set 
to give 20 mm per revolution but only 10 mm per day then this would be 
valid and would cause the machine to make one revolution every 48 hours 
giving an AVERAGE application of 10 mm per day. 
The switching of a motor on an end tower to achieve the desired application 
rate is calculated either from the fixed flow rate for the particular 
machine which is input at the time of machine setup, or it may be 
calculated from the optional flowmeter. During machine setup, the 
apparatus length, end boom length, wheel speed and machine origin are also 
input and this information is also used to calculate the end tower timing 
to achieve the desired application rate. 
Movement of the end tower is sensed by a spider wheel and information 
derived from this is sent back to the master device 110 at the centre of 
the pivot. The master can also obtain information of machine position from 
stakes around the perimeter of the pivot and from the optional shaft 
encoder mounted at the centre point. 
All information input during machine setup, current machine position, 
settings input by the user and all other variables are retained in battery 
backed RAM in the event of power failure. Low battery warning is given on 
the LCD display when the battery is in need of replacement. 
ON/OFF FUNCTIONS 
Various On/Off functions are programmable at the master control panel. The 
outputs on the master, towers and end tower may be used to switch any 
electrically operated ancillery at these points. Similarly, the inputs can 
signal to the master control unit an event that has occurred at the point 
of the relevant control unit allowing the master to take appropriate 
action. The available inputs and outputs are as follows: 
i. Master 
Outputs: 
4*Motor 
4*Fertilizer 
2*Water Valve 
2*Pump 
1*Tower Communication 
1*Pivot Unifier Computer Communication 
Inputs: 
1*Generator Fault 
1*Low Temperature 
1*Water Pressure outside limits 
1*Angular Position incremental pulse 
1*Tower Communication 
1*Pivot Unifier Computer Communication 
1*Auxiliary data Communication 
ii. Tower 
Outputs: 
2*Tower Communication 
2*Motor 
8*Tower auxiliary 
Inputs: 
2*Tower Communication 
2*Power Supply 
2*Tower auxiliary 
1*Tower door open 
iii. End Tower 
Outputs: 
1*Tower Communication 
2*Motor 
8*Tower Auxiliary 
6*End Tower Auxiliary 
Inputs: 
1*Tower Communication 
2*Power Supply 
1*Moved 0.5 m Interrupt 
2*Tower Auxiliary 
1*End Tower door open 
4*Marker Stake 
2*End Tower Auxiliary 
1*Auxiliary Interrupt 
SEQUENCE PROGRAMMING 
Using information derived from a spider wheel of the end tower spider 
wheel, the stakes and the optional shaft encoder, the pivot may be 
programmed by the operator to carry out different operations for various 
sectors. For instance the operator may set different application rates for 
different sectors (even running dry over a sector), the fertilizer may be 
selected for specific sectors and the machine may be programmed to 
auto-stop or auto-reverse at a predefined point. 
DISPLAY 
The display is a 32 character by 8 rows Liquid Crystal type. This shows the 
current status of the machine and the values that have been input. The 
display is also used to give details of safety shutdown should a problem 
occur. 
The apparatus has several safety shutdown features, these are: 
(1) "SAFETY SHUTDOWN--TOWER NO.xx" This occurs when tower number xx goes 
out of alignment (i.e. travels outside its safety limits). This could be 
caused by a tower that will not start (possible motor, contactor or 
gear-box fault) or by a tower that will not stop (possibly stuck 
contactor). 
(2) "SAFETY SHUTDOWN--COMMUNICATIONS FAULT" This suggests a break in the 
communication or power supply lines to the towers. 
(3) "SAFETY SHUTDOWN--END TOWER MALFUNCTION" This might be caused by the 
end tower not stopping (stuck contactor) or a lack of end tower movement 
indication (wheel slip). 
(4) "SAFETY SHUTDOWN--TOWER xx MALFUNCTION" This occurs if the shaft 
encoder ceases to perate correctly. 
(5) "SAFETY SHUTDOWN--TOWER xx BOX OPEN" This is a safety feature built 
into the system to ensure that the system cannot be operated whilst a 
tower box is open thus reducing the possible risk of electric shock. 
The tower control device detects the angular position of an intermediate 
tower relative to the next tower and stops or starts its motor accordingly 
to keep the angle as close to 180 degrees as possible (i.e. to keep the 
pivot in a straight line). The optical sensors 26 also give signals to the 
tower microcontroller if the angle exceeds safety limits and also 
directional information to indicate whether the angle between a tower and 
an adjacent one is less than or greater than 180 degrees. This information 
is used by each tower, together with directional information sent out by 
the device at switch on and during a change direction cycle, to allow the 
tower to switch the motor on at the correct position for the specific 
direction. Directional information is also used to allow the machine to 
align itself automatically. 
If the safety limit is exceeded, the tower motor is immediately switched 
off by the tower microcontroller and the device controller is also 
informed of the problem thereby allowing the complete system to be shut 
down. 
Each tower has a unique identification number which is automatically 
designated to it by the master controller each time the machine is 
switched on. This number is transmitted with a safety code should a tower 
go out of alignment thus allowing the device to display the tower number 
at which the problem occurred. The identification number is also used for 
other purposes such as machine alignment. 
In a further development of the system each tower sends back incremental 
angular data to the master controller allowing the exact position of each 
tower to be retained in the master controller's battery backed RAM in the 
event of power failure. This allows the angular switching sensitivity to 
be set by the operator at the Master Control Panel. An alternative 
embodiment of angular direct shaft sensing as opposed to mechanically 
amplified shaft position sensing is used. 
The flexible joint at each tower between one span and the next is an 
integral part of the sensor. For a centre pivot or a linear irrigator to 
operate there must be freedom of rotation at each joint about the x and y 
axis as well as limited freedom to rotate about the z axis to accommodate 
yaw. 
The kind of joint may vary according to the type of machine to which it is 
being fitted but it may be either of the ball joint type or the universal 
joint type. 
The end tower has no alignment sensing and is wholly controlled by the 
master controller. However, the end tower does send back information from 
its spider wheel and perimeter stakes thus allowing the device to make 
informed switching decisions. 
In every embodiment, referring to FIGS. 9 to 11B specifically, the sensor 
device 26 comprises a housing 130 with a lid 131 which has an entry for 
the main 22 shaft to which is fixed a timing pulley 129, the shaft 22 
being parallel to a shaft 132 mounted for rotation in a bearing 133 and 
mounting a further timing pulley 134. There is an endless member or belt 
135a in the form of a timing belt trained round the pulleys 129, 134. The 
shaft 132 mounts above (as viewed in FIG. 9) the pulley 134 a disc 135 of 
U-shape in cross-section. The disc 135 has a peripheral wall 136 in which 
there are slits, or areas 137 able to transmit light, the wall being 
arranged to pass through depending legs 138 of an optical sensor 139 which 
essentially is mounted on, or carried by, or is adjacent the underside of 
the lid 131 with an associated PCB 140, there also being equipment such as 
a capacitory 141, voltage regulator 142 and heat sink 143. The disc 135 
allows two sensitivities namely using slits or slots 137 in the wall 136 
set around orthogonal lines as shown in chain dashed lines in FIG. 11B, 
and by using slits or slots 137 set around diagonal dotted lines when the 
disc 135 is rotated through 45.degree. . In either case, rotation of the 
joint 6 rotates the shaft 22, this in turn rotates the disc 135 via the 
belt 135a and this rotation causes the slits or slots 137 to actuate the 
sensor so that an electrical signal is generated which can then be used by 
the microprocessor to correct the alignment to provide and maintain the 
required linear arrangement. 
Referring now to the embodiment of FIG. 16, there is shown part of a mobile 
irrigator having adjacent elongate members or spans in the form of pipes 
161, 162. One end 163 of one pipe 161 is adjacent the other end 164 of the 
adjacent pipe 162, which pipes 161 and 162 are arranged and desired to lie 
maintained substantially in a single plane, though it will be understood 
that the pipes 161, 162 are shown offset for clarity in the FIG. Secured 
to this one end 163 of the pipe 161 as by saddles 165 there are spaced 
bearings 166 which receive a bolt 167 of a cleve 168, which has a channel 
defined by two spaced flanges 169 each with a hole 170 therethrough, the 
holes 170 being aligned vertically. Mounted in the aligned holes 170 is a 
pin 171 which fixedly mounts a shaft 172 of an optical encoder 173, that 
is an encoder which produces an electrical output on rotation of the 
shaft. The shaft 172 is secured in a hole 174 in the pin 171, which in 
time is received in an aperture 175 in a mounting block 176 received 
between the flanges 169. The block 176 itself has trunions 177 at opposite 
sides (only one of which is shown) and these receive hooked ends 178 of 
plates 179 secured to the pipe 162. The pipes 161 and 162 are thus 
articulated together for rotation as before any such rotation being 
detected by the optical sensor and converted into electrical impulses for 
transmission to the master control unit for actuation to produce 
re-alignment. 
The low voltage power supply used by the remote tower and end tower 
electronics is supplied as a DC voltage which is simply regulated 
independently by each circuit board. Future electronics may have 
independent power supplies from the 415/480V AC power rails currently 
available for the motors. 
It will be understood that the invention above described with reference to 
the drawings can be adapted to convert an existing machine, or it can be 
purpose-built. Moreover, virtually identical components would be used for 
a linear irrigator as for the pivot irrigator described, but the 
arrangement would be modified. 
It will be understood that connections between the various sensors and 
motors can be by any suitable means such as fibre optic cables, to provide 
tower to tower communication. 
Moreover, the master control unit may be controlled by a radio device such 
as a VHF transmitter operating say at 300 m/cs from a central station. The 
invention therefore provides a controlled loop communication, e.g: 
EQU Master Control.revreaction.Tower. 
Several irrigators can also be controlled from a single, remote station, by 
VHF as before, or a cellular telephone system. 
Thus each tower could have its own number which when rung causes the tower 
to move in a particular way to effect alignment. 
It will be understood that in apparatus as hereinbefore described and 
embodying this invention, all operations can be carried out from the 
Master Controller including isolation of towers and adjustment of 
individual motor switching sensitivity. The machine may also be controlled 
from a central computer via the optional computer link allowing a large 
network of irrigators to be operated from one remote point. 
Periodic adjustment is eliminated and very high switching precision is 
achieved. 
By incorporating computer technology at each tower and at the master, the 
quantity and complexity of wiring has been drastically reduced, thereby 
reducing installation time and increasing machine reliability. 
An optional PC based irrigator network facility offers a complete 
irrigation management package. This Computer Managed Irrigation Network 
gives central irrigation management from anywhere on the farm or anywhere 
in the world. The PC displays the current status of all irrigators in the 
network and allows any settings to be changed. The PC also shows 
accumulative water application, slip correction etc. Further facilities 
may be incorporated as appropriate offering virtually unlimited management 
potential. 
It will be understood that each node of the system (i.e. towers and master) 
is "intelligent" and is therefore capable of operating independently. 
However, all of the towers communicate with the master controller using a 
computer data communications system. This allows the number of power 
conductors to be reduced from 11 to 4. 
When the system is started, the master carries out a diagnostic check of 
the complete system and initializes the slave units at each of the towers. 
Each slave then controls its own tower motor. Simultaneously, the master 
communicates with each tower several times a second and instructs the 
tower to switch its auxiliary outputs if they have been programmed at the 
master. It also checks the angle at the tower and the auxiliary inputs and 
acts on them accordingly. 
The end tower is switched by the master using the same communications 
system and the master compensates for slip by making further calculations. 
If the slip becomes excessive, the master controller shuts down the system 
and displays the message "SAFETY SHUTDOWN--COMMUNICATIONS FAULT". 
Because the tower angles are measured using digital transducers, there is 
complete flexibility of switching sensitivity which may be set up from the 
master keypad. This also allows the machine to compute its current 
position in the field allowing it to stop, reverse or change application 
rate, fertilizer or auxiliary output settings at any pre-programmed point 
without the need for marker stakes. 
It will be further understood that the Hall Effect referred to herein is 
that effect achieved whereby when a thin sheet of metal with an electric 
current flowing along its length is placed in a magnetic field at right 
angles to it, an electromotive force is developed which is at right angles 
both to the direction of the magnetic field and the electric current.