GB2281419A

GB2281419A - An irrigation system

Info

Publication number: GB2281419A
Application number: GB9401069A
Authority: GB
Inventors: Kenneth Crawshaw
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-06-22
Filing date: 1994-01-20
Publication date: 1995-03-01
Anticipated expiration: 2014-01-20
Also published as: GB2281419B; GB9312895D0; GB9401069D0

Abstract

An irrigation system for automatically watering soil, eg. in a greenhouse, including a resistive-type soil moisture sensor 18, 19 for producing a first electrical signal representative of actual soil moisture content, a comparator 22 for comparing the first electrical signal with one or more further electrical signals representative of required moisture contents, and for producing an output signal in the event of a deviation from a required moisture content, and a pump 14 adapted to supply water to the soil and actuated in the presence of the output signal. The duration of water flow is determined, in one embodiment, by the output signal from the comparator 22 or in another embodiment, by a timer 55. Hysteresis between switching on and off the water supply can be provided. In one embodiment the measurement of moisture is disabled whilst the pump is actuated and a display indicates which of sampling and pumping is currently being effected. A manual override and means to determine when the sensors are incorrectly located in the soil can also be provided. <IMAGE>

Description

AN IRRIGATION SYSTEM THIS INVENTION concerns a system for automatically irrigating a growing medium such as soil, potting compost or the like (hereinafter referred to as "soil") and preferably, though not exclusively, a system for irrigating the soil in a domestic or commercial greenhouse.

Certain devices to enable a greenhouse owner to ensure, during his absence, that the soil is kept sufficiently moist to prevent deterioration of plants, are already known and these consist in the main of timer devices placed in the water supply line to ensure that water flows to an irrigation outlet which may be a series of nozzles, for repeated like periods. The main disadvantage of such a system is that water is supplied to the soil irrespective of the moisture content thereof so that in cold and/or wet spells the soil may become overwatered or alternatively in hot dry spells may receive insufficient water to produce the required moisture level.

An object of the present invention is to provide an irrigation system including means for monitoring the soil moisture content and actuating a supply of water only when it becomes necessary in order to maintain a certain predetermined and substantially constant moisture content.

According to the present invention there is provided an irrigation system comprising a soil moisture sensor adapted to be located in the soil and to produce a first electrical signal representative of actual soil moisture content, means for comparing the first electrical signal with at least one second electrical signal of predetermined value representative of a required soil moisture content, and for producing an output signal in the event of a deviation from said required moisture content, and a device adapted to supply water to the soil and actuated in the presence of said output signal.

In one embodiment, said comparing means may be preset with two of said second predetermined electrical signals, one representing a minimum acceptable moisture content and the other a moisture content above the latter, said output signal being produced when said first electrical signal indicates a moisture content at or below the minimum such that water is supplied to the soil until the other of said second signals is reached, and is then discontinued.

There may be provided a timing device whereby soil moisture content is sampled intermittently and for a predetermined period. Also, water may be supplied to the soil until the first electrical signal equates to a second electrical signal representative of a required soil moisture content.

The system preferably includes one or more indicators which provides separate indications respectively when soil sampling occurs, when irrigation is taking place, when a power supply to the system is reduced, and when the soil moisture sensor is not properly located in the soil.

Embodiments of the invention will now be described further, way of example only, with reference to the accompanying drawings in which: Fig. 1 is a component diagram of a first embodiment; and Fig. 2 is a component diagram of a second embodiment; Referring initially to Fig. 1, an irrigation system made in accordance with the invention may be adapted to provide a water supply to, for example, a trough 10 containing a bed of soil 11. A manifold pipe 12 containing an array of spray nozzles 13 is positioned appropriately above the soil 11 and is fed by a pump 14 with water, for example, from a supply tank 15 via an inlet pipe 16.

Alternatively, mains water supply may be connected directly to the inlet pipe 16, via a shut off valve in place of the pump.

An electronic control system generally indicated at 17 is provided to operate the pump 14 as required to maintain a substantially constant moisture content in the soil 11.

A first component of the system 17 comprises a soil moisture sensor 18 having a pair of electrode probes 19 the remote tips 20 of which are exposed to detect moisture content at a position beneath the surface of - the soil 11. The unexposed parts of the probes 19 are insulated to avoid providing a false moisture indication.

The sensor 18 is preferably of the resistive type which is adapted to produce an electrical signal representative of the soil moisture content. This signal is fed via line 21 into the main control unit of the control system 17 and specifically to a soil humidity measuring element 22 therein. The system 17 is connected to a power source 23 which may be a mains supply with transformer, or a battery, and provides a 12 volt supply on line 24. An on/off switch 25 enables the 12 volt supply to be fed to a voltage regulator 26 feeding a 721 volt supply line 27.

A master timer 28 is adapted to produce a signal on output line 29 every 5 seconds, each for a period of 5/8ths of a second thus to produce a duty cycle of 1 in 8. The signal on line 29, during the duty cycle actuates an electronic switch 30 providing supply to the element 22 which acts as a comparator to compare the signal on line 21 representative of the actual soil moisture content, with two predetermined second electrical signals pre-set in the device 22 and representative of the upper and lower limits of an acceptable range of required soil moisture content.

The element 22 has an output line 31 which transmits a signal when appropriate to a further timer 32 adapted to produce an output signal on line 33 for consecutive 7-second periods. The line 33 is connected to a power transistor 37 switching power line 24 to the pump 14 to actuate same.

A device 34 which is connected to the master timer 28, the power line 24, the power line 27 and the line 33 controls the operation of an LED indicator 35 as will be described. Connection to the power line 24 is via a sense line 36 containing a Zener diode 38.

The device operates when the power switch 25 is closed to provide repeated duty cycle signals via master timer 28 and electronic switch 30 to the soil humidity measurement element 22. If during such a sampling cycle, the soil moisture content represented by the signal on line 21, is low i.e. drier than that represented by the lower of the two predetermined second signals in the element 22, an output signal on line 31 actuates the timer 32 and in turn the pump 14 to deliver water via the spray nozzle 13 to the soil 11.

When as a result of the added water, the soil moisture content as detected by the sensor 18 produces a signal which equates to or exceeds the upper of the two second electrical signals in element 22, the output on line 31 is discontinued so that at the end of the current 7-second irrigation period determined by the timer 32, the signal on line 33 is discontinued and the pump 14 is stopped.

The master timer 28 ensures that the soil moisture content is measured at discrete intervals rather than continuously.

This ensures feedback stability and prevents the system from "hunting" which might otherwise occur owing to complex soil mechanics and other delay factors which could result in the pump switching on and off rapidly and at random.

The soil moisture content is sampled every 5 seconds for a period of 5/8ths of a second as described. The 7-second duration output signals on line 33 overlap slightly so that the pump 14 appears to run continuously until the soil moisture content has reached the required level. The intermittent sampling function maximises power saving at the power source 23. It is envisaged that a battery would provide sufficient power to operate the system for a period in excess of 2 weeks or longer according to the type of battery used, the type of pump, and the environmental conditions prevailing.

If required, the lower and upper acceptable soil moisture content limits may be adjustable in the element 22 although this is more likely to be factory-set and may be determined by the "hysteresis" of the control system.

Preferably, the sampling electrodes 19 are driven by an AC signal to avoid plating of the electrodes.

The LED logic and driver unit 34 having connections as indicated to lines 24, 27 and 33 as well as to master timer 28 is able to provide, by a single LED 35, an indication of various modes of operation of the system. For example, if the master switch 25 is open no power is supplied to the unit 34. When switch 25 is closed, the sense line 36 may provide an indication of low power from the battery (where fitted). In this case, the unit 34 will cause the LED 35 to be illuminated once a second, providing a "battery low" indication. When the pump 14 is being driven, the signal from line 33 enables the unit 34 to cause the LED 35 to be illuminated continuously.When soil moisture is above the acceptable threshold, repeated sampling as determined by the timer 28 causes unit 34 to illuminate the LED 35 in rapid succession during each 5/8ths of a second period as determined by the timer 28. This intermittent rapid flashing of the LED 35 thus provides an indication that the system is operating properly even if the soil is adequately moist and the pump 14 is therefore not actuated. Thus, the unit 34 and LED 35 provide an indication of 4 operational modes i.e. "off", "sampling", "pump running", and "battery low".

It will be appreciated that an irrigation system as aforesaid includes the facility of active feedback thus to maintain soil moisture content regardless of external influences and weather conditions such as air humidity, strength of sun, air flow across the soil, plant moisture consumption, and surface evaporation.

It will be appreciated by those skilled in the fields of electrical and electronic engineering that a dc electric motor of the kind used to drive the water pump 14 is itself a source of electrical interference. The operation of such a motor in close proximity to, and from the same power source as, a sensitive electronic measurement comparison circuit can cause malfunction of the latter.

It is known to incorporate electrical noise suppression means into a motor circuit and appropriate filters into an electronic circuit.

However, these means cannot always be relied upon to ensure satisfactory operation, given the different characteristics of electric motors and various types. Furthermore, electrical interference can increase with time as the motor brushes become worn.

It is therefore possible for an excessively "noisy" motor to cause sufficient interference to confuse the measurement and comparison circuitry of the present system and thereby inhibit the action to switch off the motor when the soil has been sufficiently irrigated.

To overcome this problem the output signal placed on line 31 as described above should be disabled during any period when the pump motor 14 is running. Thus, in a modified embodiment of the invention when the sensing circuit indicates that the soil is too dry, the motor will be arranged to run for a pre-set period independent of any signal received from the measuring and comparison circuit. This operating time may be pre-set at manufacture or alternatively a manual control may be provided for operator selection.

A further advantage to be seen from operating the motor only under a timer control is that "hunting" of the control system is avoided.

It will also be advantageous to inhibit pump operation to provide sufficient time for the water to percolate through the soil.

This will further contribute to the avoidance of hunting.

Referring now to Fig. 2, the system 17 is connected to a power source which as with the previous embodiment may be a mains supply with transformer and rectifiers, or a battery, providing a 12volt supply on line 40. An on/off switch 41 and rectifier 42 provide the supply, the rectifier serving to protect the circuit from damage in the event that the supply is inadvertently connected with incorrect polarity.

A voltage regulator 43 regulates the nominal 12-volt supply to 9 volts on line 44.

A fast oscillator 45 operating at a frequency of for example 1 kHz, provides alternating energising current to the moisture sensor 18 via a current limiting resistor 46. An alternating voltage is developed across the sensor and represents the moisture content of the soil. A small voltage indicates that the soil is very wet whilst a large voltage indicates the soil is very dry. A dc reference voltage supplied at 47 is pre-set to a value which represents the peak value of the alternating voltage which would occur across the moisture sensor when the moisture content has fallen to the point that watering is required. This level may be pre-set at manufacture or by the provision of a manual control for operator selection.

The measured alternating voltage across sensor 18 and the dc reference voltage 47 are both fed to a comparator 48. An output from the comparator on line 49 is arranged to remain high (logic level 1) when the soil requires water, and low (logic level 0) when the moisture content is satisfactory. Comparator 48 may be provided with a small hysteresis but this is not essential as "hunting" is inherently prevented by operating the pump on a timed basis independent of moisture condition.

A further reference voltage at 50 is pre-set to a value which represents the peak value of the alternating voltage which would occur across the moisture sensor under conditions when the latter has become accidentally removed from the soil and is excessively dry.

The measured alternating voltage across sensor 18 and the dc reference voltage 50 are both applied to a further comparator 51. A signal from the comparator 50 indicating that the probe is not properly located in the soil is arranged to remain low (logic level 0) when the probe is properly located and thereby moderately moist (with a low resistance) and high (logic level 1) when the probe is out of the soil and thereby excessively dry (with a high resistance).

The output from comparator 51 passes to an inverter 52 and then to the base of the transistor 64 via a rectifier 63. It will seen that when the probe is out of the soil the cathode of rectifier 63 is held low. Therefore, when the probe is not properly located the transistor 64 has its base drive current diverted via rectifier 63. The transistor is consequently prevented from energising a relay 65, the normally open contacts of which are connected by line 66 to the pump motor circuit. By this arrangement, overwatering which might occur when the probe is not properly located in the soil, is avoided.

A slow oscillator 54 operating at a frequency in the region of 6 Hz has its output connected to the input of a divider 55 which is arranged to provide signals on two output lines 56 and 57.

The signal on line 56 is arranged to be a short duration pulse every 21 minutes, and that on line 57 a short duration pulse every 160 seconds. The repetition rates of the aforesaid pulses are chosen to suit the characteristics of the particular installation and are given here by way of example only.

The signals on lines 49 and 56 are both fed to the inputs of AND gate 58, the output of which on line 59 will go momentarily "high" once every 21 minutes, whenever the signal on line 49 is "high" indicative that the soil is dry and requires water.

A set/reset latch 60 has line 59 applied to its set input, and line 57 applied to its reset input. When the latch is set its output on line 61 is high, and when reset is low. By this means, a signal on latch output line 61 will go high every 21 minutes during the time when the soil is dry and requires watering. The line will always reset thus returning to a low state, 160 seconds after being set high. The latch output line 61 is applied to the base of transistor 64 via current limiting resistor 62.

It may be seen therefore that whenever the soil requires watering, the pump motor will be energised for a pre-set period of 160 seconds, every 21 minutes. Further it may be seen that once the motor has been switched on it ceases to be under the control of the measurement and comparator circuit and is solely under time control from the slow oscillator 54 and divider 55. The possibility of the watering cycle being foreshortened or extended as a result of malfunction of the measurement and comparator circuit due to, for example, interference from the electric motor is thereby eliminated.

Equipment may be provided with further features or increased operator convenience, as will now be described by way of example only.

The supply voltage to regulator 43 is compared with a further pre-set reference at 67 in a comparator 68 to provide an output on line 29 when the supply voltage (for example from a battery) is lower than the pre-set reference.

The divider 55 is arranged to provide a pulsing output on line 70 having a duty cycle typically 0.25 seconds "on" and 2 seconds "off". The pulsing output is supplied to one of the two inputs of each of three AND gates 71, 72 and 73. The output from an inverter 74 is arranged to be high whenever the supply voltage is normal (adequate). The output from AND gate 73 will therefore pulse whenever the supply voltage is normal. The output from OR gate 75 will similarly pulse and so cause an "on" indicator 77 to be illuminated.

The output from AND gate 72 will pulse whenever the supply voltage is low. The output from OR gate 76 will similarly pulse and so cause a "battery low" indicator 78 to be illuminated.

The output from AND gate 71 will pulse during a condition when the probe is incorrectly located as previously described. The output from AND gate 71 is fed to OR gates 75 and 76 such that both the "on" and "battery low" indicators 77 and 78 will be illuminated to indicate this condition.

Thus the two indicators 77 and 78 provide visual indication of the following four system operating states namely: Both indicators off - no supply connected, non operational.

Indicator 77 illuminated- supply connected, battery charged, probe in soil, fully operational.

Indicator 78 illuminated- supply connected, battery needs charging, probe in soil, operational but needs attention.

Both indicators flashing - probe out of soil, non operational, needs urgent attention.

Further indications may be provided for operator convenience. One might include an indicator in parallel with the motor terminals to indicate that the pump is running or has been instructed to do so. Furthermore, a push-button switch such as indicated at 79 may be connected in parallel with the relay contact and may be pressed to provide an initial charge of water to the soil independent of the timer. It also provides for manual control (override) in the event of any failure in the entire control system.

Claims

1. An irrigation system comprising a soil moisture sensor adapted to be located in the soil and to produce a first electrical signal representative of actual soil moisture content, means for comparing the first electrical signal with at least one second electrical signal of predetermined value representative of a required soil moisture content, and for producing an output signal in the event of a deviation from said required moisture content, and a device adapted to supply water to the soil and actuated in the presence of said output signal.

2. An irrigation system according to Claim 1, wherein said comparing means may be preset with two of said second predetermined electrical signals, one representing a minimum acceptable moisture content and the other a moisture content above the latter, said output signal being produced when said first electrical signal indicates a moisture content at or below the minimum such that water is supplied to the soil until the other of said second signals- is reached, and is then discontinued.

3. An irrigation system according to Claim 1 or Claim 2, including a timing device whereby soil moisture content is sampled intermittently and for a predetermined period.

4. An irrigation system according to Claim 3, wherein said timing device is adapted to actuate said comparing means at periodic intervals and for a predetermined period.

5. An irrigation system according to Claim 3 or Claim 4, including a further timing device adapted to actuate the water supply for consecutive periods of predetermined duration.

6. An irrigation system according to Claim 5, wherein the consecutive periods determined by said further timing device are adapted to overlap whereby the pump, when operating supplies water continuously.

7. An irrigation system according to Claim 2, wherein at least one of said two second predetermined electrical signals is adjustable.

8. An -irrigation system according to any preceding claim, in which the soil moisture sensor includes a pair of electrode-probes mounted in juxtaposition and adapted to be inserted into the soil, remote tip regions of said probes being exposed thus to produce said first electrical signal representative of the electrical conductivity of the soil depending upon its moisture content.

9. An irrigation system according to any preceding claim, including an indicator which provides separate indications respectively when soil sampling occurs and when- irrigation is taking place.

10. An irrigation system according to Claim , wherein further means are provided to ensure that the output signal from the comparing means is disabled during any period when water is supplied to the soil.

11. An irrigation system according to Claim 10, wherein said further means includes a still further timing device adapted to operate the water supply device, once actuated, for a predetermined period irrespective of any output signal from the comparing means.

12. An irrigation system according to any preceding claim, including means adapted to sense when the soil moisture sensor is not properly located in the soil, and accordingly to prevent excessive operation of the water supply device.

13. An irrigation system according to any preceding claim, including a plurality of indicators which provide separate indications respectively when a power supply to the system is reduced and when the soil moisture sensor is not properly located in the soil.

14. An irrigation system according to any preceding claim, including an override switch adapted to actuate the water supply device independently of any control signals produced by the system, thus to provide manual control (override) when required.

15. An irrigation system substantially as hereinbefore described with reference to and as illustrated in Fig. 1 or Fig. 2 of the accompanying drawings.