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Tough sensors for agricultural machinery

01 April 2015

Where robotic agricultural machinery is concerned, sensors rule in more senses than one: Garford’s Robocrop precision guided weed control equipment applies position, pressure and temperature sensor technologies throughout.

Peterborough based Garford Farm Machinery designs and manufactures advanced environmentally sustainable agricultural equipment that solves large scale weed control and crop cultivation for farming worldwide. Its ‘Robocrop’ range employs a specially developed precision guided vision system – for weeding, hoeing or spraying - that uses real-time digital image analysis in conjunction with a hydraulic servo ‘sideshift’ assembly that laterally positions various multiple tools on or within the crop rows as the tractor mounted equipment is driven forwards.

The precision guided tools are used for mechanical weed control; to remove individual unwanted plants, hoe between rows of plants, or spray rows of plants – with impressive lateral positional accuracies of better than 10mm at average tractor speeds of 12km/h.

With this type of agricultural machinery experiencing extreme environmental and physical conditions, Garford has turned to Variohm EuroSensor to provide durable precision linear and rotary sensors for steering angle and lateral shift position feedback. To complete the package of sensors on the Robocrop range, Variohm also supplies combined pressure and temperature sensors for hydraulic system monitoring.

The Robocrop InRow Weeder is the latest in the Garford range and utilises the tried and tested video image analysis technique to locate crop plants through pixel and colour recognition and then use this information to control individual rotating weeding discs which actually hoe the area between the plants (inter-row) or in line with the plants (inter-plant), eradicating competing weeds but leaving crops intact.

The InRow Weeder features multiple individual discs which operate across several crop rows with a maximum width of six metres. The rotation of each disc is synchronised with the forward speed of the tractor and the plant positional information from an imaging camera which can cover a two metre width. Each weeding disc is coupled to an hydraulic motor, driven via a proportional valve controlled by the cab mounted Robocrop computer, which includes an HMI display to select modes and display operational information.

To achieve accurate lateral guidance of the equipment, a Variohm SP2800 series rotary potentiometer is located on the ground-contacting steering discs and feeds back steering angle information to the system. Garford uses the Variohm LWG series linear potentiometer on the sideshift to feedback lateral position information to the inter-row hydraulic servo system. It also provides a steering aid for the driver when the machine is set up as an in-row weeder. Both of these Variohm sensor products have been used extensively for agricultural equipment applications.

The 28 mm diameter SP2800 series angle sensor features conductive plastic technology with multiple layer tracks in combination with precious metal multi-fingered wipers arranged in a unique preloaded design that ensures a very high resistance to shock and vibration.

For Garford, the sensor is supplied in an IP67 sealed Duraplast plastic housing, although an aluminium housed alternative is available. With a repeatability of 0.01 percent of measuring range and an operating temperature range of -40°C to +150°C, the SP2800 ensures a long working life of more than 50 million cycles with excellent reliability, competitive costs and straightforward installation.

The LWG linear position sensor, from Variohm's German distribution partner Novotechnik, has a pedigree of high reliability under arduous dynamic conditions. The sensor’s high resolution potentiometric output is achieved with a conductive plastic resistance element and a precious metal, multi-finger wiper system with elastomer damping mounted directly on the actuating rod - which is supported with twin bearings for maximum durability.

With a shock rating of 50g and vibration to 9g at 2,000Hz, the robust LWG linear position sensor provides a resolution of better than 0.01mm over a temperature range from -30°C to +100°C – with an operational life of more than 50 million movements.

For this application the IP65 rated unit is actually mounted on the outside of the sideshift assembly on backlash-free pivot heads with the anodised aluminium housing and stainless steel actuating rod fully exposed to the elements. Electrical connection is through M12, IP67 rated sealed connectors. Across the LWG series, electrical stroke ranges from 50mm to 750mm are available. 

Completing the range of sensors supplied on the Robocrop InRow Weeder, Variohm’s EPT3100 Pressure and EPTT5100 combined pressure and temperature sensors are used to monitor the input and output hydraulic pressures and return temperature information from the machine. These pressure sensors feature high level ASIC output circuitry with a well proven thin-film strain gauge pressure measuring cell covering a wide range of pressures.

For the combined versions, temperature measurement in gas or liquid is via a Pt1000 compatible sensor with a rated range of -50°C to +150°C. With IP66 or IP67 sealing and a high strength stainless steel finish they feature industry standard port interfaces with sleeved electrical cable and optional boot protection. Through both ranges, a wide choice of voltage and current output options is available.

The sensors supplied to Garford are part of a comprehensive range of sensor technologies that Variohm EuroSensor designs, manufactures or sources for linear and rotary position, load, force and vibration measurement in demanding applications throughout industry, agriculture, construction, autosports and research.


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