ISHS Acta Horticulturae 406: II IFAC/ISHS Workshop :Mathematical & Control Applications in Agriculture & Horticulture
CONTROL OF A PNEUMATIC VISION GUIDED ROBOT MANIPULATOR FOR HORTICULTURAL APPLICATION |
| Author: | N.D. Tillett |
| Abstract:
Research has indicated that it is feasible to automate many horticultural tasks using robotic techniques in a way that has not been possible with traditional machinery. Industrial manipulators are however in many respects over-specified and too expensive for horticultural use. Accordingly, a low cost manipulator for horticultural application has been developed using novel direct-drive rotary joints incorporating flexible pneumatic elements. The compressibility of air, combined with other non-linecarities, means that traditional techniques alone are not adequate to control this manipulator. A dynamic controller was developed, based on proportional and integral of error feedback combined with differential pressure across the actuator, which is analogous to an acceleration term. Pressure was used as it could be measured by a low cost piezoresistive sensor, avoiding the need to differentiate the output from the position transducer, a low cost plastic film potentiometer. The algorithm gain schedules according to a stability index which is defined by a combination of error and velocity ranges. In addition there are a number of rule-based filters relating principally to the build-up of two integral terms. The first acts over the short term, building up quickly but reset to zero if error or velocity exceed preset values. The second corrects long term variations in system neutral values. Hysteresis compensation, dependent on the polarity of movement, is also provided. A typical step move is achieved in just over one second and is repeatable to about 3 mm. The 2 degree-of-freedom manipulator with a 700 mm reach, weight only 8.5 kg, and has been successfully tested with payloads of up to 2 kg. The system has been demonstrated packing randomly arranged tomatoes under vision guidance into plastic trays of eight, a task it performs reliably in 21 seconds. | |
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