Hydrostatic Drive Is "Berry" Precise

June 4, 2014
A high-tech strawberry harvester, with vision and closed-loop dc-motor drives, relies on an electrohydraulic propulsion system for precise and efficient speed control.
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Industrial robots have had a profound effect on manufacturing by improving productivity, quality, and consistency. But robots are no longer confined to the factory floor. Agricultural robots are beginning to take the place of human hands in tasks much more sensitive than those typically found in manufacturing. Agrobot, Oxnard, Calif., has been working with local growers to test and perfect its SW6010 robotic strawberry harvester in California.

Agrobot’s automated harvester uses optical sensors as part of its AGvision system to pick only the ripest berries based on color, size, and other parameters determined by the farmer.

AGvision is an artificial vision system that identifies fruit with high accuracy and consistency. It implements a real-time protocol for morphological and color analysis to systematically return the ripeness of the fruit. The software then singles out only those strawberries that meet the quality standards previously set by the farmer.

The Agrobot SW6010 harvester has 24 robotic arms, with transverse and vertical movements generated for each arm by two 24-Vdc motors. Each motor uses a rotary encoder to provide position feedback to the machine’s main controller. The optical sensors work in conjunction with the motors to monitor the strawberry bed at 20 images/sec, scanning for ripe red berries. The arms and their wires are protected from tangling by plastic cable carriers from igus inc., East Providence, R. I. After a berry is picked, it is transferred to a conveyor system for subsequent packaging.

Hydraulic mobility

Juan Bravo, CEO of Agrobot, explained that the SW6010’s hydrostatic drive and hydraulic steering systems play an integral role in the harvesting process. Precise speed control is essential to ensure the vehicle’s speed remains consistent, thereby maintaining synchronization with the vision and harvesting system.

Fran Rodas, of Agrobot, revealed that the harvester is powered by a Perkins 403D-15 diesel engine, which transmits up to 25.1 kW (33.7 hp) at 3,000 rpm, and drives a Parker Hannifin PV01 variable-displacement axial-piston pump. The pump delivers up to 24 lpm (6.3 gpm) of flow at 1,500 rpm for the harvester’s hydrostatic drive. Pump displacement is controlled by a load-sensing circuit, which saves energy (fuel) by limiting the pump’s output to only that needed to achieve the required drive speed and torque. Fluid from the pump flows to a Parker D1FB electrohydraulic proportional valve that provides precise electronic speed control from its zero-lapped spool to four Poclain MGE02 steerable wheel motors.
The Agrobot SW6010’s hydrostatic transmission is driven by Parker Hannifin’s PV01 axial-piston pump. Its load-sensing feature matches fluid-power output to load and speed demands, thereby conserving fuel.

The motors have a dual-displacement function. High displacement is used when low-speed (500-m/hr) operation is needed for harvesting. The low-displacement setting is used for the harvester’s travel mode, which can achieve speeds to 5 km/hr. This function is switched through a hydraulic integrated circuit designed by Agrobot.

Rodas provided details on the hydrostatic drive circuit: “The pump feeds two pairs of motors through a flow divider connected in parallel. When harvesting, each pair of motors is connected in parallel, so each motor receives one fourth of pump flow and rotates at low speed. When we need high speed, we switch the configuration to series, so each motor receives half of the pump’s total flow. We use one proportional valve upstream of the flow divider, so it provides precise speed control for both pairs of motors.”

For the steering circuit, the Agrobot harvester uses a pair of fixed-displacement pumps mounted on the side of the diesel engine. Each pump feeds a double-acting hydraulic cylinder, which controls the front and rear steering.

Harvester reaps benefits of plastics

According to Ellen Rathburn, of igus, the SW6010 robotic strawberry harvester also incorporates hundreds of engineered plastic igus iglide bearings and Energy Chain cable carriers to keep the machine’s cost and weight down. They also lower maintenance and downtime to maximize profits for strawberry growers. The plastic components also allow for food safety in the conveyor and packaging areas of the harvester.

Poclain’s MG02 wheel motor has multiple features that simplified integration with the Agrobot harvester. Its dual displacement allows two-speed operation, and its integral mounting framework with pivots simplifies incorporation of a steering cylinder. It also has an option for an integral brake.

Rathburn explained, “The plastic components do not require external lubricant; therefore there is no grease or extra attraction of dirt and debris to the delicate fruit. The plastic components are also unaffected by the dirt and dust that is unavoidable in agricultural applications.” Bravo said the price of engineered plastic components had a dramatic impact on the overall cost of the SW6010 harvester. He explained that he and his team had agreed that the cost of typical steel components would be far too expensive, and chose to work with plastics from the beginning of the project.

Agrobot’s automated harvester is the first of its kind. Until now, laborers had to spend hours in the field picking strawberries by hand. Now, with the SW6010 harvester, an operator steers the harvester tight along the rows of plants, but the delicate nature of the berries still requires they be packed by hand. Still, automated harvesters of this nature could significantly decrease the labor requirements for berry crop growers — a welcome change as labor shortages have affected strawberry growers in recent years. According to Bravo, there is no other harvester available like his.

Agrobot’s automated strawberry harvester is still in the final prototyping stages, but Bravo says his company is close to bringing them to the market, “hopefully in a year or two.”  

Agrobot is headquartered in Huelva, Spain, with a U.S. facility in Oxnard, Calif. For more information, including videos showing the robotic strawberry harvester in action, visit Refer to the online version of this article posted to our website to see a schematic of the hydrostatic transmission circuit and links to videos.

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