Dutch producer Emsflower currently has some 26 hectares of greenhouse under glass and grows over 500 million plants every year for shipment to destinations around Europe. The challenge to produce a cost-effective automation system for plant watering and handling was answered by automation company Riwo Engineering in conjunction with the Control Techniques Drive Centre in Rotterdam.
Riwo Engineering had to meet a demanding brief. The new system had to encompass the transport of plants, watering, spraying and pest control, autonomous function of individual sections as well as a central control, precise positioning over runs of 250 m, the means to transport both power and water along 150 m, ease of maintenance and all of this at a limited cost.
System proposal and implementation
The scheme proposed, and subsequently tested and accepted, comprised trolleys with plastic wheels running on overhead rails along each 8 m wide greenhouse bay. Two wheels on each trolley are powered using asynchronous motors with encoders controlled by 1,5 kW Unidrive SP AC drives from Control Techniques.
However, position control is not a simple matter. The temperature in the glass houses can vary between 1 and 50°C, with the track itself expanding and contracting as much as 600 mm over the 250 metre length. In addition, when under heavy load, there can be considerable slip on the driving wheels. To meet the low cost objectives, Riwo and Control Techniques came up with a simple solution implemented using absolute reference marks along the track coupled with simultaneous temperature measurement. Both are fed back to a program running in the SM intelligent application module fitted to each drive, where the software provides error compensation in response to temperature changes. This closed loop control, complemented by the intelligence in the drives, has produced an excellent level of accuracy and repeatable positioning under all conditions.
Wireless communication
To minimise cabling, yet facilitate communications with every drive in each greenhouse, a Wi-Fi system was chosen. Each trolley has a Wi-Fi bridge that is connected to each drive via a serial interface. Each greenhouse has full network coverage that is used for telephone links, PDAs and other production machines.
The primary task of each trolley is to provide water spray of the plants under the control of a program on the master computer. Each is also used to carry tools to provide extra functionality. The possibilities are diverse; currently there are 10 handling modules that can be slung under a trolley, to provide insecticide or fungicide spraying for instance and different modules are used for picking and placing blocks of plants. In addition, personnel can call up trolleys to transport them along the greenhouses to points where they need to work.
The trolleys and modules, powered by Control Techniques Unidrive SP drives, communicate through CT-Net with the master drive and thence by Wi-Fi with the master computer. CT-Net allows multiple drives to be connected to a master, while the onboard programming enables the required degree of accuracy and repeatability to be achieved. The open structure based on the Modbus protocol provides a low cost interface to the master. Unidrive SP is an advanced ‘solutions platform’ AC drive, configurable into five operating modes – open and closed loop, vector, servo and regenerating modes – compatible with most industry standard networks, configurable for all types of AC motors, including linear motors, and accepting 14 position feedback protocols. It has a range of plug-in module options for adding connectivity, feedback, extra I/O and on-board programming.
The drives exceed expectations
The testing of a prototype by Riwo proved to be successful and, which led to an order and the installation of 140 trolleys with more to follow to meet proposed expansion plans for the complex. Subsequently there has been so much interest in Emsflower, with so many people wanting to look around, that the complex has been developed into a tourist attraction with over 200 000 visitors per year now touring the tropical houses and visiting the restaurants, shops and gardens. Tom Kuipers who runs the enterprise says, “As Emsflower expands, our criteria for drives will always stay the same. The price and maintenance must be low and performance must meet our targets with absolute reliability. Control Techniques is now our partner of choice.”
For more information contact Bill Tedd, Control Techniques, +27 (0)11 462 1740, [email protected], www.controltechniques.com
| Tel: | +27 11 462 1740 |
| Email: | [email protected] |
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