Motor controllers take charge of plant watering

Edited by the Engineeringtalk editorial team Dec 27, 2006

Southern Glasshouse Produce asked Curtis Instruments to modify a custom-designed automated watering system imported from the Netherlands at considerable expense.

Southern Glasshouse Produce (SGP) is one of the UK's largest suppliers of cut flowers to the supermarket trade, and the company recently approached Curtis Instruments regarding the modification of its automated watering system, a custom-designed unit imported from the Netherlands at considerable expense.

The original system consists of a beam supporting the spray jets, suspended below two runners the length of the greenhouse.

The wheels attaching both ends of the beam to the runners are driven independently by two small PM DC motors.

The user interface is by means of a handheld (tethered) control box with a selection of buttons to implement the different functions, and two potentiometers to limit the speeds of the traction motors and the reel.

The system is initialised by the user, and then does a complete cycle to the end of the bay and back again automatically.

There is also the option for a manual override.

The overall components feature the propulsion system, a hose reel, batteries and charger on the moving mechanism - with a linear potentiometer balancing system and inductive sensors to indicate end of travel.

The solution from specialist manufacturer Curtis was based on two different types of motor speed controllers: a 1310 unit to control the logic of the system and 1210s for power (one each for the drive wheels, and one to control the hose reel).

The 1210s are controlled through the 1310's analogue outputs, using the Wig-Wag throttle type to allow for drive in both directions.

The inductive sensors were also wired into the 1310 so that it could use these inputs to change states.

The core method of ensuring that both motors moved at the same speed is a linear pot fitted on the beam to indicate when it is not straight (ie when one end is ahead of the other).

This was also fed into the 1310 to use as for error feedback.

In turn, the VCL computer software takes care of all of the simple operations, such as determining operating state (automatic, manual, maintenance etc), as well as calling on a "balancing routine" to ensure that the beam remains straight on the runners.

The main challenge encountered with this system was in the synchronising of the motor speeds to avoid torsion on the cross beam causing derailment.

Curtis focused on a linear potentiometer fitted to the beam to indicate misalignment, and the balancing of the speeds of the two motors under differing loads was a technical highlight.

For SGP the advantages of the Curtis system are considerable.

The project uses off-the-shelf products with a two year warranty, replacement products can be ordered and installed by their on-site engineer, and the offer of additional features in the future at the customer's request.

As Keith Dulake of SGP observes: "We approached numerous companies with this challenge, and Curtis impressed us with their enthusiasm and expertise".

"The equipment represents excellent value for money, and it can deal with a six acre site in just three hours".

"This of course represents a major logistics and financial benefit, and there is significant future business potential for this system within our group throughout the UK".