Microwave imaging system can detect when fruit is ripe

Robot fruit pickers could one day replace humans in the fields thanks to an imaging system that identifies ripe crops.

The technology, developed by the National Physical Laboratory (NPL) uses microwaves and certain radio, terahertz and far-infrared frequencies to measure the water content of fruit and vegetables and assess whether they are ready to pick.

‘Riper fruit has a higher water content, so we can see where the crop is and how close it is to ripeness,’ NPL’s lead scientist, Dr Richard Dudley, told The Engineer.

This provides a more accurate way of judging the ripeness of fruits such as strawberries than assessing their colour. It could also lead to faster picking methods, because it can see through leaves and other visible obstructions.

Microwaves in particular are used because they are safe in low doses and pass through many substances but are stopped by water.

‘With the microwave approach you’re looking a little bit below the surface, so potentially the strawberries could be red but they might still be hard and not very full of water,’ said Dudley.

‘We can see where the strawberries are and see through the leaves. Certainly in strawberries, the leaves don’t have a particularly heavy water content so are fairly transparent and underneath the strawberries stand out very nicely.’

The system could even be used to detect disease or determine whether the crops need different amounts of fertiliser or water by continually monitoring them to see how their water content changes.

‘You can start thinking about how you can manage your crop in a more efficient way, certainly in an enclosed area such as a greenhouse, where strawberries would be,’ said Dudley.

‘But we’re also considering it for on tractors and in farm fields. That’s perfectly feasible if you can mount this technology in the right way.’

NPL has spent about two years developing the imaging system with farmers and a farming equipment company, and has built several prototypes based on technology used for material analysis that have been tested in the fields on cauliflowers and lettuce.

The biggest challenge was designing the microwave detectors to produce a high-enough resolution at a fast-enough speed to be useful.

The next step will be partnering with companies from the sensor and agricultural industries to develop the technology at a low-enough cost to mount on farming equipment, while limiting false positive readings.