COP Outcome development and evaluation

The basic problem facing the designer of a carrot-crowning machine is the variability in the position of the carrots, relative to any point on the machine. There is no fixed datum from which the cutting height can be measured. Despite the best efforts of the grower when preparing the ground, the bed will not be exactly flat, or level. Nor will it be at a fixed height above, for example, the tractor wheels point of contact with the ground.

Even if this level of precision could be achieved, the carrots themselves can grow at different heights in the bed, so the point-of-intersection between the body of the carrot and its green top is only known within a few centimetres.

In cases such as this the designer has to resort to approximations. It was observed that the variation in the level of the soil surface itself, V1,was much greater than the variation of the point-of-intersection relative to the soil surface,V2 (Diagram 1).

V1 = variation in height of surface from datum

V2 = variation in height of bottom of crown from the soil surface.

This lead to the decision to locate the cutter blades relative to the soil surface. This leads naturally to the need for a way of following the soil surface. This was achieved by having large-diameter rollers at the front and back of the machine, with the blades mid-way between.

The large diameter of the rollers reduces the tendency of the machine to sink into the soil, or to damage the carrots. Their distance apart averages-out any change of level in the soil surface, ignoring short-wave-length disturbances, but following long-wave-length ones.

The way in which the machine is attached to the tractor allows the machine to follow changes in the contour of the bed, both lengthways and side-to-side, without constraint. This contributes greatly to its ability to crown the carrots accurately.

The second big challenge is the actual cutting mechanism. Anyone who has tried to cut a carrot with a blunt knife will be very surprised to learn that a very sharp edge is not required, providing the edge is moving fast enough. This is why rotary lawn mowers work so well. This is a very fortunate phenomenon, because no matter how sharp a blade you start with, a few minutes in contact with soil will soon blunt it.

Unfortunately, the cutting action of a lawn mower blade (which is like that of an axe) is no good for carrots, because it tends to split the carrot near the cut. A further problem is the tendency of such systems to scatter the carrot tops far and wide. This can only be overcome by fitting additional mechanisms to collect the tops, at extra cost and complication. What is needed is a much gentler action, rather like a saw. There are many ways of achieving this, most of which are too complicated to work for long near (let alone under) the ground.

The solution is to use small circular blades revolving around their own centres and also around a much larger circle. Such motions are usually described as orbital. This was one of the best features of the original machine. Note that the blades do not have teeth, even though they use a slicing action.

As the orbiting discs advance, they slice into the carrot, rather than just striking square-on. The discs themselves are adaptations of disc-plough blades, easily obtainable, made from 3mm thick spring steel, which remains sharp enough in-use to give clean cuts. This arrangement works well in conjunction with flaps fastened to the big wheel, which push the cut-off tops to the side of the bed.

The ingenious mechanism, which causes the blades to rotate as they orbit around the 'big wheel', is shown in the photographs, and involves a friction drive via pneumatic tyres. These have the advantage of being wholly resistant to clogging by debris, and thus stalling. This is a huge benefit in the field.