Making the case for brushless wheels

Written by: Tom Shelley | Published:

Tom Shelley reports on a breakthrough brushless wheel motor that could prove a suitable case – for suitcases



A new brushless wheel motor – only 36mm wide – is all set to give a welcome extra lift to the wheels on a suitcase.
Able to help get a 30 kg case up a kerb, the motor comes with an ingenious controller, built into the wheel hub, that can be used to deliver torque, rather than speed. It is also very low cost and the makers see it as ideal not only for suitcases, but also golf trolleys, folding wheelchairs, toys and industrial applications.
PML Flightlink in Alton, Hampshire, is already well established, with a solid reputation for its very thin pancake motors, with printed circuit armatures. In 2001, Martin Boughtwood purchased the company with the specific intention of expanding it into the manufacture of brushless wheel motors for electric vehicles. This led to the development of the highly efficient EW series of motors – three-phase brushless motors, with separate electronic drives, specifically suited for traction and turbine applications. This has now led on to the ‘Hi-Pa Drive’ motors, which sales engineer Simon Peaty believes “will cause a revolution in the electric vehicle market”.
These motors are the power source behind the Mini QED hybrid car, first shown in 2006, which uses four 120kW units to accelerate from 0 to 100kph in 4.5 secs. While BMW has not endorsed the design, it led to development projects with other major car manufacturers and an enquiry from Clive Hemsley, the chairman of Live Luggage, based in Henley-on-Thames, as to whether the company could develop smaller wheel motors for a power-assisted suitcase.
This PML Flightlink has done – and it is now producing 2,000 motors a month for the product, as well as working on two other, as yet undisclosed, projects.

At first sight, the suitcase looks like a conventional plastic one, apart from having a rather nice adjustable handle. However, within is a battery pack containing 10 AA nickel metal hydride batteries. On the outside are the two wheels, which look like many other suitcase wheels, except they are slightly larger at 191mm in diameter and incorporate PML’s patented airless tyres. Also, there is a series of skeletal elliptical shapes between the hub and rim to absorb shocks, as an alternative to air-filled or large thickness rubber tyres.
The motors and wheels, designated ‘Nanowheels’, weigh just 1kg each. Inside is an internal stator, with wound, laminated iron poles, acting on neodymium iron boron magnets on the interior of the rotor. The magnets are very thin and backed by an equally thin iron ring round the entire rotor, in order to complete the magnetic circuit. The drive delivers a maximum of 12V, 12A and the motors deliver a maximum torque of 2N-m, although they can, in fact, be run at up to 18.5V.
Pulling on the suitcase handle delivers a force that is measured by what Peaty describes as a “Carbon pill” sensor. The drives, based on 16 bit PIC micro-controllers, are in each wheel hub, with Hall Effect sensors on the undersides of the printed circuit boards to detect the positions of the rotors (from sensor magnets on their insides). The drives deliver just enough current to the motors to give them sufficient torque to reduce the force detected in the handle to a low level.
Eureka carried out its own ‘test drive’ by pulling a catalogue-filled suitcase first up a slope and then a kerb, which is normally the hardest part of getting a suitcase to or from an airport, or train station. The suitcase negotiated the kerb with no noticeable increase in pulling force. We were told the suitcases have been tested to 60kg of content – almost double the maximum weight allowed by airlines – and have survived various drop tests. The electronics on both sides of the boards are potted in epoxy to provide environmental protection, while the wheels are made of a fairly rugged blend of polycarbonate and ABS. This we found to be fairly hard, although the airless tyre design was sufficient to help the cases get through the drop tests. Should softer suspension be required – say, for power assisted pushchairs - the structural parts could be made of some softer material, we were assured, over-moulded with a suitable elastomer.
Having proven that the motors and drives work, the effort now is to find them new markets. According to Peaty, “the whole thing is quite scalable”, with the motors selling for “only tens of pounds” in large quantities. Manufacturing is inevitably in China. We were shown a design for a 6N-m motor - within the same diameter, but with triple the width - made by stacking three 2 N-m motors side by side, with a common external rotor. Moving up to a 10 N-m version is regarded as not being a problem.
Bearings are plastic with steel balls and were specially designed for the application by injection moulders Cobb Slater, now part of BNL. They run particularly smoothly, which is especially useful if the batteries happen to be flat – when a suitcase would have to be moved without power assist.
These wheel motors are not suitable for fast road applications, like the Mini QED, but they do have the potential to be developed for wheelchairs. At present, ride-on and ride-in vehicles for the elderly are either chunky vehicles that run along like small electric quad bikes, but in no way can be lifted into a car or bus, or are hand-operated affairs – while the wheelchairs offered by the NHS are devices that have to be propelled along by somebody else.
The new wheel motors are small enough to replace the current wheels on NHS chairs and it would then only require removal of a suitable battery pack to allow them to be folded for transportation in the boot of a car. Alternatively, a design that still folds would solve the problem - but without having to remove the battery packs. As regards the torque control system (to allow power-assisted operation of machines), these could be used for a whole host of industrial applications.

Pointers

* Present motors deliver 2Nm of torque to assist the movement of heavy luggage. They are 36mm wide and 191 mm in diameter, with built-in drives

* Power is delivered by 10 AA nickel metal hydride batteries

* Cost in large-volume quantities is in terms of tens of pounds

* The wheel motors can easily be scaled up to deliver more torque or higher speed


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