Clockwork energy harvesting
The in-wheel energy harvesting system under development uses the centrifugal force experienced by a mass rotating within the car wheel to wind a spiral spring; this spring then acts as the energy source for driving a micro-generator. This is achieved in practice by a hinged mass that rotates outward as the wheel speed increases, Figure 1a.
Figure 1. (a) schematic of in-wheel harvester.
Figure 1. (b) build up of spring torque with stop start motion.
The mass is attached to the arbour of the spiral spring via a slip clutch that allows the mechanism to return to its rest point without unwinding the spring when the car slows down. Each time the car stops and starts the spring is progressively wound to its maximum torque, Figure 1b. The energy stored in the spring is given by
where m is the winding mass, r the radius of the car wheel, D is the distance of the centre of mass from the spring arbour, g is the spring rate and ? is the radial speed of the wheel. Springs do not have a high energy density and will only store enough energy for hours of operation rather than years. The stop start nature of most car journeys though ensures that the spring will be recharged on a regular basis providing power for that journey.
For such a system with an optimised 0.1Nm/rad storage spring, a car speed of 75 KPH and a winding mass of 30g, the energy available is ~2mW-hrs. Far in excess of the few tens of microwatts a TPMS requires, allowing hours of operation on a fully wound storage spring.
One of the key factors of the system is the controlled release of the spring energy to drive the microgenerator. In most clockwork systems this is governed by an escapement mechanism. An escapement will also be required in this harvesting system to provide a gradual release of energy from the spring. The long term reliability of the escapement could turn out to be the weak point in the system when the vibration of the wheel is taken into consideration.
Energy harvesting watch technology
Clockwork has had a resurgence since the introduction of the Trevor Baylis windup radio. However the technology for powering TPMS needs to be at a scale an order of magnitude smaller. The spring based energy storage technology has been available in the form of automatically wound watches for over a century, however the generation of electricity from the spring storage at the milli-scale is a more recent addition to watch technology e.g. the Seiko kinetic watch Fig 2a. Kinetic watches house a tiny dynamo system, Fig 2b, that is normally driven by the motion of the arm and recharges a built in battery. The rotor is a rare earth magnet of only 2.6mm diameter and 95x gearing from the weight to the rotor results in very high rotation speed of the rotor.
Figure 2. Seiko kinetic watch showing (a) half weight and generator coil; and (b) generator coil and rotor.
The characteristics of the generator were determined and the generator output power was measured into a matched load using a lathe to provide a known input rotation speed, figure 3a. The lathe speed was monitored using a laser tachometer. The generator output saturates at a maximum power of ~3mW into the matched resistive load at high speed, figure 3b. From the linear part of the power curve it is determined that the generator torque is ~1.6µNm.
Figure 3. (a) Measurement of generator output power; (b) output vs. half weight rotation speed
At first sight automatic watch technology offers a simple solution to the storage spring and controlled release of energy, design requirements. However watches are designed to work at very low torque and the automatic watch springs are nearly 5000x softer than the required 0.1Nm/rad spring. The watch spring at full winding only stores ~11 µW-hrs. The escapement mechanism of the automatic watch is also a problem as it is designed to operate at much lower torques than those that would be developed by the 0.1Nm/rad spring. A hybrid automatic-kinetic watch prototype of the harvester is therefore not possible without the use of a stronger spring and a redesigned escapement.
The car motion data can be downloaded here.