The concept of an integrated brake

In order to remedy to the disadvantages of friction brakes, the integrated brake concept was developed as part of the present research (Fig. 3).

Fig6.1: Integrated braking system

The integrated brake combines a friction brake with an eddy-current brake on the same caliper. This combination has several advantages:

- Reduced wear of friction pads: the eddy-current brake can provide a large fraction of the braking force, thereby reducing the amount of kinetic energy dissipated at the pads and consequently reducing their wear. The eddy-current brake is contactless and therefore wear-free.

- Reduced sensitivity to fading: the eddy-current brake can assist the friction brake when the rotor is hot. The combination of two sources of braking torque compensates for their respective loss of effectiveness at high temperature. Furthermore, it is possible to increase the effectiveness of the friction brake by keeping the pads cool. This is achieved by relying as heavily as possible on the eddy-current brake.

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- Reduced sensitivity to wheel lock: the eddy-current brake reacts faster to control inputs than a friction brake. Therefore, the brake‟s control system can prevent wheel-lock more easily than with friction brakes. Furthermore, the friction brake is mostly used at low speeds. The effects of wheel lock are much less severe at low speed than at high speed.

- Faster control dynamics: the eddy-current brake is directly controlled by its excitation magnetic field. The response time of an eddy-current brake is counted in milliseconds, whereas the response time of mechanical systems is counted in tenths of seconds. This is particularly true of power assisted and pneumatic brake systems.

- Easier integration with vehicle electronic driving aids: ABS, traction control and dynamic stability systems require fast response times for more precise and safer vehicle control.

The fast response time of eddy-current brakes makes them more suitable for interfacing with these electronic driving aids.

- Reduced fuel consumption of power assistance: the primary reliance on eddy-current braking reduces the maximum braking force required from friction brakes. The power assistance requirement is consequently decreased, making it effective to replace the hydraulic actuation and vacuum assistance by an electric actuation, which drains power only when actuation is needed.

However, their requirement for a large excitation current is a major disadvantage. The most significant drawback is the lack of failure safety. The excitation current may not be available for a variety of reasons, in which case the retarder is totally useless.

Furthermore, the excitation current is necessarily supplied at a low voltage, which induces high ohmic losses in conductors, diminished bus voltage, and renders electronic control challenging. Additional resulting problems include heavy wiring from the battery to the retarder, heating of the coils.

In order to palliate to these disadvantages, it is possible to replace the electromagnets by permanent magnets. However, in gaining a loss-free, powerless permanent source of excitation, controllability is lost. Indeed, permanent magnets cannot be “turned off” or controlled directly. The magnetic flux crossing the airgap to excite the disc must be

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controlled by a variable magnetic circuit on the stator. Most patents for permanent magnet retarders revolve around variable magnetic circuit architectures.

One commonly encountered flux control scheme involves a drum brake exited by permanent magnets facing the inner side of the drum and attached to a ring. The ring of magnets is moved in an out of the volume inside the drum to modulate the surface area of magnets facing the drum. Thus, the amount of flux crossing the airgap to excite the rotor can be varied continuously. This scheme has been implemented and tested by Isuzu in Japan [4]. There are several disadvantages to this architecture: the magnets are in the airgap and thus exposed to the heat generated on the drum, the whole stator has to be moved and when completely disengaged, nearly doubles the length of the retarder.

US Patent #6,237,728 relates to a drum brake, using two rows of permanent magnets.

Each magnet is included in a horseshoe ferromagnetic circuit. One row is attached to a fixed stator inside the drum. The other row is attached to a stator, which can be rotated slightly. The rotation brings the horseshoes from each row to present the same polarity to the drum or gets the horseshoe of one row to short-circuit the horseshoes of the other row. While this system is compact and does provide the ability to turn-on and off the flux in the airgap, it doesn‟t provide the ability to control the flux linearly between the two extreme positions. Furthermore, the magnets are only minimally preserved from the heat generated on the rotor.

US Patent #6,209,688 and 5,944,149 relate to a drum brake with two rows of permanent magnets. One row is mounted on a fixed stator, while the other is mounted on a stator that can be rotated slightly. A ferromagnetic plate is between the magnets and the inner surface of the drum brake. If two magnets with different polarities are paired, then the flux shunts through the ferromagnetic plate. If the polarities are similar, the flux is pushed towards the drum and braking is induced. This structure is no more capable of linearly varying the flux between the on and off positions than that claimed in the previous patent. The magnets are also located close to the heated rotor.

44 6.2 Novel eddy-current brake concepts

Existing eddy-current brake concepts all have several disadvantages that make it difficult to integrate them with a friction brake in a same unit. We developed a novel concept of eddy-current brake suitable for integration with friction brakes. The novel eddy-current brake uses rare-earth permanent magnets instead of electromagnets to generate the excitation magnetic field without dissipating energy. Rare-earth permanent magnets are very compact sources of magnetic flux, much more than electromagnets.

TABLE 2 shows a comparison of rare-earth permanent magnet materials with conventional permanent magnet materials.

Neodymium-Iron-Boron (NdFeB) magnets have a higher energy product than Samarium-Cobalt (SmCo) magnets. They are also cheaper because neodymium is a much more commonly occurring metal than samarium. NdFeB is thus the preferred permanent magnet material for a low-cost, light, compact and powerful flux source.

However, it has a much lower curie temperature than either Alnico or SmCo.

Furthermore, neodymium magnets cannot practically be operated without a significant loss of their magnetization beyond 100ºC.

There are therefore two challenges in using permanent magnets as flux sources in eddy-current brakes: controlling the magnitude of the flux and preserving the magnets from the high temperatures. Two variable-geometry magnetic circuit structures were developed to control the flux from permanent magnets: the shunted magnet structure and the rotated magnet structure. In the shunted magnet structure, a ferromagnetic bar is used

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to bypass the airgap and short-circuit the permanent magnet. The flux density in the airgap is varied from zero to a maximum by sliding the bar from a shunting position to a non-shunting position. Fig. shows the shunted magnet structure in shunting position and Fig. shows the same structure in non-shunting position.

Fig. : Shunted magnet structure, shunting position Shunted magnet structure, non-shunting position

In the rotated magnet structure, the magnet is rotated from a position of alignment with the magnetic circuit to a position in quadrature with the magnetic circuit. When aligned with the magnetic circuit, the permanent magnet delivers all its flux through the airgap. When the permanent magnet is in quadrature, its flux is short-circuited by the magnetic circuit without ever reaching the air gap. Fig. shows the rotated magnet

structure in aligned and quadrature position.

:

Fig: Rotated magnet structure, aligned position Rotated magnet structure, quadrature position

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Both structures provide thermal protection for the magnets by keeping them away from the airgap and by providing some room to insert a heat shield. Additional thermal protection is required to limit heat conduction from the poles of the magnetic circuit.

Variable-geometry magnetic circuits provide a simple, compact, cost-effective, and fast way of controlling the flux from a permanent magnet over a broad dynamic range.

Only a small electric actuator is required to control the geometry of the circuit.

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REFERENCES

1. R. Limpert, Brake Design and Safety. Warrendale, PA: Society of Automotive Engineers, 1999.

2. Telma. (2004, December). Nos Produits. [Online]. Available: www.telma.com.

3. H. Sakamoto, “Design of permanent magnet type compact ECB retarder,” Society of Automotive Engineers #973228, pp. 19-25, 1997.

4. J. Bigeon and J.C. Sabonnadiere, “Analysis of an electromagnetic brake,” IEEE Journal of Electric Machines and Power Systems, vol. 10, pp. 285-297, 1985.

5. . J.H. Wouterse, “Critical torque and speed of eddy current brake with widely separated soft iron poles,” in IEE Proceedings-B, vol. 138, no. 4, pp. 153-158, 1991.

6. wikipedia: http://en.wikipedia.org/wiki/Eddy_current_brake

7. wikipedia: http://en.wikipedia.org/wiki/Electromagnetic_brake

8. Youtube : http://www.youtube.com/watch?v=TxYh6TodacM&feature=related

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