Tom Shelley reports on a rotary bi-stable actuator
that is effectively a new kind of motor, specially developed for
opening and closing valves in smarter car engines
A new type of electrical device is able to function either as a
rotary actuator, swinging back and forth, or as a stepper motor,
able to run at up to 4,000 rpm.
It comes out of a bi-stable valve actuator technology that
started life as a means to produce anti-noise pulses to silence
jet engines and improve their efficiencies.
In its new embodiment, however, it has been specially developed
to produce variable poppet valve movement in a new generation of
internal combustion engines. This will allow valves to be opened
and closed at just the right moment and by just the right amount
to achieve maximum fuel economy, or if required, maximum
acceleration.
Details of the anti noise suppression technology developed by Mr
Wladyslaw Wygnanski and his company, Cambridge Concept, later
renamed Camcon, were first revealed in the May 1998 edition of
Eureka. However, it was not until April 2001, that we could
describe Camcon's breakthrough in the design of its valve
actuators. In these, potential energy stored in springs is
converted into the kinetic energy of the armature, which is
subsequently converted back to potential energy. The changeover
action is triggered by a very short electrical pulse. The double
spring action initially accelerates the armature, ensuring rapid
movement, and subsequently decelerates it, ensuring a soft
landing. All the many variants of the basic concept include
permanent magnets, spring and coils. Achieved performance
benefits in each case include: speed, low energy consumption, due
to energy recycling and zero energy consumption in stable
positions, and long life, due to the soft landing.
While the basic devices could undoubtedly move engine poppet
valves very quickly, they could not do so very quietly. Engine
manufacturers have been looking for years for an electric valve
actuator technology that would combine low cost, fast response,
small size, and variable movement with low seat landing speed.
This has to be no more than 240mm/s, equivalent to 0.01mm/cam deg
in a conventional valve train at an engine speed up to 8000 rpm.
Other requirements include: a 50,000 mile reliability and the
ability to move a 36g, 29mm diameter valve against 6 bar cylinder
pressure over 1 to 10mm with a 2.5ms reaction time. Operating
temperatures are -40 to +150 deg C while valve head temperature
may be up to 700 deg C. A normal 12V power supply is preferred.
The resulting design has a four-pole permanent magnet rotor and
eight coil windings arranged as four opposed pairs. It thus
resembles a conventional stepper motor, but being based on
Camcon's bistable technology, the device only requires input
energy to change its position. This is in contrast to a
conventional stepper motor, which exerts either no or only a very
small position holding force when not energised.
A unique feature is the use of two camshafts. One, interacting
with a leaf spring is used to effect the energy recycling. The
other, which is of very different shape, moves the valve and is
described as being 'desmodromic', in that it opens and closes the
valve rather than the more usual practice of opening the valve
against coil spring pressure which is then relied on to close the
valve. The new device achieves two dynamically stable conditions.
One, when the valve is closed, is based on mechanical
equilibrium. The other, then when the valve is fully or partially
open occurs when the permanent magnets in the rotor provide
locking torque equal to the spring force. Only a very small
electric pulse is needed to disturb the equilibrium.
Each actuator has its own positional sensor in the form of a
reluctor ring on the rotor read by a Hall effect sensor, which
inputs to the control system. One full rotation results in the
valve opening and closing with a maximum lift of 10mm. Smaller
valve openings are achieved by oscillatory movements. The
actuator has a maximum speed of response of 7ms for a complete
valve event. At 7000 rpm, this equates to a valve open period of
296 deg crank angle, whereas at 1000 rpm, 7ms equates to a valve
open period of 42 deg crank angle. Where longer valve opening
times are required, the rotor is dwelled for the necessary times.
The magnitude of the current pulses required to initiate and slow
the motor have been reduced from those initially encountered
during the development process by improved software and PWM
(Pulse Width Modulated) control. The actuator has the capability
for valve opening within 3.5ms, but this is not required under
all operating conditions. Therefore, in the interests of energy
management, it is favourable to slow the valve opening and
closing time rather than having a fast opening event followed by
a dwell period and a fast closing event.
When a braking pulse is used, the actuator effectively becomes a
generator. This energy can then be fed back into the vehicle
electrical system. The work done during one complete actuator
cycle is 1.5J. A four cylinder valve train using 16 actuators
operating at 67Hz (8040 rpm) would require 1.6kW of electrical
power. This compares favourably with a direct operating
mechanical valve train, which typically consumes 3kW.
The development work was conducted in conjunction with
Powertrain Ltd, who prior to the demise of the MG Rover Group,
successfully packaged it into a 'K' series engine with valve
centres of 35mm and cylinder bore centres of 88mm. Under the
intellectual property agreement, all IP has since reverted to
Camcon which is thus now in a position to offer it to MG Rover's
successors or other companies.
The mechanism design was undertaken using SolidWorks.
The original jet engine noise suppression project is still
continuing in co-operation with Rolls Royce and leading
laboratories with support from the DTI.
Other valve developments include a design to admit air to the top of helicopter blades to increase lift during the backwards part of their rotation and special purpose valves for the oil and gas industry. One of these is a Camcon "Roller" valve with key ring springs to apply sufficient force to allow the crushing of debris when the valve operates. Crushing/sealing force is 500N and magnetic locking force is 25N. Working pressure is 60bar and orifice size is 6mm. The specification requires reliable operation over 1 million cycles.
More information from www.camcontec.com
(Camcon Technology) and www.solidworks.com
(SolidWorks)
Pointers
* Actuator resembles a stepping motor, but with a much higher
position holding force at no current and low overall power
consumption
* Device has been developed specifically for the car industry but
is equally appropriate for other applications
* Design has a four pole rotor and eight stator coils. Response
time for a complete revolution is 7ms but could be reduced to
3.5ms. Maximum rotation speed is 4,000 rpm, corresponding to a
maximum engine speed of 8,000 rpm.
For more technical
developments see www.eurekamagazine.co.uk