Figure 1. Outline of the HABR actuator.
Résumé
Les organes de manoeuvre robotisés destinés à l'usage spatial doivent satisfaire à des exigences rigoureuses en matière de température de survie et de fonctionnement, de lubrification, de fiabilité, de douceur et de linéarité du mouvement. Cela complique en particulier de manière très sensible la conception des freins et boîtes de transmission, et contraint à employer des configurations et des matériaux inusités. Cet article présente un nouveau principe d'actionneur compact à transmission directe adapté aux rigueurs du milieu spatial grâce à un dispositif électromécanique simple mais efficace.
Contractors
Tecnospazio (I)
Phase Motion Control (I)
Funding
Basic Technology Research Program
Robotic actuators intended for use in space must meet very demanding requirements on survival and operational temperature, lubrication, reliability and smoothness and linearity of motion. In particular, these requirements severely complicate the design of brakes and gearboxes, forcing the use of special materials and unconventional configurations.
This article presents a new concept for a compact, direct-drive actuator which meets the demanding requirements of space with a simple but efficient electro-mechanical arrangement. The main aim of this new development has been to realise an intrinsically reliable unit, which can replace the traditional mechanical joints found in robots and other mechanisms used in space. Standard units generally consist of a miniaturised motor, a gearbox assembly, a safety brake, one or more (commonly two) position sensors, and an associated mechanical train of bearings, friction surfaces and couplings. The resulting high number of components limits the achievable robustness and reliability of the device, while non-linearities in the gearbox degrade the smoothness and precision of the motion control.
The development described in this article offers a substantial improvement in terms of high torque, high accuracy, safety- braking and direct position control in a device contained in a simple mechanical housing; it has an accurate and sturdy bearing system which allows direct implantation of the drive in the driven mechanism.
The mechanism uses a high-torque, low-speed, axial gap brushless robotic (HABR) actuator. This device comprises a permanent magnet motor with two axial air gaps; a thin rotating disk, fitted with rare earth magnets on each side, is suspended between two multiphase, multi-pole, disk stators. Maximum use is made of the magnetic energy of the rotor, and by minimising the leakage flux, an exceptionally high torque density is achieved, which is fundamental for direct drive applications.
In the geometric configuration chosen, the rotor is in unstable equilibrium and it assumes a position in the centre of the air gap. By introducing a little axial play in the design of the shaft, two stable working points are created, slightly displaced with respect to the centre of symmetry of the air gaps. At one of the working points, the rotor disk bears against a rotating member mounted on the shaft and is thus free to rotate; at the other working point, the strong magnetic attraction, inherent in its construction, presses the rotor disk against a friction pad and generates a self-braking action. The motor is also an intrinsic bistable safety-brake, i.e. a brake which requires no electric energy.
Switching between the two stable states does not require a separate solenoid or control system, as in conventional safety brakes. Instead, this is done with the windings and drive of the main motor, by simply short-circuiting one of the two stators and shifting the current vector in the other stator by 90 degrees with respect to the motor electromagnetic flux, thus generating an axial, or lift-off force, akin to the forces used to control magnetic bearings.
The overall shape of the actuator, which comprises a sandwich of three disks, is naturally short and wide, allowing the use of a direct pancake sensor, large bearings and a generous central orifice for routing the wiring of the actuated machine. The short and compact geometry of the joint requires only one bearing assembly, made of two preloaded, angular contact ball bearings. This mechanical solution is extremely stable and can withstand large temperature excursions, while retaining high accuracy and load carrying capacity in all directions.
A demonstration prototype of the actuator (Figure 1), capable of up to 10 Nm torque, was developed and tested in the laboratory by Phase Motion Control and the expected performance targets were met. The unit measures only 110 mm in diameter by 87.5 mm in length, and has a through-hole 25 mm in diameter; it weighs 3.3 kg with an 18-bit absolute position sensor included. The unit, as all direct drives, has all the advantages typical of non-mechanical actuators, such as a control bandwidth in the region of 100 Hz. Its only source of friction is the main bearings and it functions almost independently of temperature with little wear to its mechanisms.
Figure 1. Outline of the HABR actuator.
This prototype is now being used for vacuum-thermal testing the microwave imaging radiometer intended for ESA's Metop satellite. In this application, the actuator must rotate an instrument test jig weighing over 70 Kg at a speed of 30 degrees per second for a period of several weeks.
A larger unit has been developed for machine tool applications in which a direct drive torque in excess of 1000 Nm has been obtained.
Figure 2. Demonstration model of HABR
(courtesy of Tecnospazio).
Our novel direct drive system has characteristics which make it well suited to a number of space missions. Its performance makes it appropriate for high-precision applications such as pointing platforms or the manipulator of a geostationary servicing vehicle. It has only two friction contact points, making it ideal for use in a high vacuum environment (e.g. deep space probes). The simplicity of its design, with only one moving part, provides the extended lifetime and high reliability which are required in long duration missions, such as manipulators for the Space Station. It is also suitable for planetary exploration missions such as the exploration of the Moon or Mars, where its flat cylindrical shape and hollow shaft allow it to fit inside the wheel of a planetary rover vehicle.
Preparing for the Future Vol. 7 No. 2