465 Holding brakes for Space Mechanisms with M inimum Power Requirement  

Ted Hopper *, Christoph Stuckmann*, Scott Starin ** and Walter Whitehead ** 

Abstract  

In space applications , the rotational axes of mechanism s that are driven with electric motors  need to be  secured, not only during launch but very often during most of their operational life or cycle.  Holding brakes   are therefore required, which are mostly engaged in the un- energized condition.  Such brakes should,  however , require a minimum of elect rical power  during release, as energy  is at a premium in space. It is  also important to avoid or to  minimize  thermal losses . 

In this paper various alternative holding brake technologies are presented with their characteristic advantages and disadvantages.  

Introduction    One preferred motor  type for space use  is the hybrid stepper as this  electric motor  type has good torque  density . It can further operate without angular position feedback  and exhibits a high intrinsic detent torque  at many holding positions  per revolution (typically 200) . However , there also many applications  that require  servo or torque motors  that do not or should not exhibit a ny natural salient holding- torque positions.  In this  case or when a stepper motor is  used for the design of a specific mechanism but its detent torque is   insufficient, a brake must be implemented, which must  also fulfil a major secondary requirement – low or  better zero power  consumption in the holding condition. We consider four interesting br ake implementations   that can meet this requirement :  • Locking and release mechanisms   • Friction holding brakes   • Reluctance brakes, passive   • Reluctance brakes, active    Locking and release mechanisms -  electromagnetic , pyrotechnic  (when only one- time operation is required)   etc. – are common in space applications but  are not  described here further .   Friction holding brakes, usually with spring operation and electromagnetic release, are frequently  used in  industry. Two suitable  configurations for space use are presented  below . This type of brake has the  advantage of allowing the motor shaft to be held in any position but it has the significant disadvantages of  mechanical interference and wear; it  also requires a mechanical separation movement of two brake plates .  It is often a major challenge to ensure safe movement of the brake plates over the wide and demanding   environmental conditions of space application.    The passive reluctance brake is also  employed in space. It has a symmetrical  geometry using highly salient  poles on stator and rotor , which cause the brake to relax automatically to preferred positions with a high  detent torque, thereby holding the s haft in one of these positions .   The fourth option, the “active reluctance brake”, we believe to be unique. This has been  developed to meet  the requirement of ze ro or minimum torque or damping when excited. Like the passive rel uctance brake,  it  also has no wearing or  mechanically interfering parts  and consumes no power when active.                                                     * MACCON GmbH, Munich , Germany    ** Avior Control Technologies, Inc. , Longmont , CO  

Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016