Motion and positioning with magnetic direct drives

Louise Smyth

It is possible to benefit from the properties of magnetic direct drives in numerous fields of application; examples range from semiconductor production, biotechnology and medical technology to tip/tilt mirrors, dispensing, test and focusing applications and photonics or space technology. In addition, their outstanding features include relatively large travel ranges, high velocities and longer lifetimes of the drives.

Voice coil actuators and magnetic linear drives

Basically, different drive technologies can be used for magnetic direct drives. Voice coil actuators and magnetic linear drives take advantage of the fact that the force acting on a current-carrying conductor in a magnetic field is proportional to the strength of the magnetic field and the current. The electrical energy is converted into mechanical energy and generates a force which can act bidirectionally, depending on the direction of the current. Voice coil drives are characterised by high dynamics but relative low holding forces and travel ranges. They are used primarily in scanning applications with travel ranges of up to several tens of millimetres.

In principle, nonferrous magnetic linear motors correspond to a sequence of several voice coil actuators; individual coils can be controlled according to a position-dependent, defined pattern (commutation). This means basically, that either the motion of the coils or the magnet assembly is possible and therefore virtually unlimited strokes can be achieved. These types of motors can be used for both very high or very low feed velocities and work accurately in a range from below 0.1μm/s up to more than 5m/s. If combined with air or magnetic bearings, a position resolution down to a few nanometers is possible.

Position and force control

Because they are current-controlled and the driving force is linearly dependent on the current, it is not only possible to operate magnetic direct drives based on position or velocity control, but also force control. Force control allows magnetic drives and stages to be operated at a defined holding and driving force. The force and position sensors can be read simultaneously and the values processed. In addition to pure force control, subordinate position and velocity control is also an option. An auto-zero function defines the holding current, at which the drive outputs an open-loop force of 0N, eg, for compensating the weight force.

Specially designed drive technologies achieve high dynamics

A special approach to each solution allows further optimisation that goes beyond standard technologies such as voice coil and linear motors, in particular with respect to the force density, energy efficiency and size. In this case, PI relies on resonant drives with up to 60g acceleration or reluctance drives for extremely compact sizes. Special magnet arrangements (eg, Halbach arrays) can also contribute to reducing weight of the moving components, which in turn allow optimum dynamics and increased efficiency.