Abstract— Electrical drives employing motors able to directly manage multiple degrees of freedom may result of relevant interest in several application fields, e.g. robotics and tooling machines. This paper recalls first the features of a rotary-linear brushless machine recently introduced, highlighting the ideal possibility to achieve a decoupled force/torque regulation through a suitable supply strategy. A control system for such machine is then presented, using adaptive yet relatively simple regulators. A complete drive structure is finally analysed by simulation using a simplified model suitably developed in Matlab-Simulink®. Index Terms— electrical drives, rotary-linear brushless motor, multi-degree-of-freedom machine I. INTRODUCTION Articulated controlled movements involving multiple degrees-of-freedom (DoF) are increasingly required especially in advanced industrial applications, such as robotics (e.g. arms, end-effectors), combined tooling machines (e.g. tools exchange, simultaneous machining) pick-and-place positioning systems etc.. In most cases, such movements are obtained by mechanically combining linear and/or rotary elementary motions, generated independently. Nevertheless, this requires a suitable mechanical structure able to perform such combination, equipped with as many standard single- DoF drives as the total number of elementary motions. Usually, such solutions result in expensive, heavy and cumbersome apparatuses. In such applications, the employ of actuators able to simultaneously manage more DoF is then expected to permit a significant simplification of the mechanical structure, an overall reduction of weight and volume, and eventually an improvement of performance in terms of both dynamics response and precision. Nevertheless, till now in the technical literature a small number of configurations of multi-DoF electric machines has been proposed. The most diffused one is probably the Sawyer step planar motor, which was conceived for the precision x-y motion capability required in the semiconductors manufacturing processes [1]. Spherical motors have been also proposed, especially referring to applications such as pointing of micro-cameras [2] for automation purposes. For what concerns instead rotary-linear machines, the conceptually simplest solutions proposed consist in mechanically coupling a standard induction rotary machine, having a prolonged bulk mover, with a linear machine, such as a voice-coil actuator [3] or a linear induction motor [4]. Nevertheless, the poor integration level of such solutions leads to modest overall performances. A more integrated machine is proposed in [5], featuring 4 linear poly-phase induction stators evenly surrounding a cylindrical bulk prolonged mover. Here the rotary torque is obtained by suitably time-shifting the supply current terns relative to each stator. Anyway, four 3-phase inverters are required to supply the windings, while a true decoupled regulation of force and torque appears problematic. In [6] a "screw- thread" cylindrical linear reluctance motor is allowed to also rotate and is supplied with the superimposition of a low frequency tern and a high frequency tern of currents. In fact, the linear motion results primarily due to the low-frequency component, while the high-frequency component is mainly responsible of eddy currents induced in the bulk mover, giving rise to a net torque by induction effect. A grossly decoupled regulation becomes so possible by separately controlling the amplitude of these 2 components, although at the price of a relatively complex control system requiring a fast switching inverter. A wider description of multi-DOF machines is provided in [7]. II. MACHINE STRUCTURE A novel rotary-linear machine, featuring a fully integrated structure exhibiting interesting properties, was recently introduced [8]. In its simplest configuration, the proposed structure consists of the following parts: - a ferromagnetic core featuring an external yoke and 2 sets of 4 identical salient poles evenly spaced along the tangential direction; the 2 sets are in turn positioned side-by-side along the axial direction, with a half-pitch angular displacement; - a stator winding composed of 8 identical coils, wound around the poles, which are grouped into 4 phases by connecting in series, with reverse magnetic polarities, each pair of diametrically opposed coils; - a ferromagnetic cylindrical sleeve featuring a length about triple of the axial pole pitch a , fitted onto a mechanical shaft supported by rotary-linear bearings; - a permanent magnets ring (or a set of separated magnets) fitted centrally onto the sleeve, featuring an axial length about equal to the pole pitch and divided into 2 equal angular sectors having opposite radial magnetization and separated by small transition bands. In fig. 1, 2 are reported a front view and an axial Paolo Bolognesi, Member, IEEE, Alberto Landi, Member, IEEE, Lucio Taponecco Department of Electric Systems & Automation - University of Pisa - Via Diotisalvi 2, 56126 Pisa - Italy e-mail: p.bolognesi@ieee.org, landi@ieee.org, lucio.taponecco@dsea.unipi.it Control of an Unconventional Rotary-Linear Brushless Machine