Three degrees of freedom (3DoFs) precision positioning wedge air bearing stage system (WABSS)
A three degrees of freedom (3DoFs) Wedge Air Bearing Translational Stage System (WABSS) for positioning of miniature components using a parallel kinematic mechanism. The system has three kinematic sub-chains situated at 120° apart from each other, where each sub-chain has one prismatic joint and one planar joint guided by air bearings. One central stage assembly (CSA) having three wedges with wedge planes oriented 45° to the horizontal plane is capable of translations along the X-, Y- and Z-axes for precision positioning of the targeted workpiece material held on the CSA. Following the parallel kinematic principle, the coordinated translations of the three slide bars produce 3DoFs translations to position CSA in each sub-chain. The linear geometric error along the Z-axis is independent from the angular tolerances due to its unique vertical structure.
1. A three degrees of freedom (3DoFs) precision positioning Wedge Air Bearing Stage System (WABSS), comprising a movable central stage assembly (CSA) connected to a stationary base platform (BP) via three slide bar assemblies (SBAs) corresponding three carriage assemblies (CAs) and a vertical carriage column (CC), where the CSA can translate along three axes for precision positioning by controlled and coordinated motion of the three SBAs via parallel kinematic principles, when a linear air bearing based active prismatic joint (T i , ∀i=1,2,3), and a flat pad air bearing based planar joint (P i , ∀i=1,2,3) are provided in each kinematic sub-chain, (M i , ∀i=1,2,3) such that linear actuation is provided through an ironless linear motor at each prismatic joint (T i , ∀i=1,2,3) to make cogging free frictionless translations of slide bars along a Z-axis within corresponding CA; the linear actuation is controlled via position feedback obtained from an optical linear encoder with reading head and optical scale, such that the planar joints (P i , ∀i=1,2,3) can maintain a stiffness in the presence of a magnetic preloading at each of the flat pad air bearing based planar joints; a pneumatic counterbalance attached to each prismatic joint (T i , ∀i=1,2,3) can maintain quick settling as well as gravity compensation for achieving high precision and fault tolerant in sudden power off situation, in addition to provision for further enhancement of precision via three plane mirrors attached to the CSA with the help of mirror brackets and mounts, so that the Abbe error-free measurement, which is is followed along the three directions of translations, and a vertical topological design of the WABSS are responsible for independence of linear geometric error along the Z-axis from angular tolerances and a relatively small footprint of the system, to achieve smaller thermal and geometric errors.
2. The 3DoFs WABSS as claimed in claim 1 , wherein three prismatic joints are formed between the corresponding CAs and the SBAs; and three flat pad air bearing based planar joints are formed between the corresponding SBAs and three stainless-steel plates, respectively, where one prismatic joint and one flat pad air bearing based planar joint are present in each kinematic sub-chain (M i , ∀i=1,2,3).
3. The 3DoFs WABSS as claimed in claim 2 , wherein each of the prismatic joints is actuated for coordinated and cogging free translation of the SBA via iron-less linear motor, having magnetic track and stationary motor coil, by following the kinematic principle and are further responsible for the translations of CSA along the X-, Y- and Z-axis directions via three planar joints to achieve precision positioning.
4. The 3DoFs WABSS as claimed in claim 2 , wherein each of the prismatic joints has a modular and detachable linear aerostatic bearing consisting of four aerostatic bearing plates with nozzles to facilitate pressurized air flow, where the plates are connected with a first aerostatic bearing plate and a third aerostatic bearing plate, at two ends via screws, and subsequently, the first aerostatic bearing plate is connected with a corresponding side wall of a stationary vertical CC via screws for obtaining a frictionless motion between a stationary CA and a translating SBA.
5. The 3DoFs WABSS as claimed in claim 3 , wherein each of the three planar joints is a flat pad air bearing with a flat pad air bearing plate and preloading via permanent magnets to maintain the required stiffness at a joint for frictionless motion between a bearing surface of each of the stainless-steel plates of CSA and a flat surface of the corresponding bearing plate flat pad air bearing plate of the respective SBA.
6. The 3DoFs WABSS as claimed in claim 1 , wherein following the kinematic principle, input translational motion is provided by a translatable magnetic track that is through energizing of a stationary motor coil with a required amount of current, in the corresponding SBA in each of the prismatic joints so that a respective motion is tracked by the optical linear encoder, for feedback to a motion control loop, whose reading head is mounted on a left linear air bearing plate of the stationary CA via a mounting plate.
7. The 3DoFs WABSS as claimed in claim 1 , wherein a pneumatic counterbalance, having a stationary cylinder through which the air flows, is mounted on the corresponding CA with the help of a mounting plate and a piston mounted on the corresponding SBA with the help of a mounting plate, is provided with each of the prismatic joints for compensation of gravity force, so as to increase load to weight ratio, for quick settling and also to protect the system from sudden power off situations.
8. The 3DoFs WABSS as claimed in claim 1 , wherein geometric translational error along the Z-axis is independent of the angular tolerances and amount of translation along the Z-axis.
9. The 3DoFs WABSS as claimed in claim 1 , wherein the three plane mirrors provide the Abbe error free measurement of the position of the CSA along three axes (X-, Y- and Z-axis), respectively, using three laser interferometers.