New Way’s Porous Media Technology Enables Arc-Second Precision in Custom Goniometer

IST received a customer request for a motion control system capable of precisely rotating and tilting large parts weighing up to 300 lbs. The system needed to provide 360-degree rotation with arc-second control and tilt the parts close to their center of axis with +/- 15 degrees and also arc-second control. To meet these requirements, we designed a compact solution: a custom rotary stage integrated into a conventional goniometer design, utilizing concave radial air bearings from New Way. This article provides an overview of the design.

The CAD image below provides a first look at the fully assembled motion control system. Note that the part itself is not shown here; it is secured to the servo rotary stage using a Schunk hydraulic expansion arbor. This arbor fits within the part’s inner diameter and expands slightly under hydraulic pressure, ensuring the part is centered and locked in place relative to the rotary servo axis. While we’ll discuss the rotary axis in more detail later, our primary focus here is the goniometer and its utilization of New Way air bearings. This design incorporates both concave radial air bearings and flat round air bearings, which we’ll describe below.

Figure 1: Custom goniometer and rotary stage

For those unfamiliar, a goniometer is a rotational device with an offset rotational axis. This offset, positioned at a specific distance from the track, typically aligns with the part’s center of rotation or a desired location to assist in a manufacturing process. This design minimizes potential offsets during tilting, while simultaneously aiming to keep the entire design compact for a larger scale process.

In our case, a goniometer was necessary to minimize part offset during tilting. Our standard practice is to explore off-the-shelf solutions, but this application’s constraints require a custom design.

To achieve the goniometer function, we incorporated monolithic 250 mm radius tracks into the base of a large aluminum part (shown as green curved tracks in the figure below). These tracks were machined to a low surface finish using a 5-axis CNC machine tool. Additional low surface finish tracks were added to the side of the part and discussed later in this article. The opposite side of this aluminum part housed the rotary servo stage, with cables and a hydraulic line routed through the base. The final dimensions of the machined aluminum part were 475 mm x 373 mm x 54 mm, with a weight of 79 lbs (36 kg).

Figure 2: Goniometer carriage CAD model with green highlighted tracks for the air bearing pads to glide on

The goniometer carriage is mounted on four concave radial air bearings from New Way. To support this, we designed a ~131 lb aluminum base (shown below as a CAD model). The red datum surfaces on the base indicate the resting positioning of the air bearings during assembly. Each bearing block features a fine adjustment screw with a ball underneath, supplied by New Way. This screw couples into the back of the aluminum bearing block, enabling precise alignment of the goniometer during assembly. The design includes air lines connecting the blocks to a main air trunk. Additionally, a Renishaw Resolute absolute encoder with single-digit nanometer motion is integrated into the system.

Figure 3: Goniometer base with radial air bearings, encoder read head and adjustment screws shown in the CAD assembly

The goniometer carriage features a Renishaw encoder tape securely fastened beneath its arced surface. This encoder strip is read by the Renishaw Resolute read head located in the bottom housing. On the opposite side, a machined cavity houses a custom rotary servo axis (illustrated below). Due to space constraints and other design considerations, we opted for a traditional THK cross roller radial bearing (295 mm OD, 160 mm ID) for the bearing element. The design incorporates a custom shaft with a Tecnotion QTL-210 frameless torque motor. Additionally, a precision Renishaw absolute rotary encoder (150 mm OD RESA) is positioned at the bottom of the shaft, providing sub-10 arc-second resolution.

Figure 4: Goniometer housing with frameless motor, precision encoder and traditional cross roller bearings.

Following the assembly in Figure 3, the unit is carefully lowered onto the radial arc bearings using lift hoists and a custom 80/20 jig (not shown). This ensures a gentle placement onto the graphite bearing surfaces. Subsequently, Akribis ACR motors are mounted to each exterior surface of the frame. The arc motor magnets are attached to the goniometer carriage, while the coil motors are secured to the frame. An external triangular bracket is then mounted. This bracket’s apex houses a New Way flat bearing pad (depicted as a blue round puck in the CAD model), which features a fine adjustment screw and ball socket on its backside. The bearing is designed to float freely while remaining contained within springs and wave washers. The primary function of these side bearings is to restrict lateral movement of the carriage while allowing for the tilting motion.

Figure 5: Assembly of goniometer carriage, ACR motors and New Way flat air bearings

The custom-designed goniometer system, incorporating New Way air bearings, exceeded performance expectations and seamlessly integrated into a specialized metrology machine for the aerospace sector.

The utilization of New Way air bearings proved to be a pivotal factor in achieving the desired arc-second level motion control. Their frictionless operation, combined with the precision of the Renishaw encoders and the stability of the goniometer structure, enabled the system to deliver exceptional accuracy and repeatability. This level of performance is essential in aerospace applications, where even minute deviations can have significant consequences. The successful implementation of this goniometer system not only highlights the capabilities of IST Precision but also reinforces the importance of air bearing technology in advancing the field of precision motion control.

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