Hexapods offer agile motion for micromachining

Some consider it exotic and pricey, but a nimble, six-legged device has the moves to make it a useful addition to many micromachining operations.

The M-810 miniature hexapod provides six axes of motion and submicron precision. Image courtesy Physik Instrumente.

The device is a hexapod, and it consists of six parallel actuators directly connected to a bottom plate and a top plate that serves as a moving platform. The actuators can move this platform in six axes: three linear (X, Y and Z) and three rotational (pitch, roll and yaw). Either a workpiece or a tool can be mounted on the platform, with the other being stationary.

The more common way to get 6-axis motion for micromachining or other applications is to stack three linear and three rotational stages. Compared to conventional stacked stages, hexapods offer a smaller package. “The beauty of the hexapod is you can get all six axes in a compact structure,” said Beda Espinoza, senior manager for motion for Newport Corp., Irvine, Calif., which manufactures and distributes hexapods and other equipment.

In addition, the hexapod is a parallel kinematic system, so the positioning errors of the individual actuators aren’t added together. With stacked serial stages, however, the errors of the separate stages add up, adversely affecting overall positioning accuracy.

Hexapods also feature a moving platform as well as high system stiffness because all the actuators support the load. The stiffer the system and the lower the moving mass, the faster the system can be stopped, explained Stefan Vorndran, vice president of marketing for PI (Physik Instrumente) LP, Auburn, Mass., which offers hexapods in different sizes and configurations.

Referring to positioning systems made of six stacked stages, Vorndran pointed out that “every time you add something, the system gets ‘softer,’ and you also add mass. If something is soft and heavy, it wants to keep moving and it takes more time to stop it. But if something is stiff and lightweight, it stops quickly when you want to stop it.” The actual time difference between the two might only be a fraction of a second, he added, but that difference can be important in meeting motion requirements.

Shown is a hexapod on a robotic workcell for manufacturing fiber-optic components. Image courtesy Aries Innovations.

Other hexapod advantages include:

• No moving cables. In stacked-stage systems, each stage is connected to a motor cable. When the stage is in motion, it must overcome cable inertia and friction, limiting accuracy and repeatability.
• Virtual centers of rotation. Both PI and Newport hexapods come with controllers that let users choose any point in space as the pivot point for the rotational axes.

The case for stages

Stacked stages, however, have their advantages. These include superior linear motion accuracy, according to Espinoza, who noted that linear stages can be very straight and equipped with linear encoders to improve accuracy. On the other hand, he said there is no closed-loop position measurement of a hexapod platform. “You can only infer where the platform is from the position of the actuators,” Espinoza said. “And since a hexapod depends on the motion of six actuators, you may not be able to define a straight path.”

In addition, stage systems can provide greater motion range. With a hexapod, rotational travel range is typically limited to 60°, according to Vorndran. If more rotation is required, users must add conventional rotation stages.

The HXP100 is for complex applications that require six axes of motion. A set of two programmable coordinate systems enhance flexibility when orienting a tool or workpiece. Image courtesy Newport.

As for linear movement, the motion of one stage in a stack is independent of the motions of the other stages. So, if there’s full travel in the X-axis, full travel in the other directions is available as well. But that’s not the case with a hexapod, because “everything is interdependent,” Vorndran explained. As a result, full travel in one linear direction limits the travel distances in the other directions.

Then there’s the cost. If four axes or fewer are needed to provide the required motion, hexapods are at a cost disadvantage, according to Vorndran. But if your machining application requires six axes of motion, he added, hexapod prices are competitive with those of stacked-stage systems. “For a complete system that includes a controller and software, the price of the two [options] is very similar if you want the same precision,” he said.

Regarding precision, those considering the two options must factor in the motion-error accumulation of stacked stages. “Sometimes people look at one axis [of a stacked-stage system] with 1µm precision and think that it’s so much better than a 3µm precision hexapod. But in the end, it’s not,” Vorndran said.

Vorndran’s firm offers two miniature hexapod models that cost around $50,000 each. Measuring about 4" in diameter and height, PI’s M-810 offers minimum incremental motion of 0.5µm and repeatability to ±0.5µm.

Machining with hexapods

For micromachining, Espinoza believes the hexapod is useful because it lets users orient a workpiece or tool in different axes. He often sees hexapods enlisted for managing laser beams, and a hexapod could move a beam that shapes a micropart. Newport researchers, for example, are examining laser micromachining of uneven, curved surfaces with a hexapod. One purpose might be to create optical elements that mimic the eyes of insects.

To the best of Espinoza’s knowledge, however, hexapods are not yet widely used in micromanufacturing. “People think of the hexapod as expensive and complicated,” he said. “But we are moving toward making it more affordable.” For example, Newport hexapods are driven by actuators that are variations of the company’s proven actuator designs for positioning applications, rather than special actuators developed just for hexapods. This reduces system cost and improves reliability, according to Espinoza.

In addition, Vorndran indicated that PI is working on a hexapod in the $20,000 range. Expected early next year, the new system won’t be quite as precise as the M-810, but Vorndran thinks it could be good enough for applications that don’t require the highest precision.

Espinoza and Vorndran are optimistic about the future of the devices. “The technology is still rather new and not too widely known,” Vorndran said. “But prices are beginning to come down, more products are being introduced and manufacturers have more experience making them. It is a very accurate technology and will find more applications.” µ