What kind of bearings and mechanical transmission are you using?

"When you're talking about moving at a rate of one nanometer per second, the ball on a ball bearing is hardly moving. Any kind of stickslip is your enemy," says Floresta. To combat this stiction, he recommends very careful selection of linear bearings, for low disturbance, low friction, and high precision. With ball screws, stiction and compliance will likely cause problems at extremely slow speeds. Instead of this mechanical transmission, consider using a direct-drive linear motor. Also appropriate might be air bearings, which provide smooth constant motion with very low disturbance.

What's your vibration environment?

In the sub-micron positioning world, even the softest footstep or the quietest voice can introduce unwanted vibrations into the application environment, notes Floresta. Here, consider an active isolation system to control the machine's environment, such as a 6-ft. deep concrete pad for the tool or measuring stage to rest on. Also, remember that anything attached to your actuator is a potential disturbance, including cables and cable carriers that introduce vibration with rolling and unrolling movements. EMI and other electrical noise are other considerations.

How's your control system?

Think about the controller itself and its ability to sense and reject disturbances. A controller with an extremely high bandwidth that can react quickly using sophisticated control algorithms is your best weapon against system turmoil in slow speed applications, advises Floresta.

Oil film eases slow motion

Think about driving in the rain at a high speed. But be careful. You may encounter the phenomenon of hydroplaning, which creates a thin water film between tires and road, meaning a loss of car control. In the machine world, an oil film between metal-on-metal interfaces is a good thing, as it protects surfaces from wear. But what happens when machines move very slowly? This isn't such a good thing, because the oil film breaks down. When speeds are greatly reduced, adjustments are needed to maintain smooth motion by keeping the oil film stable.

Yoshiro Oishi, standard product engineering manager at THK America Inc., offers two ways to keep linear motion working smoothly in slow speed applications. One is to take a good look at lubrication. THK has developed a new grease, AFJ, which creates a strong oil film. When speed becomes very slow, such as 0.1 m/sec, standard greases show significant wear in comparison to the AFJ grease, according to Oishi. The mineral oil-based grease contains a urea-based thickening agent plus a special additive to keep the oil film strong and stable in lowspeed applications.

Another tactic the company uses in slow speed applications is caged ball technology, in which the use of a cage allows lines of evenly spaced balls to circulate, thereby eliminating friction between the balls. Grease held in the space between the ball circulation path and ball cage (in a grease pocket) is applied to the contact surface between each ball and ball cage as the balls rotate, forming an oil film on their surfaces. As a result, the oil film does not easily break down, helping ensure smooth slow motion.

To learn more about speed-extreme solutions, visit motionsystemdesign.com