Leisurely linear actuation

Sept. 1, 2007
How would you like to move a row of 10 water molecules at the rate of human hair growth? If watching paint dry or grass grow is on your list of party

How would you like to move a row of 10 water molecules at the rate of human hair growth? If watching paint dry or grass grow is on your list of party favorites, this extreme slow-speed challenge might be right up your alley. If you have more practical concerns — for instance, precise industrial measuring applications — you'll want to know what's involved in this extraordinarily slow linear actuation.

Slow motion in the nanoworld

A trip to the nanoworld brings designers to the home of nanometers — tiny units of just a millionth of a millimeter. One of the most important tools for exploring this world is the electron microscope. Now, a joint project taking place in the Netherlands called “NewMotion,” between microscope manufacturer FEI and Bosch Rexroth, has tightened new microscope process speeds to 1 nm per second. At the heart of these electron microscopes are FEI's mechatronic specimen-manipulator stages — and FEI's current goal is to design stages that will make it possible to manipulate the nanometer world in three dimensions. This new manipulator requires a movement and positioning accuracy down to the atomic level.

FEI already has their motion control system in place: a modular system made of various hardware and software packages from Bosch Rexroth's NYCe4000 industrial system. The Technical University of Eindhoven (TU/e, in the Netherlands) is aiding the project with research into the necessary control techniques. For this next step, TU/e is developing new measurement and control algorithms to be used in the motion control system for increased accuracy and fluid motion in the nm/sec range.

To do this, sensors and controls must constantly monitor, analyze, and correct the speed and position of several axes. As you might imagine, this places huge demands on the calculation capability and master control software tuned to do the job. Because only a small number of encoder steps are made per increment of time, the regulator in the control unit must be capable of generating a homogenous speed profile so that the actual speed of the sample being manipulated remains constant.

On the mechanical side, new mechatronic linear transducers are used to achieve virtually backlashfree power transmission with an accuracy of +/- 0.1 nm/min, with an ultrasonic piezomotor used as an actuator to achieve the super-slow rate of 1 nm/sec.

However, accuracy alone isn't good enough — the movement must be absolutely jerk free. So, FEI switched off mechatronic vibrations as much as possible, and uses the control system to compensate for any remaining vibration. Also in development is a balanced thermal-compensation system, which will hold temperature constant as a function of time.

Tips and tools

“To achieve smooth linear actuation at the sub-micron level, many factors must be considered, because everything becomes a potential system disturbance,” says John Floresta, vice president of engineering at Anorad, a division of Rockwell Automation, Milwaukee, Wis. Here are some tips to keep in mind when tackling extremely slow motion applications, such as those found in semiconductor manufacturing.

What kind of feedback do you have?

“A good rule of thumb is that the feedback resolution should be 10 times the speed or resolution of what you're trying to move,” advises Floresta. So, if you're trying to move at a rate of 100 nm/sec, you'll want 10 times that information coming back in order to accurately control the motion. High-resolution optical encoders or laser inferometers can work well to capture the data you'll need to make adjustments.

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 low-speed 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.

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