Applications pinpoint positioning solutions

02 Jun 2003

Confused about the range of positioning equipment on offer? Jacqueline Hewett investigates the competing technologies of flexures, piezoelectrics and bearings.

From Opto & Laser Europe June 2003

Whether you want to move an optical fibre by a fraction of a micron or shift a microscope sample a few millimetres, there's an alignment system on the market that can meet your needs. The tricky part is choosing the one that best suits your application.

This month Opto & Laser Europe has delved into the technology behind different types of alignment system - otherwise known as the positioning stage. Over the next two pages we describe the choices on offer, complete with what we think are their pros and cons.

A system of two parts The first important point is that a positioning stage is actually composed of two parts: the raw stage that mounts the sample; and a series of actuators that control the motion of the stage. At least one actuator is needed for each axis of motion. Alignment systems vary greatly in their capabilities and performance, from the simplest of set-ups offering motion along a single axis to the most sophisticated six-axis (x, y, z, pitch, yaw and roll) devices.

Here's the lowdown on the common variations on stage and actuator technologies that you are likely to encounter during your search for the perfect product.

1. Stages Typically a stage resembles a small black metal cube, with a flat surface for mounting the sample or optical part that needs to be moved. The mounting surface often has screw holes for firmly securing the sample.

Dovetail-based stages A dovetail slide or joint is one of the simplest and cheapest components found in positioning equipment. It comprises two close-fitting metal parts that move along one another, giving translation along a single axis. One piece of the joint is usually V-shaped, while the other is a slot. An actuator, such as a manual micrometer or motor, controls the motion of the joint. Several joints can be "stacked" together at 90° to each other to allow motion in more than one axis, if required.

* Pros: the joints are small, compact and cost-effective. They offer a long travel range of several millimetres and are ideal for the relatively undemanding alignment of optical parts that only weigh a few grams.

* Cons: all positioning equipment has an error associated with its motion. If dovetail stages are stacked the error is translated into every axis, making accurate measurements very difficult. The nature of dovetail motion means that the joint will suffer from wear and tear over time owing to friction between the surfaces. This joint is not suitable for high-precision applications and its load-bearing capacity is limited.

Ball-bearing and cross-roller bearing stages If you are looking for larger travel ranges or you want to translate heavier parts, you might want to consider ball-bearing and cross-roller stages. Like the dovetail, these only offer single-axis motion. Ball-bearing stages replace the sliding motion of a dovetail with a lower-friction rolling motion. Cross-roller bearing stages substitute the ball-bearings for a set of steel rollers, which can cope with larger loads.

* Pros: interchangeable micrometers are often available for these stages, so you can adjust the resolution according to your application's demands. These stages can deal with travel ranges of the order of 1m.

* Cons: friction is associated with the motion, which causes wear and tear in the joint. Dirt can also cause problems in the case of the cross-roller bearing, increasing friction and making the motion more prone to error over time. Be aware that errors will be multiplied if these stages are stacked.

Flexure stages A flexure stage relies on the elastic deformation (flexing) of a lever. This eliminates the friction that is inherent in dovetail and bearing-based stages. An actuator works against a series of levers that are built into the body of the stage; the principle of operation is similar to what happens when you flex a business card between your fingers. The stage is constrained so that the resulting motion is only in the desired direction.

* Pros: a big benefit is that the resolution of flexure-based stages is limited only by the actuators that drive them. As a result, they can be very precise. A single stage can also provide multiple axes of motion if it is equipped with more than one flexure.

Flexure stages have a high load capacity and can cope with a sample weighing anything from a few grams up to a kilogram or so. They are also less sensitive to shock and vibration than other positioning stages.

* Cons: flexure stages have a limited travel range - typically no more than a few millimetres - and can be expensive.

2. Actuators An actuator is a device that controls the motion of the various types of stages. Actuators fall into three basic classes: manual drivers, stepper motors and piezoelectric-effect based drivers.

Manual drivers A manual driver is the simplest way to control a stage and is the workhorse for repeatable motions. These drivers come in the form of adjustable screws that are mounted onto the stage to control motion around a desired axis. They are marked with a micrometer scale that typically permits up to 50mm of travel.

These actuators come in either direct or differential micrometer form. Both essentially work in the same way, using a thread with a fine pitch. In a direct micrometer the length of the thread on a shaft determines the length of travel, and the pitch of the thread - coupled with the diameter of the drive knob - determines its resolution. Differential drives use a slightly different design of shaft to provide extended travel and higher resolutions.

* Pros: manual drivers are easy to use, high resolution and interchangeable. A direct micrometer can provide precision travel from many millimetres to around 1µm, while a differential micrometer can provide 50nm resolution over a range of 300µm.

* Cons: these actuators have to be controlled manually. Longer travel ranges are available from electrically powered actuators such as stepper motors.

Stepper motors Stepper motors use a series of electrical pulses to drive the stage in small linear increments. The "steps" are generated by converting the small rotary motion of a screw into a linear translation. If your application demands a long travel length but nanometre-level precision is not essential, a stepper is a good alternative to a piezoelectronic-based actuator. Vendors will be able to sell you the control electronics for the device and you can either buy or write your own software to program the exact motion of the stage.

* Pros: stepper motors can offer a travel range from a few tens of millimetres up to the metre scale. A typical stepper has a range of travel of 25-150mm and a resolution of a few tens of nanometres.

* Cons: steppers require control electronics and can suffer from a mechanical positioning error called backlash. This affects the repeatability of the positioning.

Piezoelectric drivers This type of actuator uses a piezoelectric material - one that expands or contracts when a voltage is applied. Piezoelectric drivers are ideal for producing the small movements required for nanopositioning.

The most popular piezoelectric material is a polycrystalline ceramic called lead zirconate titanate (PZT). Drives based on PZT ceramics offer nanometre resolutions but only over a 10-100µm range. An actuator of this type is usually made from a stack of several layers of PZT. A 10mm stack of PZT can give about 10µm of travel.

* Pros: very fine step size and resolution. Movements of just a few nanometres are achievable and the resolution of a piezoelectric actuator is only limited by the electrical noise of its control electronics. PZT drivers have a fast response: the ceramic responds within microseconds to the applied voltage, and there is no friction associated with the motion. The repeatability of a piezoelectric drive is an order of magnitude better than a stepper motor.

* Cons: PZT ceramics are nonlinear and suffer from hysteresis, so the crystal will not expand and contract along the same path as the applied voltage.

Before you buy... Before you order your positioning device, think very carefully about the requirements of your application. To help you pinpoint your needs, here's a summary of the key information you will need to have to hand when you are making a purchasing decision.

1. What length of travel do you need?

2. How many axes of travel do you need?

3. What is the minimal incremental motion or smallest step size that you require?

4. What is the load-bearing requirement of your application?

5. Do you want a fully manual system?

6. Do you want a computer-controlled system?

Jargon buster With such an enormous amount of equipment on offer, some of the terminology associated with it inevitably causes confusion. Opto & Laser Europe has compiled a lexicon of some of that jargon.

Backlash defines all of the mechanical errors that can occur in stepper motor and cross-roller-bearing systems. In a stepper, the error comes from the small difference between the screw thread and the mating thread. In a bearing-based system, there is small amount of "looseness" that needs to be taken up every time the stage moves.

Repeatability A measure of how accurately the system can locate to a common position after moving elsewhere. Unidirectional repeatability is how accurately you can get back to the same position when you are approaching it from the same direction, whereas bidirectional repeatability is how accurately you can get to a point when approaching it from any direction. A typical test would be to move to a point, go past and then try to come back.

Resolution/minimal incremental motion The smallest step-size that the positioning system can make. Although it is quite common to find actuators that offer high resolutions, many stages may not be able to support the smallest movements. A complete system is defined by its lowest-specification part.

Roll is rotation around the x-axis. The x-axis is generally defined as the optical axis.

Pitch is rotation around the y-axis. The y-axis is generally defined as the horizontal axis.

Yaw is rotation around the z-axis. The z-axis is generally defined as the vertical axis.