Piezo Stages in Imaging and Alternative Technologies
Piezoelectric actuators are currently the dominant technology for objective focusing, despite new high-performance, lower-cost options that are recently available. What exactly are piezo stages, what are their advantages and drawbacks, and what is different about new alternatives? Learn about these new stages, and why they are poised to become the new go-to technology for automated nanopositioning and high-magnification imaging applications.
What are Piezo Actuators?
Let’s consider each word independently. Actuator or stage refers to a surface that holds material for inspection by a microscope or other imaging technology. The material can be anything from human cell samples screened for abnormalities to industrial materials tested for quality. To bring the material into focus, the stage is moved in very tiny increments with great precision.
Enter the “piezo” of the piezo stage. piezo is an abbreviation of piezoelectric. The word piezo comes from the Greek word “to squeeze or press.” In 1880, the Curies discovered that when they squeezed certain types of materials, especially crystals, the material gained an electric charge. When they released the pressure, the crystalline material would give off the charge. They could harness the resulting electricity to develop motors that could make very miniscule and precise incremental movements.
Thus, a piezo stage, or piezoelectric stage, is a surface which is controlled by a piezo motor. The piezoelectric actuator is composed of a piezo motor (motion created by applying voltage inverse piezoelectric effect) plus mechanical elements which allow the movement of the stage. To control the movement of the stage, modern piezo stages are fitted with a controller and position feedback device and manipulated via a computer interface.
Applications for Piezo Stages
For many years, the piezoelectric stage (actuator with a piezo motor and flexure bearing to control movement) has been the industry standard for objective focusing applications. Piezo stages are commonly used for high-performance imaging tasks in a wide variety of industries, from medical diagnostics and DNA sequencing to materials engineering and semiconductor inspection.
Why Piezo Actuators have been the Dominant Technology
Piezo stages are a time-tested and reliable technology. The first piezoelectric motors were produced over 50 years ago. They remain the dominant technology partly because of their satisfactory reputation and because until recently, robust non-piezo alternative solutions had remained elusive.
There is also a misconception about the definition of a piezo stage, or a piezo nanopositioning stage. Some incorrectly define the term to mean “a positioning device capable of nanometer or sub-nanometer resolution” disregarding the underlying technology completely. It is important to note that not all nano-positioning stages are piezo stages.
While piezo technology does have some advantages, it also has significant drawbacks. Those in the market for nanometer and sub-nanometer level positioning devices should seriously consider non-piezo positioning devices when evaluating which devices suit their needs the best.
Advantages of Piezo Stages
There are some clear advantages of actuators with piezo motors and flexure bearings over stages using other technologies (such as ball bearings or cross rollers) for very specific use cases. For applications with light payloads and very small motions (< 100 um) that need sub-nanometer resolution, direct piezo actuators work well. The inverse piezoelectric effect is effective for quickly moving very small distances when a precise voltage is applied to the piezo motor.
Piezo motors are able to move in tiny increments in rapid-fire bursts, and then become very stable. Also, after a voltage is applied to move the piezo stage, it remains in that position even when power is lost. In order to move larger distances typically amplification, a spring, and a flexure are employed, and this approach comes with drawbacks that will be detailed later.
Disadvantages of Piezo Stages
A typical imaging application involves a piezo stage moving an objective vertically for focusing with a separate sample motion using an XY stage. The use of piezo and flexure-based stages for focusing in this situation has many drawbacks.
Typically, a piezo stage can only move 100 – 300 nanometers. It moves very tiny amounts, very precisely; however, the initial steps of focusing often requires larger movements. The larger moves are necessary for avoiding objective collisions with the sample or for finding the optimal focusing plane due to sample variations. Because of their movement limitations, piezo stages may have difficulty accommodating thicker samples such as tissue samples.
Also, the XY stage motions or other vibration sources such as pumps and fans in the microscope or diagnostic instrument can impart off-axis forces on the flexure which results in instability of the piezo stage. The stiffness of a system is referred to as bandwidth, and piezo stages with flexures are typically lower bandwidth compared to a stage using a linear motor or screw combined with a crossed roller bearing.
Another drawback to piezo stages is that the advanced crystalline materials (PZT) used in piezoelectric motors are expensive to produce and frequently include lead and other potentially hazardous materials. Piezo controls are also costly and are frequently complex to operate.
|Piezo Stage Advantages||Piezo Stage Disadvantages|
|Good for applications with light, very thin payloads and only nano-level motions.||Because of their movement limitations, piezo stages may have difficulty accommodating thicker samples such as tissue samples.|
|Remains in place if power to the unit is lost.||Vibrations of integral pumps, fans, and even the motions of the stage itself can impart instability of the piezo stage.|
|Advanced crystalline materials used in piezoelectric motors are expensive and potentially hazardous.|
Alternatives to Piezo Stages
Because the name piezoelectric stages has become synonymous with imaging stages, alternative solutions have had a difficult time gaining a foothold in the market. There have been some attempts over the years to engineer new solutions which would solve some of the drawbacks of piezo stages and unseat the piezo stage’s dominance.
One solution to the piezo stage’s movement limitation problem is to approach focusing in two phases. The first is a macro-level focusing typically using a screw driven stepper motor stage, followed by switching to the piezoelectric stage for the micro-level focusing. This solution adds complexity and cost. The focusing process takes longer and these solutions are more expensive because you are essentially purchasing two separate focusing systems.
Some piezo stage alternatives use lever amplification, a spring, and a flexure to scale up the travel distance. By adding a mechanical lever component to the piezo actuator, travel can be increased to 400 – 500 nanometers. Adding levers reduces axial stiffness, resulting in reduced servo bandwidth and a reduction in step and settle performance.
Other piezo stage alternatives employ unlimited travel piezo actuators, sometimes known as resonant piezo actuators. Piezo fingers oscillate against a ceramic strip to create motion. Drawbacks of this type of solution are their considerable transfer function non-linearity, which makes closing a stable servo loop difficult. The performance of resonant piezo actuators degrades as the contact points weaken and wear down the ceramic strip. Also, the oscillating motion creates a loud screeching noise which is unpleasant for lab workers.
Why aren’t direct-drive linear motor stages more common in focusing applications? These stages use a non-contact linear servo motor to adjust the stage precision. As motion control solutions have advanced, nanometer-scale resolutions are easily achieved with this type of stage. Crossed roller bearings provide very high stiffness with none of the out-of-plane compliance commonly experienced with flexure stages. In addition, direct-drive linear motor stages offer a significantly longer travel range compared to piezo/flexure stages. Other benefits include:
- Very efficient step and settle times
- High servo bandwidth with a critically damped response
- Long service life with no need to adjust the servo tuning
The direct drive linear motor stage is well-suited for automated imaging applications in which objects are examined at a microscopic level by positioning them along two axes so they can be scanned by a high-magnification imaging system.
A Dover XYZ stage solution increased the client’s productivity and helped them acquire FDA approval for their cancer screening instrument.
Drawbacks of the Direct-Drive Linear Motor Stage
For vertical applications, direct-drive linear motor stages need to have enough force to counteract gravity, and will lower when power is removed due to gravitational forces on the payload. In some designs, damage to the sample and objective lens could occur during a power outage, when the motor no longer acts to maintain position. To compensate for gravity, past approaches have been to use a large linear motor which adds size, complexity, cost, and motor heat to maintain position or to add a pneumatic counterbalance which adds the additional requirement of needing a clean dry air source.
Dover Motion has recently overcome these drawbacks of using direct-drive linear motors for vertical applications. Dover Motion has developed very compact, constant force passive magnetic counterbalances which are a cost-effective method to optimize linear motor stages for vertical use.
Re-Engineering the Direct-Drive Linear Motor Stage for High Performance Automated Focusing
The Dover Motion DOF-5 stage provides a fresh alternative to the piezoelectric stage. As mentioned above, Dover Motion has developed standard linear motor stages with passive magnetic counterbalances and engineered such models for custom applications. In light of the success of these existing products, Dover Motion wanted to push the boundaries of XYZ direct-drive linear motors further. The goal was to engineer a novel and disruptive nanopositioning product with a breakthrough price that delivers high performance and increased travel.
This began by conducting customer research to identify areas in which existing models fell short. It was clear that two parameters needed work: volumetric efficiency and cost. Dover Motion’s standard stages lacked some of the specialized “wish list” features that, if included, would set an improved model apart from its peers as a world-class solution. Armed with a set of goals for the new stage, the engineering team set out to make the dream stage a reality.
Advantages of the DOF-5
Unveiled in the spring of 2018, the DOF-5 exceeds the goal specifications for the stage. The device includes the first ever built-in servo controller in a linear motor stage. Its compact constant force embedded magnetic counterbalance enhances the reliability and safety of the unit.
Some of the most compelling highlights of the DOF-5 include:
- Internal, high-performance servo controller and drive; no bulky and expensive external electronics.
- User-friendly GUI which simplifies servo tuning and initial setup.
- High servo bandwidth that provides very nimble move and settle performance.
- High-speed digital I/O signals for simple integration with laser autofocus systems.
- Generous travel allowance of 5 mm to accommodate most use cases.
- The objective mount is designed with threaded inserts to support integration with all standard microscope objectives, while three objective mounting locations deliver maximum flexibility.
- Much lower price point, selling at ½ to ⅓ the price of existing piezo-based solutions.
While piezoelectric stages are currently the dominant technology for focusing, new focusing options have recently come to market which are poised to challenge the status quo. Ideal for automated imaging applications, lower-cost Z-axis focusing stages using direct-drive linear motor technology deserve a thorough review.
Dover Motion designed the DOF-5 with the goal of overcoming customer-defined drawbacks to both traditional piezo stages and existing direct-drive technologies. The result is a user-friendly, very cost-effective stage which is adaptable to integration with a wide variety of objectives and imaging workflows in many different industries.
To request pricing for the Dover Motion DOF-5, please contact firstname.lastname@example.org.