What is the benefit of piezo drives?

20th May 2013

Steffen Arnold outlines the advantages piezo drives bring to laboratory automation.

Laboratory automation is a very special field of automation engineering. It is concerned with the automation of laboratory processes in chemistry, bio-engineering, pharmaceutical and food technology as well as medicine aiming to improve the reproducibility of the processes and save time and costs through automation.

Undoubtedly quite different from each other, the various markets have one crucial thing in common: they need fast and precise drive systems. If the application so requires, they must also be as compact as possible or operate reliably even in strong magnetic fields, at low temperatures and in a vacuum.

Piezoelectric drives, scanners and positioning systems are a safe bet here, ranging from pumping and dosing tasks through sample manipulation to fully-automatic screening.

The precise supply of substances is one important field of laboratory automation. The compact dimensions, high precision and dynamics of piezoelectric drives combined with their low purchasing and maintenance costs make them indispensable for micropumps and nanolitre dosing. The same applies to piezoceramic disks for generating fast oscillations as well as for actuators with strong acceleration at strokes of 10 to 1000µm, and piezomotors for fast motions in the range of several millimetres.

Micro-diaphragm pumps are a typical exemplary application of piezoceramic actuators. They work in a similar way to piston pumps but the medium to be transported is separated from the drive by a diaphragm. The drive can therefore not interfere with the pumped media and vice versa. Highly dynamic, disk-shaped, piezo elements mounted directly onto a metal disk lend themselves to the miniaturised version of this type of pump. High delivery rates can thus be realised even under backpressure by varying the switching frequencies or the amplitude of the piezo displacement by means of a corresponding control.

Micro-dosing valves operate in a similar way to pumps but the forces required are higher. The common dosing frequencies in this application are usually in the kilohertz range and can be implemented only with piezo actuators. A variety of piezo components are used in the micro-dosing valves, depending on their size. For example, small piezo tubes are used for drop-on-demand methods, like in ink jet printers.

Miniaturised valves for dosing tasks in the nanolitre range are produced using piezo disks, for example, which are also used for lab-on-a-chip applications.

Valves requiring larger strokes can be implemented by piezo actuators with a lever system. They are suitable for longer travel and thus also for applications in which certain drop sizes have to be achieved as a function of the material properties.

Automation in laboratory environments is often necessary because ambient conditions restrict the accessibility, eg in vacuum, magnetic fields, radiation or because components are integrated deeply into the system. Small, precise, and reliable piezo drives are the proper solution for this task.

Typical applications include, eg, multi-leaf collimator apertures, in which small lamellae are readjusted individually by compact drives in order to predetermine a certain shape and eliminate undesired peripheral effects of the illumination and/or radiation.

Similar requirements apply to sample manipulation or alignment of optical imaging systems by miniature drives.

For this purpose, PI offers a large selection of appropriate positioning and handling systems based on different piezoelectric working principles: for example ultrasonic piezo drives are well-suited for many applications requiring high dynamics.

Working as direct drives, they dispense with the mechanical components of classic motor spindle-based drive systems such as clutch or gearhead to the benefit of costs and reliability. They are lightweight and suitable for travel speeds up to about 500mm/s, their accuracy is in the range of approximately 50nm.

Since they are offered in various steps of integration, they are easy to adapt to the application on hand.

Piezo-based inertia drives are considered to be particularly compact and inexpensive micro-drives. They utilise the stick-slip effect for fine steps with step heights of less than a micrometre.

A piezoelectric actuator expands and takes along a mobile rod. In the second part of the motion cycle, the actuator contracts so rapidly that it slides along the moved part, which cannot follow this motion due its inertia, and thus remains in the same position.

The electric control is easy and similar to a saw-tooth voltage. The drives are small, which opens up many application areas.

Typical applications for this drive principle range from soldering tip positioning to shutter adjustments in micromanipulation.

High forces and a high resolution are achieved when piezoelectric inertia drives are used to move screws, which allows the automation of alignment tasks in inaccessible places. Depending on the design, they are operated at a frequency of 20kHz, which means that they cannot be heard, and reach velocities of up to 10mm/s.

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Steffen Arnold is head of Marketing and Products, Physik Instrumente (PI) GmbH & Co KG, Karlsruhe, Germany.




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