fem modelling of a bellows and a bellows- based micromanipulator

I. Introduction FEM modelling of a bellows and a bellows-based micromanipulatorlimit the amount of liquid and thus, to decrease problems arising from temperature changes.The bending of the bellows facilitates a joint-free structure. Using the bending character of thebellows instead of ball joints simplifies the structure and the manufacturing process. Tests andmeasurements have proven that it is possible to control a bellows-based parallel manipulator.By changing the lengths of the bellows the orientation of the mobile platform, and thus theposition of the end-effector can be controlled.The objective of this thesis was to develop a computer-controlled micromanipulator thatfacilitates automatic and semi-automatic operations. Its main application are currently inbiotechnology where needs for automating operations have emerged. For example in celltoxicology, hundreds of cells are typically injected and therefore automatic micromanipulatorwould speed up the process remarkably. Since the size of biological objects vary frommicrometers to hundreds of micrometers, the micromanipulator should be capable of bothsubmicrometer resolution and work space of several hundreds of micrometers.1.1.2 THE BELLOWSThe bellows is a spring type of passive component, i.e. force required to deform the bellowsis directly proportional to the displacement. The bellows is manufactured using electrodeposition,where spring-quality nickel – nickel plus cobalt at 99,85% and maximum 0,04%of sulphur – is first deposited onto an aluminium mandrel. In the end of the process, themandrel is dissolved out.Electro-deposited metal bellows have the following advantages. Because the bellows wallcan be thinner than other types – up to 8 [µm] – they are extremely sensitive, which makesthem excellent for very accurate instrument applications requiring a high degree of sensitivity.They can provide large deflections with only very tiny forces. They are up to 25 times assensitive as hydroformed bellows in the same size range. Servometer ® manufactures bellowswhich can be fully compressed by a force as small as 4 grams.Their stroke can be 60% of their extended length. They can be designed for infinite lifeexpectancy and have a normal minimum life of 100’000 cycles. They are seamless and nonporous.So, they are fully impervious and can be filled with oil without losses. Therefore, theycan be used in micromanipulations of cells without any contamination of the environment.That’s why they were chosen for this application.1.1.3 AIM OF THIS PROJECTThe micromanipulator can be controlled using either open or closed-loop control. In teleoperation,open-loop control is typically sufficient, since the operator closes the loop. Inapplications that are not critical to imperfections of the manipulator, open-loop control cantherefore be used. However, an increasing number of micromanipulation applications,especially those that involve automatic operations, require high absolute accuracy, high speedand elimination of drift.Actually, to satisfy these requirements, closed-loop control must be applied. Thedisplacement of the actuators are measured using strain gages, to compensate non-linearity.The position of the end-effector is controlled at a second level using Hall sensors. Of course,this hardware implementation increases remarkably the costs. Thus, it should be moreproficient to enhance the precision of the closed-loop control. But the bellows have alsobending characters when non-axial forces are applied to them. This might be a problem incontrolling the manipulator’s movement. Therefore an accurate model that shows the bendingand behaviour of the used bellows was needed.2