Fundamentals Series: Micromanipulators Part 1
Our Fundamentals Series of articles will endeavour to discuss basic concepts related to tools, techniques and technology, primarily focused on the neurosciences.
Key factors to consider when investing in a new micromanipulator
This is the first topic in Our Fundamentals Series. It consists of three informative chapters, with the first offering an overview of several broad design features of micromanipulators and at the same time introducing different micromanipulator types currently existing on the market.
The second chapter will describe the most important criteria for evaluating micromanipulator movement, many of which should be considered when choosing to invest in a new micromanipulator. The final chapter on micromanipulators will give an overview of the most popular driving mechanisms currently implemented by micromanipulator manufacturers and introduce the mechanisms involved with each. It will also evaluate some of the implications of these driving mechanisms as it affects the criteria listed in the other articles.
The factors and variables considered in this piece can be applied to micromanipulators engineered for a wide range of applications, from life sciences to physics and engineering. All of these are not equally important for the different applications, and some aspects of function have an increased financial cost implication which can’t be justified considering a relatively moderate/negligible benefit as it relates to a particular application.
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Basic Micromanipulator Components
Micromanipulators consist of two basic parts – the control interface and the micromanipulator body.
The micromanipulator body is the structure where movement is realised and onto which the electrode or probe (hereafter only referred to as probe) is mounted. It is therefore typically mounted near the sample.
Picture: An example of a manipulator body – the MCI CleverArm motorised manipulator
The control interface includes the hard and software components used by the operator to achieve the required movement on the micromanipulator body and the probe.
Remote controlled systems usually include a remote-control interface which may be a separate component from the micromanipulator itself, with control wheels and/or buttons. Additionally, computer software may further be included through which an operator can instruct micromanipulator movement. Electrically controlled remote interfaces often also include a separate external power supply to provide electrical power to the relevant micromanipulator component/s. In this case, the micromanipulator body, remote control interface and power supply unit will all be connected with electric cables, although some recent models use wireless connection.
Picture: An example of the MCI CleverControl Wireless Control Interface with the manipulator body to the left
Furthermore, electric cables may be replaced by hydraulic tubes when the micromanipulator is hydraulically driven.
On the other hand, manual micromanipulators will have manually adjustable threaded interfaces (a fine thread screw or micrometer) attached to the micromanipulator stages for control of movement.
Micromanipulator Body – Design Principles
The micromanipulator body always incorporates one or more “active elements”, or actuators. This is the hardware component on the micromanipulator body responsible for translating force or a digital command into linear or rotary motion. Actuators are often complex devices, but for this discussion two basic parts are considered, namely the fixed component and the moving component. The former doesn’t move when the actuator is effecting movement, and is often part of the actuator casing or enclosure. The moving component is the part which can move forwards, backwards, and rotate. Micromanipulator actuator types are typically:
- Micrometers or fine-thread, precision screws (for manual micromanipulators), where movement is achieved by the operator turning a precision screw fixed to the micromanipulator body in a specific orientation to effect movement
- Hydraulic drums, which will move forwards or backwards depending on the direction of pressure applied by the operator
- Piezo actuators, which will alter its dimensions in response to a change in voltage amplitude and direction across the piezo element
- Stepper motors, where movement can be achieved through electrical commands which rotate the motor by programmable steps for precise positioning and speed control
Please see the third chapter of Our Fundamentals Series: Micromanipulators, for more detail on these actuators.
Each actuator is coupled to, or forms part of, a micromanipulator stage. The micromanipulator stage is essentially an extension of the actuator in that it has similar fixed component and moving component parts. When the stage moves, the fixed component remains fixed relative to the surface onto which the micromanipulator is mounted, whilst the moving component can move either forwards or backwards. Rotating stages often only consist of the fixed component. Here, the actuator’s moving component will then act as the rotational moving component of the micromanipulator stage.
The integration between actuator and stage is done in such a way that the fixed component of the actuator is connected to the fixed component on the micromanipulator stage, with the actuator’s moving component attached to that of the stage. The movement of the micromanipulator stage is therefore driven through its coupling to the actuator.
The main reasons for using the stage/actuator assembly, instead of actuators alone, are due to the following advantages of the stage:
- Compliments various aspects of actuator movement – some of which will be described in the second chapter of Our Fundamentals Series: Micromanipulators
- Provides a stable, low or no-drift platform when the actuators are stationary
- Protects the actuators and related electronics from the environment
- Adds volume and weight to the micromanipulator body – provides better damping of vibrations generated by the micromanipulator movement and from other components on the workstation
- Further adds to surface area – useful for other functional components to be mounted onto the micromanipulator, and for the stages to be merged on a multi-axis setup
Stage design and assembly
As mentioned above, ideally the stage needs to be built in a way for it to compliment, or at least not impede the actuator movement. There are various strategies for achieving this, but a central strategy here is to have two tracks of precision-engineered roller bearings integrated into either the fixed or the moving component. This design will include a complimentary track or groove on the opposite component of the stage on which the bearings can roll when the actuator drives the stage.
By analogy, if the actuator were the engine of a train locomotive, the bearing tracks would be the wheels and the track or groove would be the rail. For the sake of stability, and extending the metaphor to road transport, manufacturers usually design these bearing tracks to be as far apart from each other as the stage size and design would allow, for the same reason that a wide-axis, large vehicle will be more stable than a motorcycle.
The tension on the bearings between the fixed and moving components should allow the two components to be driven across the intended travel range of the actuator with equal and little resistance. Equal, because even small inconsistencies in resistance would cause, at microscopic levels, a noticeable change in how the actuator drives the stage. Low resistance, since having to drive against unnecessary force may cause the actuators to stall, cause unwanted vibrations or extra heat (a possible cause of drift). On the other hand, the resistance between the two stages should be adequate to provide consistent movement and stability when the micromanipulator is stationary.
Setting tension and resistances on bearings are therefore important aspects in micromanipulator assembly. It requires high tolerance machining of the metalwork, high quality bearings and a high precision assembly process.
In order to ensure stability and smooth motion across the travel range, the bearing tracks should cover as much of the length of the stage interface as possible. Therefore, some manufacturers use long tracks of more expensive but highly advantageous cross-roller bearings to increase the bearing footprint.
Materials used and shielding
Since aluminium is an easy material to machine, it’s the material of choice for building stages. Nevertheless, due to changes in the aluminium characteristics in unusual environmental conditions, some micromanipulator applications may be hindered (which can be minimised by using surface modification treatments). This applies to the materials used for the stage, actuator, and any on-board electronics, as well as the integration of these parts . With regards to the latter, sometimes the actuators or electronics require shielding/isolation from the environment, or placement in a position where it can stay cool or dry. These design choices are all important, since expansion coefficient, temperature stress, and condensation or frost can all affect micromanipulator function under unusual temperature and/or humidity conditions.
Even under normal room temperature conditions, relatively minor fluctuations in ambient temperature can affect the stability of a micromanipulator. This is perhaps an opportunity for manufacturers to research and test other materials that may be resistant to micro-scale changes in response to temperature changes, which may ensure higher quality built micromanipulators in future.
Size of the micromanipulator body and mounting it onto the workstation
Most workspaces have limited scope for mounting of a micromanipulator, both in terms of space availability and how the micromanipulator can be mounted securely in close proximity to the sample. The micromanipulator body should therefore be as compact as possible, but still have sufficient mounting surface for stability.
The absolute limiting factor with trimming down stage size is the size of the actuator, as this needs to be integrated with the stage. Therefore, the stage-to-actuator assembly will have at least the same dimensional footprint as the actuator by itself.
Design requirements which further increase the body size are the need for stable mounting surface/s for the probe and an interface to fix the micromanipulator body onto the workstation. In addition to this, stable mounting interfaces between different stages when it’s a multi-axis micromanipulator may also need to be incorporated. Considering these size-increasing factors, most manufacturers attempt to keep the micromanipulator size to a minimum as a general design principle, both to save on material cost and to allow the micromanipulator to fit into restricted spaces.
Some micromanipulators are specifically built for workstation configurations with very little available space, for example inside the vacuum chamber of an electron microscope or where multiple probes are used around a small sample area. Whilst these smaller micromanipulators are sometimes the only option for these applications, some other important micromanipulator features often is compromised by this. As such, smaller actuators with less driving force and less travel range are used in these smaller micromanipulators. Therefore, becoming less versatile in terms of the probe type which can be mounted with regards to the size, weight and mounting mechanism used. Furthermore, the ease and versatility when mounting it onto the workstation and general stability of the micromanipulator will be impaired.
Micromanipulators with larger bodies also tend to be more resistant against vibrations from the environment, since a greater force is required to move the greater mass.
Dimensions and Axes
Another functional factor determining the body design and dimensions is the number of movement axes on the micromanipulator. A single axis meets the requirements of some applications, but more versatility is often required. Three-axis micromanipulators are probably most popular and these axes are named X, Y and Z.– The X axis moves left to right from the operator’s perspective, whilst the Y axis moves towards and away from the operator. The Z axis, on the other hand, moves up and down.
Another fourth axis in a diagonal direction is often necessary and added to produce four-axis micromanipulators. Some manufacturers save cost and space by implementing a “virtual” axis for the diagonal direction, where the diagonal movement is achieved by entering the probe angle into the control electronics. This can be done either automatically with an angular sensor or manually by the operator taking a measurement from a calibration printed on the micromanipulator body.
The diagonal movement is then generated by implementing a combined movement of the X and Z stages to generate the diagonal direction. This has space and cost-saving implications, but it needs to be well-designed and -implemented for the fourth axis to function properly.
Further complexity in movement direction can be added with one or two rotational axes, required by a number of applications.
Brackets are often employed to mount the probe, change the probe angle to a steeper slope, or to adapt the micromanipulator for mounting in a specific environment. It can also be used for crude adjustments to probe position or to facilitate easy access to the probe, by means of sliding or rotating brackets. It is noteworthy, that the use of brackets may change the movement and stability characteristics of the micromanipulator, and that the published specifications of many manufacturers won’t necessarily be with brackets attached.
With brackets being added, it’s critical to avoid long levers and ensure that there’s enough rigidity, overlap and suitable fixing points between surfaces of the bracket and the micromanipulator body. Poorly designed brackets can cause instability, non-linearity in the travel direction, as well as oscillating vibrations with movement. Therefore, the bracket design should be carefully scrutinised prior to use, as this may affect data quality.
The second micromanipulator component to be discussed in this chapter is the control interface.
The main differentiating requirements for control interfaces are for it to be intuitive to the user, ergonomically friendly and designed in a way to make the full functionality of the micromanipulator accessible to the operator. As mentioned above, control interfaces can be subdivided into manually controlled and remote controlled interfaces.
Manual micromanipulator control interfaces
With regards to manual micromanipulators, some of the systems on the market have coarse and fine-scale movement on the micrometers, whilst others only have the fine-scale. This may be particularly problematic for applications where longer range movement is required, or where the user needs to swap the probe often.
To compensate for this restriction, some manufacturers employ sliding or swinging brackets to enable the user to quickly exchange the probe and to do crude adjustments. It should also be considered that some manual micromanipulators’ micrometers may not be positioned in line with its relevant axis, whilst others are not easily accessible by hand when mounted onto the workstation.
The ease with which the micrometers (and therefore the micromanipulator) can be adjusted is also important. When the micrometer is resistant to rotation, adjustment can cause unwanted vibrations or non-linear travel on a particular axis, which could result in sample damage.
Besides these aspects, there are limited differentiating characteristics on the control interfaces of manual micromanipulators, which leads us on to remote control interfaces…
Remote control interfaces
Despite there being some common themes for developing a remote control interface, manufacturers are fairly differentiated in their approach. The interface is usually housed in a single enclosure, containing a) control electronics (with the exception of hydraulic micromanipulators, where electronics are not necessarily used), b) a display screen, and c) rotary wheels and/or click-buttons or a touch-screen linked to the micromanipulator stages.
Another popular approach is for the interface to communicate with the micromanipulator body via a joystick, again with supplementary controls on the interface itself. It is also common for micromanipulators to have a software interface, with a software developer’s kit (SDK) for custom integration with 3rd party or user software. Some micromanipulator models offer options between these and/or a combination of these in a single remote control interface.
Choosing which of these interfaces would work best for you is partly determined by the application, since some important application-led requirements are a feature of some interface types. For example, when combined movement of more than one axis is required, opting for a joystick will be more intuitive than using a button interface. On the other hand, having buttons would be easier when your application requires you to make repeatable steps in only one linear axis at a time.
Some applications may also require additional control over movement, when programmability becomes important. An example of this, implemented by most systems, is the capacity to switch easily between coarse and fine speeds, with several interfaces offering additional control with several speed gears, or control over acceleration and deceleration.
One may also require the ability to make step-wise movements of user-programmable dimensions – a further control option which is often available. Most of the additional control features are incorporated with a specific application in mind.
Another example of a possible useful feature is the implementation of additional electric communication ports. This can be useful when you require a feedback loop between the probe or sample, other external devices and the micromanipulator or control interface. The operator may wish to, for example, automate an instruction for the micromanipulator to stop moving when there’s a voltage change detected by a probe attached to a voltammeter. This communication can take the form of a simple digital trigger in/out port, or something more complicated.
Another useful feature on some systems is the ability to synchronize movements between micromanipulators, saving time by not having to control each micromanipulator individually.
Intuitiveness and Ergonomics
Most users find an interface where the wheels or buttons are somehow represented in the 3-D space they work in, most intuitive. This lead to the popularity of the control cube design style. Users also tend to find joysticks easy in terms of its intuitiveness, and even new users find it easy to work with.
Manufacturers give attention to the ergonomics of the control interface, which is important not only from a convenience perspective, but also has functional implications. Long hours of use requires ergonomic awareness as a tired hand is more likely to make errors in control.
Related to this aspect is the size of the control interface. The surface dedicated to control often has limited space, limiting the size or the footprint of the interface. Some interfaces need to be able to be carried around or held on the user’s lap during operation. For this reason, some interfaces are better suited and more user-friendly than others.
Further to these factors, personal preference remains an important determinant, and for this, what the user “grew up” with is often the most important factor.
This brings us to the end of the first chapter of Our Fundamentals Series: Micromanipulators.
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