Fundamentals Series: Micromanipulators Part 2
Our Fundamentals Series of articles will endeavour to discuss basic concepts related to tools, techniques and technology, primarily focused on the neurosciences.
Functional Movement terminology and concepts relevant to micromanipulators
This is the second chapter on the first topic in our Fundamentals Series. The Micromanipulators topic comprises of three informative chapters, which started with an overview of several broad design features of micromanipulators and at the same time introduced different micromanipulator subtypes available at the moment. It described the basic components and considerations in micromanipulator design, and translated the choice of these alternatives into implications for different applications.
Chapter 2 focuses on the most important concepts for evaluating micromanipulator movement, many of which should be considered when choosing to invest in a new device. It also serves as a source for (sometimes ambiguous) terminology typically referred to in product literature when talking with sales professionals and/or product managers involved with this technology.
The third chapter, to follow soon on Neurotech Today, will provide a more in-depth overview of the most popular actuator hardware currently implemented by most micromanipulator manufacturers. and will introduce the mechanisms involved with each. Additionally, this chapter will introduce the driving mechanisms of the different micromanipulators and expand on its implications with respect to the criteria discussed in the first two chapters.
The factors and variables considered in this Fundamentals Series piece can be applied to micromanipulators engineered for a wide range of applications, from life sciences to physics and engineering. Nevertheless, these considerations are not equally important for the different applications. Some aspects of function have an increased financial cost implication which can’t be justified considering the relatively moderate or even negligible benefit, as it relates to a particular application.
Movement terminology and concepts
Movement terminology, used to indicate micromanipulator movement functionality, describes the nuances around the ultimate goal of positioning the probe on the target point as required by each application. There is considerable overlap in these terms, which evolved or were inherited from engineering terms.
We’ll first look at the basic movement terminology followed by a brief description of some typical movement artifacts, both of which should be considered when evaluating manipulators.
This term is borrowed from optical sciences, where resolution describes the minimum distance between two distinguishable points in the image, captured by an optical system. Translated into micromotion terms, resolution refers to the smallest distinguishable points in movement, as can be measured by an interferometer or a super-resolution microscope.
In the case of electrically-driven micromanipulators (e.g. stepper motors and piezo-electric actuators) the resolution is functionally important in determining the “smoothness” of the movement. Here, a lower resolution means less smoothness in the movement, where step-wise motion or even a “judder” can be easily detected. A finer or higher resolution means smoother movement, where the individual steps comprising the movement are not visible under a microscope.
Resolution is also the basic factor determining whether the user would be able to hit the target point with the tip of the probe with precision. If the current position of the probe tip is 1um from the target, but the resolution of the manipulator is 2um, it may be difficult to reach target. The determining factor here is the size or area of the target or structure. The Nyquist sampling rate theorem can be applied as a good guideline when determining the required resolution for an application. That is, a requirement for the resolution to be half or less the size of the target point.
Determining the required target size may be easy in some instances, for example when considering required resolution on an electronic probe station. The target area here may be easy to define, such as a predetermined pad size on a circuit board. But there are applications when this is more difficult , e.g. when required to position a probe tip on a cell membrane during a patch clamp electrophysiology recording. There is a certain level of compliance in the system with the membrane, despite being only a few nanometers thick It is rather fluid in some aspects and can be deformed when establishing a tight gigaOhm seal between the probe tip and the membrane. Other biology applications also often have this forgiving compliance aspect to it. It therefore requires less resolution than you would otherwise estimate.
Different resolution settings
The resolution as it relates to smoothness is also often linked to the mode of movement of the micromanipulator. Many manipulators have a course and a fine movement mode, where the former has a lower resolution when compared to the latter. This is particularly evident with motorised manipulators. The resolution in micromanipulator system specifications would, therefore, usually refer to that of the fine movement mode.
How is resolution determined?
Resolution of manual manipulators is directly related to the thread of the actuator screw and the circumference of the manual adjuster attached to it. A larger adjuster and a finer thread offers the user the ability to move more finely, with a higher resolution.
On hydraulic systems, resolution is determined by the ratio between the solution flow rate and the drum surface area. The hydraulic solution pushes into the hydraulic cylinder or chamber and against the drum. The drum in turn effects movement of the hydraulic actuator, but the speed of this movement is determined by the drum surface area. A larger area means slower movement if the flow rate remains fixed. This flow-rate to drum surface area ratio therefore determines how much the stage will extend or contract (move forwards or backwards), based on the user’s ability to control hand movement on the control interface.
On electric systems, the actuators (i.e. stepper motors or piezo devices) have certain engineered resolution limits. The electric input signal, as controlled by the device’s driving algorithm, needs to be optimised in order to realise the actuator’s resolution potential. There usually is a compromise between this and other important criteria in designing the driving algorithm. The designer may also opt not to have the algorithm push the resolution limits of the actuator.
Speed and accelleration/deceleration
The maximum speed is often an important characteristic for the sake of convenience and efficiency during operation. Many micromanipulators have one or more fast speed “gears” for course navigation to an area close to the target point, before switching to a more careful, slower final movement. For larger manipulator working areas, having a fast top speed becomes useful, saving on time and frustration when navigating to the target area.
For some applications, small but fast, square-wave like (fast acceleration/deceleration), “stabbing” movements are useful. For example, when an electrode penetrates into soft tissue for some deep-tissue electrophysiology recordings – slow “stabs” often compresses tissue instead of pushing through tissue. For such applications, speed, acceleration and deceleration should be considered carefully.
At the other end of the spectrum, the slowest speed is often one of the first considerations when evaluating micromanipulators. The slowest speed and resolution are related in terms of the desired outcome, i.e. to be able accurately navigate the probe to the desired point. Furthermore, both minimum and maximum speeds rely on the same variables as resolution for the different manipulator actuator types (see section above for explanation), with maximum speed operating at the opposite end of the spectrum.
Acceleration/deceleration on hydraulic and manual manipulators are again linked to the same resolution dependent variables. However, these manipulators are more closely tied to hand-control by the user. This control is determined by the extent to which the user’s hand motion, translating into actuator movement, generates fast or slow acceleration/deceleration. Despite control usually being fairly easy in these instances, repeatability may be more difficult.
With electric manipulators, different driving algorithms are involved in determining speed, acceleration and deceleration. This can be user-adjusted on some of the advanced control systems. Deceleration is also indirectly linked to overshoot (see below in the movement artifacts section below) in the sense that there is a drop-off time after you stop moving – this can also be programmed to intentionally overshoot.
Minimum Step size
The terms step size and resolution are often confused with each other, and more often used incorrectly as interchangeable terms in product literature. What is actually meant by step size is the minimum distance covered as a repeatable movement step from a starting position to the next nearest stationary position. With manual and hydraulic manipulators a theoretical number can be defined in the product specifications. Nonetheless, having a repeatable minimum step size may become problematic with the close link to manual control. Minimum step size, as defined above, is therefore usually not listed in the product specifications. The term is more generally used for electric manipulators, where a drive signal generating a defined step size can be sent to the actuator.
The step function on these manipulators is often used in scanning applications. For this, a probe is used to scan across (or into, in the case of soft tissue) a sample, with stops being the points of measurement and/or intervention. The theoretical minimum step size is the same as the resolution of the actuator, but the driving algorithm often doesn’t offer access to this theoretical minimum.
Movement or travel range
Travel range is the distance covered by the manipulator along any of the comprising axes from the one far end of its range to the opposite.
The requirement for more or less travel range is determined by the application’s working area and the requirement to reach all of the space within thisarea, without adjustment of the manipulator mounting orientation. The range is determined by a combination of actuator specifications and the way it is integrated into the stage. This includes consideration of limit switches and other stop components, to prevent the actuator from passing its limit and preventing it from changing direction. Having the system optimally mounted on the setup means you can use the full range of travel in the working area.
Most manipulator types with a course and fine mode have independent mechanisms for each mode. Therefore, a longer travel range in the course movement setting will be applicable when compared to the fine movement. On the other hand, manipulators that use a single stepper-motor for both course and fine modes, typically have equally long travel range in both these modes because of this function being handled be one motor.
This variable can refer to two different aspects of manipulator movement. Firstly, to the ability to stop at the point in space where you intended on stopping. This is a function of resolution, speed and deceleration of the manipulator, with its interdependencies.
Secondly, to the manipulator’s calibrated coordinates. In the case of manual manipulators, this can be where micrometers have calibration markings or when the manipulator is equipped with a linear encoder reading out a digital coordinate display. Many electric micromanipulators also have a coordinate readout, where the coordinates are generated by an encoder on the stage or control interface, or calculated digitally by counting command steps on the electric drive. Here the term accuracy refers to the level to which the coordinate display reflects the actual movement.
Several applications, where distance is measurement at sub-micron levels, rely on this type of accuracy in coordinate display. If this criterium is important for your application, a direct measurement of the stage movement by a high-resolution encoder may be a better option than the more indirect methods.
Repeatability, in manipulator terminology, typically refers to the ability of the manipulator to (be programmed to) repeatedly move the probe to the exact same spot during an application. Programming is relevant mainly when referring to electric manipulators – many electric manipulators have memory save functionality, or step functions.
Most electric manipulators that do offer this function have repeatability to within less than 5um, but some applications require this to within submicron ( nanometer) scale. This would then require a feedback loop with a high-resolution encoder on the stage or actuator, which is often seen with piezo-driven manipulators.
Unwanted movement artifacts
In the next section we’ll refer very briefly to unwanted artifacts with regards to movement, as they are often referred to in product literature. Some of these may only be indirectly related to movement.
Backlash is another term where the meaning in manipulator terms is often ambiguous:
On the one hand, backlash (also referred to as lag, lash or play), is a clearance or lost motion in the driving mechanism of the manipulator caused by gaps between the composite parts. This would be where the command to move is given, but there is a time lag before the manipulator responds.
Whereas the abovementioned meaning of backlash is the more generally accepted engineering term, a second meaning is also attributed to backlash in product literature or discussions. This second meaning can also be described as recoil, and it refers to unintended reverse movement of the probe when the user stops driving the manipulator. This can be caused by misaligned mechanics, artifacts in the driving algorithm (electric manipulators), or tension on brackets, and spring loading on the stage or the control wheels.
In both interpretations of backlash, the implications can render the manipulator unfit for purpose, since it affects the accuracy. It may further cause immense frustration with lack of control, when either the probe or the target surface is fragile.
Overshoot is the opposite problem of the recoil-type backlash. This is caused when the user stops instructing movement and the manipulator continues to move, instead of stopping immediately. The cause and implications of overshoot is the same as for the recoil-type backlash.
The engineering definition of hysteresis is where a system’s output is affected by recent input(s) in a time-dependent manner.
In terms of manipulator functionality, this is visible when the manipulator’s accuracy or repeatability is affected by a change in direction of movement just before the assessment is done. This is often caused by incorrect spring loading, assembly or imprecisions in the metalwork. Hysteresis can also be visible in sluggishness with some systems. Apparently, this may cause driving of the manipulator against greater resistance, especially when it has been stationary for a while. This effect can be due to inadequate temperature control, which affects the actuator directly, or due to use of incorrect lubricants that may require a warm-up period.
Hysteresis may appear random, and marginal instances maybe hard to detect and diagnose in a manipulator. Manufacturers should, therefore, include diagnostics for hysteresis in a quality control and testing protocol.
Inaccurate assembly may lead to a linear drive not actually driving in a straight line. This is often linked to tensions on the bearings not being set correctly or the actuator not being correctly integrated into the manipulator body. This artifact is a critical aspect for manufacturers to master.
Other movement-dependent non-movement artifacts
The terms described below are not directly descriptive of manipulator movement, but are related artifacts which may be relevant to your application:
Several applications require the probe to be settled in the same position for extended periods. When the probe noticeably moves away from where it was positioned by the user, it is described as drift – the opposite of stability. The cause of drift in manipulators can be complex. Troubleshooting is often further complicated by causes in the experimental setup other than the manipulator itself. There are several resources focusing specifically on troubleshooting for drift prevention.
When small electric currents are measured by the probe, electromagnetic noise generated or amplified by the manipulator may mask the signal. Manipulators should be grounded properly to prevent the effects of electrical noise. Drive electronics also need to be properly grounded and cables, that could act as antennae, should be shielded
Compared to manual or hydraulic manipulators, electric manipulators are more prone to generate noise if not properly grounded. Therefore, reducing noise requires extra consideration by electric manipulator developers.
Electric manipulators can cause audible noise when the actuator is moving. This can be a high-pitched buzz (typically with piezo actuators) or a lower pitched hum (usually with stepper motors). Some users prefer to hear a noise when it moves, since it confirms that the manipulator is active and receiving power.
Nonetheless, some biological applications (e.g. working with live animals) require complete silence around the setup. Furthermore, the audible noise may also become an irritation to users with prolonged practice.
Some applications are highly sensitive to vibrations. This causes concern as some manipulators may cause vibrations to be transferred to the sample, the microscope or other manipulator-mounted probes on the same setup. This is usually only visible when moving in course mode. The effects of vibration can be reduced or prevented by careful considerations in the design of the manipulator as well as suitable mounting of the manipulator in the setup.
Subscribe to Neurotech Today to receive notification of the next Fundamentals article on Micromanipulators: an in-depth overview of the most popular actuator hardware currently implemented by most micromanipulator manufacturers