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NanoDirect Series Control System
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Angular Resolution
One of the most critical factors determining the linear resolution of the NTS NanoDirect system is the angular resolution of the piezoelectric rotary motor. The minimum angular increment/step of 1 arc sec was measured using a laser autocollimator as shown in Figure 1. A mirror is attached to a table that is firmly mounted to the shaft of the motor. The test entails measuring the change in the angle of reflection (when the motor is rotated at its minimum angular increment) of a collimated light beam, originating from the autocollimator and reflected back from the mirror. The method enables angular movement to be resolved to better than 0.5 arc sec.
Torque
Another critical parameter
of the rotary piezoelectric motor is its extremely high initial torque (0.2
Nm), compared to the torque of a comparable stepper motor (e.g. 0.01 to 0.05
Nm). This high torque allows the use of a high-tension spring load, which
ensures continuous contact between the lead screw and the moving stage platform
irrespective of the direction of movement. This guarantees high resolution
of NTS NanoDirect and by minimizing backlash.
Characteristics of the Linear Translational Stage
The mechanical characteristics of the translation stage have been carefully designed to provide the highest achievable resolution. A summary of these design characteristics is given below:
Lead Screw Assembly
The lead screw assembly of the NTS NanoDirect has been precision engineered to provide the accuracy needed for precise stage movement. The design of the lead screw incorporates several important innovations. Each lead screw is manufactured from Class 12 polished chrome steel. The lead screw is then individually fitted into each assembly with the same level of tolerance being assured for all stages. Any unevenness of surface is kept to less than 0.1 mm. In addition, strict control of the shaft’s material and design ensure that any mechanical hysteresis of the stage is minimized.
Translation Stage Guide
The surface irregularity of a translation stage guide can affect the accuracy of movement. To minimize this a tight tolerance is placed on the fabrication of the surface and the quality of polishing. The maximum size of any imperfections on the surface of the guide is in the order of 0.2 mm. It can be proven that if a ball with a diameter of 4 mm steps into a crevice of 0.2 mm its sag will be approximately 0.005 nm. It will therefore have negligible effect on the performance of the stage.
Straightness of Movement
The straightness of stage movement was monitored optically by a laser autocollimator (Figure 2). A mirror, facing an autocollimator, is placed on the moving platform of NTS NanoDirect. The rotational angles of inclination of the mirror in respect to the rotational axes Z and Y were measured during a displacement of 1 mm along the X-axis. The straightness tolerance was better than 1 µm per 1 mm displacement. This parameter shows typically how accurate the grooves of the platform in space are. The guiding grooves on any NTS NanoDirect stage are individually adjusted on an optical bench during assembly and calibration.
Bearings
Any surface irregularities in the balls of the bearings or the bearing guides can affect the accuracy with which a demanded movement is achieved. To achieve the optimal requirements the ball bearings used in the stage are Class 12, which sets the spherical tolerance to better than 0.1 mm. The basic parameters that define an individual ball’s geometry, and hence its imperfection, are of different signs and are randomly distributed between different balls. Accordingly, a bearing comprising of several balls results in the tolerances being averaged, so that the final inaccuracy resulting from ball geometry uncertainties is less than the worst-case tolerance of a single ball. The NTS10 (i.e. 10 mm travel) NanoDirect stage uses 8 balls in the bearings, which ensures that the effect on accuracy arising from ball imperfections is almost undetectable.
Hysteresis
Hysteresis reflects the difference in the achieved positions, which may occur when the stage is moved to the same fixed point from two opposite directions. One factor, which may contribute toward hysteresis in the stage, is related to a non-optimal lateral tension of the guide ball bearings. If the tension applied is too small this results in a sideways wobbling effect of the stage. Conversely, if the tension applied is too great this can cause increased friction of the ball bearings resulting in a nonsymmetrical drag. The current design overcomes this problem by applying an optimal spring tension, therefore minimizing these effects. A second factor, which may contribute toward hysteresis, is caused by a deviation from colinearity between the shaft of the rotary motor and the lead screw. To correct for this effect the NTS incorporates a proprietary elastic transmission (between the motor shaft and the lead screw) providing orthogonal elasticity of approximately 1 mm/N.
Eliminating Backlash
Preloading the lead screw can significantly reduce backlash in the system, by ensuring constant contact between the moving components of the NTS NanoDirect (lead-screw and stage), irrespective of the direction of movement. NTS NanoDirect has two proprietary anti-backlash springs each with 10 N force. This design is only possible because of the very high torque of the rotary piezoelectric motor, which must overcome the spring tension when moving. To avoid any significant wear arising from the high torques used, the NTS NanoDirect is designed with all elements subjected to friction having a hardness of not less than 60 hardness units, HRS by the Rockwell scale.
"Stick-Slip" Effect
The “Stick-slip effect” is one of the major factors, which limits resolution. The effect is caused by the fact that the coefficient of static friction is greater than the coefficient of dynamic friction. When a driving force is applied to a nanopositioner, movement from rest is slightly delayed on the applied force. Initially, with finite force, there is no movement until the force exceeds the static friction. At this point there is a jump in position. Only “frictionless” devices such as solid-state actuators (piezo actuators) exhibit zero measurable friction and therefore can provide a resolution superior to “classical” mechanical positioners in the sub-micron to sub-nanometer range. However, the NTS NanoDirect series overcomes the “Stick-slip Effect” due to the unique start-stop characteristic of the rotary piezoelectric motor. Any angular position of the rotor is “locked” (held) by the self-decelerating torque of the motor. The same force “locks” the whole friction system of the translation stage. To limit the effect of any jump when initiating motion the unlocking process must occur almost instantaneously (with a time constant in the range of 10-100 µsec). The incorporated piezoelectric motor has been designed to implement a step formation within 10-50 µsec per 1 arc sec. This timing results in an angular step of the motor, which translates immediately into an equivalent linear step in the nano-range eliminating any static friction effects.
Interplay of Open and Close Loop System Design
An optical encoder is placed on the shaft of the piezoelectric motor providing a linear accuracy of about 0.125 µm in a closed-loop mode. The shaft lead-screw system is implemented through a special elastic transmission with a twist level of 1 arc sec, which corresponds to 0.4 nm in linear units. The accurate positioning in the nano-range requires step calibration on the adjusted markers of the encoder and the subsequent step interpolation in open-loop mode between the markers. NTS NanoDirect DSP-based controller and software incorporates a proprietary algorithm, which allows the value of the minimum step of approx. 0.4 nm to be measured and updated in real time during implementation of a movement. In this way NTS NanoDirect assures the necessary resolution in open-loop mode between the readings of the optical encoder.
Minimum Incremental Step of Stage
With the combination of all the factors outlined above which affect the intrinsic resolution, the NTS NanoDirect can achieve an unprecedented minimal step size in spite of its long travel range. Using an optical microscope, 2500 minimum steps of the NTS NanoDirect have been routinely measured and found to equate to 1000 +/- 500 nm, yielding a mean incremental step magnitude of 0.4 +/- 0.2 nm.
Characteristics of the NTS NanoDirect DSP-based controller
The 3-channel NTS NanoDirect controller system consists of three separate stage controllers: one master and two slaves. The 2-channel controller consists of one master and one slave. The 1- channel controller consists of one master.
Each controller comprises a DSP core processor, logic circuit and output drivers module. The master controller also includes an analogue-to-digital converter (ADC) module, electrically-erasable programmable read-only memory (EEPROM), USB interface circuit and a voltage regulator module.
The DSP processor receives directly the position-coded signal generated by the optical motor encoder and forms the signal sequences that control the piezomotor. The logic module and the output drivers module generate signals of the required amplitude for control of the motor. The communication between each linear stage and the appropriate controller is via a standard mini DIN-8 M-M cable.
The master controller provides the direct interface with the host computer using the USB (Universal Serial Bus) port. It also receives signals from an optional analogue joystick via the ADC.
Each of the embedded controllers can be re-programmed in circuit via a JTAG port (not shown).
The (nominal) 12 V DC external power module provides the prime source of power to a built-in network of voltage regulators that is used to supply power to the circuit modules in all of the controllers in operation.
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NTS NanoDirect™ Software Control
The NTS NanoDirect controller provides the communication and control link between the NTS Stage and Windows2000/XP operating software. The computer/laptop monitor displays simple menus and prompts the operator to select or enter parameters for the chosen mode of operation. The software processes the digitised input signal and applies the operator-selected information for real-time display and control of stage movement.
The initial setup screen allows the user to select the stage type (e.g. NTS10, NTS20) and then choose between PC or Joystick control for each channel on the NTS NanoDirect controller. The user can also choose to have PC control on one channel and Joystick control on another.
The following menu options are also available
Measurement Units – Allows the selection of units of measurement (e.g. mm or nm etc.) and the creation of the new units altogether.
Distance from Zero – Shows the distance that the stage has travelled from the Zero point.
Travel Indicator Bar - Travel indicator bar graduated in percentages indicates graphically the movement, approximate position, speed and direction of the stage relative to a percentage scale where 100% is equal to the maximum linear travel range of the stage.
Manual Travel - Forward or Reverse motion controlled by mouse button.
Programmed Travel - Automatic travel where the required distance and direction of travel is entered and actual travel is then initiated by clicking the Start button.
Preset Travel – Enables an unlimited number of preset positions of the stage platform to be stored and recalled so that the stage can move automatically to the chosen preset position.
Stepped Mode - Moves the stage in increments controlled by the Velocity and Step Size.
Continuous Mode – Moves the stage continuously with maximum velocity (250 mm/sec).
Velocity - Defines the speed at which the Stage moves in mm/sec.
Step Size - In Stepped Mode, this fixes the length of a single increment (i.e. a single step) of movement of the stage.
Technical Data and Specifications:
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Travel Range ~ NTS10, NTS20, NTS100 |
10 mm, 20 mm or 100 mm |
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Design Resolution |
0.2 nm |
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Min. Linear Increment |
< 0.4 nm |
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Unidirectional Repeatability |
< 0.4 nm |
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Bi-directional Repeatability |
< 0.5 µm |
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Backlash |
< 0.5 µm |
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Hysteresis |
< 0.5 µm |
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Pitch ((θy) / per 1 mm |
< 5 arc. Sec. |
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Yaw (θz) /per 1 mm |
< 5 arc. Sec. |
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Max. Velocity |
2000 µm/s |
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Reaction Time – demand to maximum velocity |
< 0.3 ms |
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Velocity Range (Stepped – Continuous) |
6 orders (0.5 nm/s to 250 µm/s) |
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Response Time |
10 msec |
Max Load –Capacity |
3 kg |
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Max. Push/Pull Force |
15/50 N |
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Max. Lateral Force |
100 N |
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Encoder Resolution |
4000 counts/rev. |
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Drive Screw Pitch |
0.5 mm/rev. |
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Supply Voltage |
12 Volt |
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Nominal Power Consumption |
1 W |
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Length ~ NTS10, NTS20, NTS100 |
185 mm, 219 mm or 293 mm |
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Weight |
0.35 kg, 0.45 kg or 0.60 kg |
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Long Term Stability – Drift |
Less than 2 nm /hour @ 200C |
Recommended Controller |
NTS NanoDirect series - 1, 2 or 3 channel |