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This article presents a novel vertical positioning stage with a dual-sensor arrangement suitable for scanning probe microscopy. The stage has a travel range of 8.4 μm and a first resonance frequency of 24 kHz in the direction of travel. The sensor arrangement consists of an integrated piezoelectric force sensor and laminated piezoresistive strain sensor. The piezoelectric force sensor exhibits extremely low noise and introduces a zero into the dynamics which allows the use of integral force feedback. This control method provides excellent damping performance and guaranteed stability. The piezoresistive sensor is used for tracking control with an analog PI controller which is shown to be an approximate inverse of the damped system. The resulting closed-loop system has a bandwidth is 11.4 kHz and 6 σ -resolution of 3.6 nm, which is ideal for nanopositioning and atomic force microscopy (AFM) applications. The proposed vertical stage is used to replace the vertical axis of a commercial AFM. Scans are performed in constant-force contact mode with a tip velocity of 0.2 mm/s, 1 mm/s and 2 mm/s. The recorded images contain negligible artefacts due to insufficient vertical bandwidth. [ABSTRACT FROM AUTHOR]

This article describes a new electrical configuration for driving piezoelectric benders. The ‘Biased Bipolar’ configuration is compatible with parallel-polled, bimorph and multimorph benders. The new configuration is similar to the standard three-wire drive method where the top electrode is biased with a DC voltage and the bottom electrode is grounded. However, the new configuration uses an alternate DC bias voltage and adjusted range for the central electrode which allows the full range of positive and negative electric fields to be utilized. Using this technique, the predicted deflection and force can be increased by a factor of 2.2 compared to the standard two wire configuration and 1.3 times for the standard three wire configuration. These predictions were verified experimentally where the measured factor of improvement in displacement and force was of 2.4 and 1.3 compared to the standard two-wire and three-wire configurations. [ABSTRACT FROM AUTHOR]

Abstract: Position sensors with nanometer resolution are a key component of many precision imaging and fabrication machines. Since the sensor characteristics can define the linearity, resolution and speed of the machine, the sensor performance is a foremost consideration. The first goal of this article is to define concise performance metrics and to provide exact and approximate expressions for error sources including non-linearity, drift and noise. The second goal is to review current position sensor technologies and to compare their performance. The sensors considered include: resistive, piezoelectric and piezoresistive strain sensors; capacitive sensors; electrothermal sensors; eddy current sensors; linear variable displacement transformers; interferometers; and linear encoders. [Copyright &y& Elsevier]

Abstract: This paper presents a new sensor arrangement and feedback controller for hysteresis, creep and vibration in piezoelectric actuators. A piezoelectric force sensor is combined with a resistive strain gage to provide both extremely low noise and high stability. The use of a force sensor also results in a system transfer function that exhibits zero-pole ordering. Such systems allow a simple integral controller to provide excellent tracking and damping performance with guaranteed stability. The proposed technique is demonstrated on a nanopositioning platform with a range of 10 m and a resonance frequency of 2.4kHz. In closed-loop, the controller damps the resonance by 33dB and provides a tracking bandwidth of 1.8kHz. Excellent tracking of a 130Hz triangular reference and reduction of hysteresis to 0.46% at 10Hz is also demonstrated. The closed-loop positioning noise, predicted from the sensor noise density, was approximately 0.67nm peak-to-peak, or 0.0067% of the 10 m range. [Copyright &y& Elsevier]

• Active AFM cantilever with piezoelectric actuation and piezoresistive sensing. • No requirement for an additional doping step for piezoresistor fabrication. • Compatible with a commercial MEMS fabrication process. • Feedthrough reduction by utilizing separate actuation and sensing phenomena. Active atomic force microscope cantilevers with on-chip actuation and sensing provide several advantages over passive cantilevers which rely on piezoacoustic base-excitation and optical beam deflection measurement. Active microcantilevers exhibit a clean frequency response, provide a path-way to miniturization and parallelization and avoid the need for optical alignment. However, active microcantilevers are presently limited by the feedthrough between actuators and sensors, and by the cost associated with custom microfabrication. In this work, we propose a hybrid cantilever design with integrated piezoelectric actuators and a piezoresistive sensor fabricated from the silicon device layer without requiring an additional doping step. As a result, the design can be fabricated using a commercial five-mask microelectromechanical systems fabrication process. The theoretical piezoresistor sensitivity is compared with finite element simulations and experimental results obtained from a prototype device. The proposed approach is demonstrated to be a promising alternative to conventional microcantilever actuation and deflection sensing. [ABSTRACT FROM AUTHOR]

The use of integrated on-chip actuation simplifies the identification of a cantilever resonance, can improve imaging speed, and enables the use of smaller cantilevers, which is required for low-force imaging at high speed. This article describes a cantilever with on-chip actuation and novel dual-sensing capabilities for AFM. The dual-sensing configuration allows for tip displacement and tip force to be measured simultaneously. A mathematical model is developed and validated with finite element analysis. A physical prototype is presented, and its calibration and validation are presented. The cantilever is optimized for use in off-resonance tapping modes. Experimental results demonstrate an agreement between the on-chip sensors and external force and displacement measurements. [Display omitted] • Faster imaging allows for the detection of rapidly changing phenomena in samples. • Indirect tip displacement can induce errors in the imaging data. • OBD measures angle and it is not linear for large amplitudes. • Integrated sensors and actuator are compatible with microelectromechanical devices. [ABSTRACT FROM AUTHOR]

• Integration of strain gauges into piezoelectric bender electrodes. • Detailed dynamic modelling of the strain gauge response. • Design of a position controller. • Experimental validation showing excellent position control performance and the ability to measure contact forces. • Applications include micro-grippers and pneumatic valves. [Display omitted] Piezoelectric benders are widely used in industrial applications due to their low-cost and compact size. However, due to the large relative size and cost of displacement sensors, bender actuators are often operated in open-loop or with feed-forward control, which can limit positioning accuracy to 20% of full-scale. To improve the positioning accuracy of piezoelectric benders, this article proposes integrating resistive strain gauges into the electrode surface by chemical etching or laser ablation. These strain sensors are then used to measure and control the tip displacement. The proposed sensors are shown to suffer from significant cross-coupling between the actuator voltage and measured signal; however, this can be mitigated by judicious choice of the sensor location and actuator driving scheme. In addition to position sensing, a method is also presented for simultaneous estimation of the contact force between the actuator tip and load. The proposed methods are validated experimentally by controlling the tip position of a piezoelectric bender while simultaneously estimating the force applied to a reference load cell. [ABSTRACT FROM AUTHOR]

• Novel designs and single-step fabrication of omnidirectional soft actuators using SLA and FDM. • Characterization of the elastic resin used with SLA and modeling of the actuators using finite element modeling (FEM). • The actuators can achieve bending, extension and contraction motions under positive, negative or differential pressures. • The omnidirectional actuators show great potential for worm and snake robots, e.g. in biomedical and pipe inspection devices. [Display omitted] Soft pneumatic actuators are usually fabricated using molding and casting techniques with silicone rubbers, which requires intensive manual labor and limits repeatability and design flexibility for complex geometries. This article presents the design and direct 3D-printing of novel omnidirectional soft pneumatic actuators using stereolithography (SLA) with an elastic resin and fused deposition modeling (FDM) with a thermoplastic polyurethane (TPU). The actuator is modeled and optimized for bending performance using the finite element method along with a hyperelastic material model that is based on experimental uniaxial tensile data. The designs inspired by fast pneumatic network actuators (PneuNets) allow for multimodal actuation including bending, extension and contraction motions under positive, negative or differential pressures. The predicted results from the finite element method are compared with the experimental results for a range of actuation configurations. These novel omnidirectional actuators have significant potential in applications such as pipe inspection and biomedical devices. [ABSTRACT FROM AUTHOR]

• This work proposes a new class of nanopositioning systems constructed from a single bimorph piezoelectric sheet. • The derivation of a finite DOF model based on the constitutive piezoelectric equations, Euler-Bernoulli beam theory, and Hamilton's principle. • Modeling the effect of the load on the nanopositioner's first out-of-plane resonance frequency. • Experimental identification of the nanopositioner's modal frequencies and shapes. • Experimental identification of the cross-coupling between the axes of the nanopositioner. [Display omitted] The article describes the design and modeling of a five-axis monolithic nanopositioning stage constructed from a bimorph piezoelectric sheet. Six-axis motion is also possible but requires 16 amplifier channels rather than 8. The nanopositioner is ultra low profile with a thickness of 1 mm. Analytical modeling and finite-element-analysis accurately predict the experimental performance. The stage was conservatively driven with 33% of the maximum voltage, which resulted in an X and Y travel range of 6.22 μ m and 5.27 μ m respectively; a Z travel range of 26.5 μ m; and a rotational motion of 600 μ rad and 884 μ rad about the X and Y axis respectively. The first resonance frequency occurs at 883 Hz in the Z axis. Experimental atomic force microscopy is performed using the proposed device as a sample scanner. [ABSTRACT FROM AUTHOR]

• Integration of strain gauges into piezoelectric bender electrodes. • Deflection sensing in response to applied voltages and external applied tip forces. • Model based tip force sensing. • Applications include micro-grippers and pneumatic valves. This article derives design guidelines for integrating strain gauges into the electrodes of piezoelectric bending actuators. The proposed sensor can estimate the actuator tip displacement in response to an applied voltage and an external applied tip force. The actuator load force is also estimated with an accuracy of 8% full scale by approximating the actuator response with a linear model. The applications of this work include micro-grippers and pneumatic valves, which both require the measurement of deflection and load force. At present, this is achieved by external sensors. However, this work shows that these functions can be integrated into the actuator electrodes. [ABSTRACT FROM AUTHOR]

• Lock-in amplifier is a widely used instrument for high-resolution sensing applications. • Derivation of the lock-in amplifier amplitude noise spectral density as a function of Gaussian input noise sources. • Application to microcantilever noise measurement with thermal force noise and electronic sensor noise. The lock-in amplifier is a crucial component in many applications requiring high-resolution displacement sensing; it's purpose is to estimate the amplitude and phase of a periodic signal, potentially corrupted by noise, at a frequency determined by a reference signal. Where the noise can be approximated by a stationary Gaussian process, such as thermal force noise and electronic sensor noise, this article derives the amplitude noise spectral density of the lock-in-amplifier output. The proposed method is demonstrated by predicting the demodulated noise spectrum of a microcantilever for dynamic-mode atomic force microscopy to determine the cantilever on-resonance thermal noise, the cantilever tracking bandwidth and the electronic noise floor. The estimates are shown to closely match experimental results over a wide range of operating conditions. [ABSTRACT FROM AUTHOR]