Abstract

Paper deals with miniature actuators. First part of paper shows the problems related to scalling effect. Next part of paper shows the some results from experimental measurement of piezoelectric actuator.

1. Introduction

Micro mechatronics is the synergetic integration of both mechanical and electronic systems based on scaling effects in the micro world. In this time the micro mechatronics and MEMS products are very often used in almost every product around us. Very often it is not visible but it is included in cameras, mobile phones, microwave oven, photocopier, washing machine, CD and DVD players etc.

Cars are full up with mechatronic influence and it causes that cars have a certain degree of intelligence. Cars are able to control brake force, it is able to control of amount of transferred torque to wheels, it is able to park into the sequence of parked cars, it can do own monitoring and diagnostic and warning before dangerous status, it is able to rescue your life through the airbags etc.

Designers have to apply mechatronics systems almost in every product to improve its properties and to add new function. It is possible to say that mechatronic is a strategic key how to reach the best products which are competitive on world market.

Situation is most complicated in very small products where you are limited with place. It is not so easy to integrate mechanics, sensors, electronics and other parts into the small space. Very often we say about micromechatronics or MEMS products.

Difference of dimensions of macro and micro world causes the difference in physical phenomenon influence, in object moving, in relative changes of system performance between these worlds. It is possible to define microphysics as science about study of motion, object structure of objects, where attractive forces are becomes dominant unlike of macro world, where gravitational and inertial forces are dominant. A lot of physics laws are differ in micro world. “Scaling effect” is used for description of miniaturization influence to system efficiency.

Analysis of scale effect helps to choose suitable type of actuator for designed micromechatronic products.

Key elements of mechatronic product can be summarized into several groups (Figure 1).

An actuator is a part of mechatronic product or part of machine that is able to make conversion of electrical (or other) energy to mechanical work. This conversion is possible only if source of energy is available and also controlling is necessary for this purpose.

The actuators can be divided into these groups by physical principle:

- Hydraulic or pneumatic actuator - uses hydraulic or pneumatic power to ensure the mechanical work.

- Electric actuator - uses electromagnetic principle based on Faraday induction law (DC and AC motors, stepper motors and solenoid actuators).

- Thermal expansion actuator and shape memory alloys uses principle of thermal expansion and contraction when temperature changes. It uses the phenomena of thermal dilatation or shape memory alloy effect.

- Piezoceramic actuator uses the piezoelectric material which deforms when electric voltage is applied to their electrodes.

- Giant magnetostrictive alloy changes dimension when magnetic array is applied (permanent magnet or electromagnet).

- Electrostatic actuator converts electrostatic energy to mechanical work.

- etc.

Mechanical work at the output of these actuators can be in two forms:

- Straight linear movement (push/pull action) - linear actuators.

- Rotary motion - rotary actuators.

Actuator converts energy (electric, light, heat, mechanical or chemical) to mechanical energy. This transformation process is very often reversible. Piezoelectric element is typical example. It can converts electrical energy to mechanical energy and reversal. Typical transformations are shown on Figure 2 ^{1, 2}.

Special attention is dedicated to microactuators which seem to be as key part for motion generation in micromechatronic products. It is not possible to ignore influence of attractive force like Van der Waals force, forces of surface tension and electrostatic force. Also friction forces and other passive resistance cannot be ignored in design of miniaturized products ^{1, 2}.

2. Piezoelectric Actuators

The most commonly used piezoelectric actuators are in two possible types as “stack” also known as “piezostacks” (Figure 3) or “bender” also known as “piezobender”.

The piezoactivity can be described in simplified form as:

(1)

where Δl is deformation (or actuator stroke); d₃₃ is piezoelectric strain constant and U is applied electric voltage. Consequently, the deformation does not depend on dimension of piezomaterial.

The piezoelectric strain is generally very small (approx. 10^-9m/V). This significant problem can be solved via using of piezostack. The piezostack is composed of many thin piezo layers, which together produce a higher stroke ^{3, 4}.

Overall stroke of piezostack can be expressed as:

(2)

where n is number of piezoelectric layers in piezostack.

Blocked force can be derived in form:

(3)

where Δl is deformation (or actuator stroke); S₃₃^E is elastic constant (m²/N), t is thickness of layers, n is number of piezo layers and F is blocked force (N).

3. Experimental Verification of Piezostack Properties

The piezostack actuator has been tested in this work. Test equipment for this testing has been developed (Figure 4). The piezostack acts mainly like an expanding element “pusher” generating a compressive force. Loading of the piezostack has been through the set of etalon weights. Loading frame has been used for this purpose. Piezoactivity has been measured through the dial indicator.

Excitation of the piezostack has been realized via using of microcontroller and the overall activation and deactivation process was automatic for obtaining of the stabile measurement conditions.

Measured characteristic (Figure 5) shows nearly linear characteristic as it is defined in math model, but polynomial function is a better model. Also hysteresis for this characteristic is the typical property of this material.

Measured piezostack characteristic projected in 3D graph (Figure 5) shows that maximum stroke of actuator was 19.5 μm. Maximum blocked force was 150N, but maximum testing load was only 60N, which causes decrease of stroke to value of 5μm.

The tested piezostack is able to operate in non-resonant cycling (<1 kHz) with maximum strain and non-resonant cycling (< 10 kHz) with reduced strain.

4. Concept of Module with Controlled Bristles

In-pipe machines are developed for inspection tasks or cable drawing into pipes ^{5, 6, 7, 8}. Bristles are non-conventional carrying elements instead of wheels or tracks. Bristled in-pipe robots have better ability to locomote inside dirty pipes than wheeled in-pipe robots. The basic principle of in-pipe robot is coming from arrangement on Figure 6.

Bristle is an elastic cantilever beam which is preloaded when in-pipe machine is inserted into pipe. The principle of bristled locomotion lies on inertial stepping principle (inertial driving comes from impact of machine bodies) or on anisotropic friction difference (Figure 6) (coefficient of friction is depend on bristle moving direction). Consequently, bristles are also used for elimination of pipe geometric deviations. Passive bristles are able to eliminate only limited interval of geometric deviations.

Bristles with regulated properties (Figure 7) would be a significant contribution to solving of these mentioned locomotion problems. It means that bristles should become to controlled compliant parts. Principle of controlled bristles leans on fact that bristle properties changes to suitable values for obtaining of higher locomotion efficiency.

The control structure allows increasing or decreasing the value of normal force between the bristle tip and inner pipe wall and consequently friction force between bristle and inner pipe wall is changes following the locomotion principle.

If linear actuator (Figure 7) elongates, friction force between front bristles and inner pipe wall should be decreased for ensuring of maximum travelled path of front bristles. However, friction force between back bristles and inner pipe wall should be increased for minimising of back bristles moving. Ideal state is if back bristles are stop and fixed in one position. This phenomenon is often called as “self-locking mechanism”.

If linear actuator (Figure 7) contracts, friction force between front bristles and inner pipe wall should be increased for ensuring of minimum or none travelled path of front bristles (front bristles are self-locked in their position). However, friction force between back bristles and inner pipe wall should be decreased for ensuring of maximum travelled path of back bristles. These steps allow obtaining maximum locomotion speed and minimum energy loss.

Change of the bristle angle or change of displacement of bristle tip is possible to realize through the compliant mechanism (Figure 7). The concept is coming from lever mechanism with combination of reduced cross-section of material. Piezoelectric actuator force is applied to compliant mechanism and it is used for regulating of normal force value applied to inner pipe wall. Compliant mechanism consists of three bristle connected with middle rosette. The compliant part is inserted into case with preloaded piezoactuator.

5. Conclusion

The displacement of piezoactuators is very low. For many applications it is necessary to increase it via using of mechanical amplifier as lever or arm. This amplified piezoactuators have a stroke approximately 10 times higher than basic piezostack structure. Also another composition is possible for many applications as Inchworm motor or ultrasonic motor for obtaining of high movement output. These structures can be used also for high precision nano positioning ^{3, 4}.

Acknowledgements

The work has been accomplished under the research projects No. VEGA 1/0872/16 and APVV-15-0149 financed by the Slovak Ministry of Education and project “Design and realization of pneumatic manipulator” financed by the Faculty of Mechanical Engineering at the Technical University of Kosice. This contribution is also the result of the project implementation: Centre for research of control of technical, environmental and human risks for permanent development of production and products in mechanical engineering (ITMS:26220120060) supported by the Research & Development Operational Programme funded by the ERDF.

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