Actuation for Mobile Micro-Robotics

John C. Tucker
North Carolina State University
 

Introduction

    Advances in precision micro-machining has led to an interest in micro-robotics. Applications of micro-robotics range from micro-assembly, to biomedics (inner space), to land mine sweeping, to city water system analysis. As with conventional robotics one of the biggest challenges is making robots that are mobile and can traverse a wide variety of terrain. Furthermore, in micro-robotics there is the problem that as the robot gets smaller the terrain obstacles seem bigger. A pebble is no problem for a six meter long HMV, but it is real challenge for a ten millimeter surveillance robot.

 Actuation systems for mobile micro-robotics must meet the following challenges:

     Obviously the actuation method must be designed to meet the needs of the robot. A robot in a desert (scorpion design) will have a different design than one in a water pipe (fish design). This paper reviews the current technologies for actuation systems and then discusses some designs for a micro-robot.

Conventional Electromagnetic Motors and Solenoids

     In the past robotics has mainly used motors and solenoids to make robots mobile. This can be done simply by using motors with wheels or tracks, or by using arms and legs powered by motors and solenoids. Designs of this type benefit from the large amounts of physical motion that can be produced. Furthermore, rolling motion like a car is very efficient and traverses simple terrain very well. The use of arms and legs adds the ability to traverse steps and other obstacles. However, electromagnetic motors are mechanically complex and do not scale down very well. Manufacturing electric motors less than a millimeter in size is very challenging. Other problems are power efficient (30-40% max.) and fragility.

Piezoelectric Linear Actuators

     Piezoelectric materials are materials that expand/contract when an electric field is applied to them. They also will produce an electric field across themselves if a mechanical force is applied to them. Common places for piezoelectrics are in gas lighters, high frequency speakers, and micro-positioners. These devices rely on the piezoelectric effect. The piezoelectric effect happens in materials with an asymmetric crystal structure. When an external force is applied, the charge centers of the crystal structure separate creating electric charges on the surface of the crystal. This process is also reversible. Electric charges on the crystal will cause a mechanical deformation. Quartz, turmalin, and seignette are common natural piezoelectrics.  Much work has gone into making polycrystalline ceramic piezoelectrics because physical properties can be tailored to the application. Furthermore, these materials can be bulk produced or deposited onto surfaces. Common ceramic piezoelectrics are lead-zirconate-titanate (PZT) and lead-magnesium-niobate (PMN). Piezoelectrics have also been made in polymer form, such as poly-vinylidene fluoride (PVDF).
    Piezoelectrics deform linearly with applied electric field. Unfortunately, conventional materials only deform up to 0.1%. Thus, for a 5 cm leg on a micro-robot, the motion will be only 50 um. Furthermore,  this happens at an electric field around 2 kV/mm. Thus, the applied voltage would have to be 100 kV. Piezoelectrics follow the equation

where E is the electric field, d is the piezoelectric tensor of the material, F is an externally applied force, and CT is the stiffness of the material. Because strains are so small, piezoelectric actuators are mainly used in speakers or precision micro-positioning applications where small, precise motion is needed. However, deflection amplification methods make piezoelectrics possible actuators in micro-robotics.
 
 

Bending Mode Mechanical Amplifiers

    Unimorph

   One amplification method is the unimorph design shown in figure 1. When a voltage is applied across the ceramic and metal plate the unimorph bends. Reversing the voltage bends it in the other direction. This device relies on the d31 piezoelectric factor. This is the change in strain induced perpendicular to the electric field. The factor d31 is typically half of d33, the induced normal to the electric field. However, a motion of 0.875 inches can be produced by a unimorph approximately one inch in diameter and 0.02 inch thick. This design is typically found in loud speakers.
 

    Bimorph

   Like the unimorph, the bimorph uses d31 piezoelectric actuation. The bimorph uses two piezoelectric plates that amplify the deflection as shown in figure 2. The two plates can be electrically connected in parallel or in series. A parallel connection produces twice the displacement as a series connection. In either case the strain is proportional to the square of the applied voltage.
 

    RAINBOW

    RAINBOWs or Reduced And Internally Biased Oxide Wafers are piezoelectric wafers with an additional heat treatment step to increase their mechanical displacements. In the RAINBOW process, developed by Gene Heartling at Clemson University, typical PZT wafers are lapped, placed a on graphite block, and heated in a furnace at 975 C for 1 hour.8 The heating process causes one side of the wafer to become chemically reduced. This reduced layer, approximately 1/3 of the wafer thickness, causes the wafer to have internal strains that shape the once flat wafer into a dome. The internal strains cause the material to have higher displacements and higher mechanical strength than a typical PZT wafer. RAINBOWs with 3 mm of displacements and 10 kg point loads have been reported.9
 

Flextensional Amplifiers

Stacks

    Similar to the bimorph is the piezoelectric stack where several elements are placed on top of each other and electrically connected in parallel. The advantage of this design is that a stack uses the d33  which is larger than the d31 effect. Furthermore, displacements are N (number of elements in stack) greater for the same applied voltage.

Cantilevers

    Other ways of producing mechanical amplification are through the use of cantilevers in figure 3.  This is just a simple mechanical amplifier that increases displacement but reduces force.
 
 
 

 

Inch Worm Motors

    Piezoceramic inch worm motors are linear motors generally used in micro-positioning applications due to the ability to make very small accurate motions. The concept is shown in figures 3.1 and 3.2. There are two clamps and one extentional element. While clamp A is on and clamp B is off the drive piezo is extended. Then, clamp A is off and B is on returning clamp B to its original position by relaxing the drive piezo. Again, clamp A is on and clamp B is off the drive piezo is extended and so on. This is done many times and the rod moves up. Reversing the clamping sequence can make the rod move down. These devices can be operated at high frequencies to achieve millimeter per second motions. Some challenges of inch worm devices are achieving high precision in manufacturing so that the clamps work properly.
 
 

Piezoelectric Rotary Motors


    Piezoelectric rotary motors have been developed that not only weigh much less than conventional electromagnetic motors but also supply much higher stall torque. Timothy S. Glenn and Nesbit W. Hagood at MIT have developed an 330 gram ultrasonic piezoelectric motor that can supply 170 N-cm. of torque1.  A 8 mm, 0.26 gram motor has also been developed that can provide 0.054 N-cm of torque2.  Piezoelectric rotary motors are also available commercially from Shinsei and Canon. Like other piezoelectric devices, these motors require a high voltage supply (~150 V).
    One possible actuator design with a piezoelectric rotary motor is shown in figure 4. The motor winds a spring up. The other end of the spring is held by a pin. When the pin is pulled back the leg moves down quickly and produces a "cricket" jumping motion.
 
 

 

Relaxor-ferroelectrics

      Relaxor-ferroelectrics are similar to piezoelectrics except the strain is produced by the second order electrostrictive effect as opposed to the first order effect. The advantages of these actuators over conventional piezoelectrics include improved stroke (quadratic relationship to applied electric field shown in figure 5), low hysterisis, return to zero displacement when voltage is suddenly removed, and insusceptibility to stress depoling3. However, they have a higher temperature dependence of 65% change in expansion 0-50 C (only 5% for piezos)4.
    All insulators are electrostrictive and produce a strain under an applied electric field. While this effect is negligible in most materials, the PMN-PT-BT relaxor-ferroelectric manufactured by Lockheed Missiles and Space Company had a 0.1% strain at 1 kV/mm.
 
 
 
 

Magnetostrictive Actuators

    Like the Piezoelectric effect where the material deforms under an applied electric filed, a magnetostrictive material deforms in a magnetic field. Induced strains and maximum stresses are on the same order of magnitude as piezoelectrics. One common magnetostrictive material TERFENOL (TER (Terbium) FE (Iron) NOL Naval Ordinance Laboratory)) produces a 0.2% strain in a 100 kA/m field5. One major disadvantage of magnetostrictive actuators is the need for a device to produce the magnetic fields. This device is typically a coil wrapped around the material. This makes the device bulky and losses in the coils can be high.

Hybrid Actuators

    Because piezoelectrics are capacitive and magnetstricters are inductive, delivering high electrical power to them individually can be inefficient and/or require matching networks. Even with with matching networks, high efficiency over a wide frequency range is difficult. However, recent work has been done using the two devices together in order to increase frequency operation6.
 

Enhanced Electrostrictive Actuators

 
 CRESCENT (CERAMBOW)
 THUNDER
 Caterpillar d33 unimorph
 

Ion Exchange Actuators

    The theory behind ion-exchange-membrane-metal composites is fairly complex. Essentially the materials are made of ionizable molecules that can dissociate and attain a net charge when a electric field is applied. These actuators have a large deformation in the presence of low applied voltage. Actuators made from these materials can deform as much as 2.5 cm under a 7 V applied voltage7 .  These actuators best work in a humid environment, but may be encapsulated.

Shape Memory Alloys

    Shape memory alloys are metals that deform when electric current is passed through them. The deformation is due to thermal expansion.

References

1. Timothy S. Glenn, and Nesbit W. Hagood, "Development of a two sided piezoelectric rotary ultrasonic motor for high torque", SPIE Conference Procedings Vol. 3041, pp.326-338, 1997.

2. A. M. Flynn, Piezoelectric Ultrasonic Micromotors, MIT PhD. Thesis, Thesis in Electrical Engineering and Computer Science, June 1995.

3. Craing L. Horn and Natarajan Shankar, "Modeling the Dynamic Behavior of Electrostrictive Actuators", SPIE Conference Procedings Vol. 3041, pp.268-280, 1997.

4. "Basis of Piezoelectric Positioning", Products for Microposionting Catalog, Physik Instrumente Co., 1995.

5. Ian W. Hunter and Serge Lafontaine, "A Comparison of Muscle with Artificial Actuators", IEEE ?, pp.178-185, 1992.

6. Bernd Clephas and Hartmut Janocha, "New linear motor with hybrid actuator",  SPIE Conference Procedings Vol. 3041, pp.316-325.

7. Karim Salehpoor, Mohsen Shahinpoor, and Mehran Mojarrad, "Linear and Platform Type Robotic Actuators Made From Ion-Exchange Membrane-Metal Composites", SPIE Conference Procedings Vol. 3040, pp.192-198, 1997.

8. E. Furman, G. Li, and G.H. Heartling, "Electromechanical Properties of Rainbow Devices", Proceedings of the 9th International Meeting on Applications of Ferroelectrics, pp.313-318, University Park, PA, 1994.

9. G. H. Haertling, "Chemically Reduced PLZT Ceramics for Ultra-High Displacement Actuators", Ferroelectrics, vol. 154, pp.233-247, 1990.

For more information on MEMS research at North Carolina State University visit the Electronics Research Laboratory.

Actuation for Mobile Micro-Robotics / John C. Tucker / jctucker@eos.ncsu.edu