Technical Article

Micro Miniature Linear Motors and their Control Simon Bramble Senior Applications Engineer, austriamicrosystems UK An Introduction to SQUIGGLE Motors In 1965 Gordon Moore predicted that transistors in an integrated circuit would double in number every two years. This statement would become known as Moore’s Law and has been applied to the general progress of the electronic industry for the past 40 years, from computer memory to processor speed. The same cannot be said for mechanical components however with motors and relays often being far larger than the electronics that controls them. That has all changed since New Scale Technologies, Inc. launched the SQUIGGLE motor. This device is the world’s smallest linear motor and the SQUIGGLE motor family has an impressive feature set. The motor evaluated for this article is less than 3 x 3 x 6 mm, weighs 0.16 gram and has a stall force of 30 grams (0.3 N). It travels at 7mm/sec, has a travel range of 6 mm and can achieve nanometer resolution. Later devices have a 50g stall force and can run on voltages as low as 2.3V. Electromagnetic motors smaller than about 6mm in diameter start to lose efficiency and reliability as power is wasted in heat and less torque is produced to overcome internal friction. MEMS (micro-electromechanical systems) can achieve sub micro motion, but at the expense of low force and travel. With the SQUIGGLE motor, engineers can add motion features to products where they could not have imagined before.

FIG 1 – the SQUIGGLE motor on a fingertip How the SQUIGGLE Motor Works The SQUIGGLE motor is based on piezoelectric material. Two pairs of opposing piezoelectric plates are connected to a nut with a mating threaded screw inside. Two out of phase square waves are applied to the piezo electric plates, causing them to vibrate in an orbital motion. This movement is similar to a hula hoop action and causes the SQUIGGLE motor screw thread to move in a linear motion back and forth through the nut.

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FIG 2 – SQUIGGLE motor drive waveforms Since there is a small amount of friction in the shaft converting the rotary motion to linear motion, the SQUIGGLE motor stays in place when the power is removed. The overall architecture means that the SQUIGGLE motor shaft can move smoothly over fractions of a micron. The motor drive is controlled by the NSD1202. This device is a highly integrated piezo electric motor driver designed to drive two SQUIGGLE motors. It incorporates a boost converter to boost a battery voltage to 40V to drive the piezo electric plates. The switching frequency of the piezo electric drivers is controlled by an internal register as is the duty cycle and amplitude of the piezo electric voltage enabling precise control of both the position and speed of the motor. The driver is addressed using I2C. The motor moves swiftly back and forth with barely any noise, making it perfect for camera lens focusing in handsets, stills cameras and pico projectors. Their tiny size also means they can be designed into endoscopes and mechanical locks. The device operates on extremely low power too consuming only 500mW when moving, thus is perfect for battery powered applications too. Operating the SQUIGGLE motor The NSD1202 is clocked by an external 20MHz oscillator. A Period Count register divides this frequency down to produce a signal compatible with the resonant frequency of the piezo material (approximately 172kHz). A Pulse Counter register dictates how long the motor is operational. A value of 0 to 2047 can be loaded into the Pulse Counter register and once the value is loaded, it starts to decrement. When it reaches zero, the motor stops. The smaller the value loaded into the Pulse Counter register, the smaller the distance the motor moves. Once the register has decremented to zero, the register has to be reloaded and the decrement process starts again, thus the length of travel can be tightly controlled. The Duty Cycle register and the Output Voltage register control the speed of the motor by varying the ON-time of the motor drive and its amplitude. Using the above registers the speed and displacement of the SQUIGGLE motor can be varied with extreme precision. Closed Loop Control – The Tracker The SQUIGGLE motor runs open loop. However New Scale Technology can provide an IC (the Tracker) that enables the motor’s linear displacement to be measured to a resolution of 0.5um. The device consists of a row of Hall sensors that measure the magnetic field of a linearly polarized strip magnet as shown in FIG 3.

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FIG 3 - The Tracker FIG 3 shows a magnet, magnetized with poles of 1mm in length placed above the Tracker. As the magnet moves over the Tracker, the sine and cosine magnetic field is sensed by the Hall elements producing a corresponding sine and cosine voltage. This is measured differentially to eradicate stray magnetic fields. The chip contains an interpolator that subdivides the 2mm pole pair length by 4096 giving an overall resolution of 489nm. Thus, over the span of 2mm, the internal register counts from 0 to 4096 and resets back to zero when the pole pair boundary is crossed. The results can be read out over an I2C bus. The magnet can be displaced until a loss of magnetic field is detected, creating a soft zero point that can act as a reference for absolute measurements. Experiments with Sub-Micron Motion The SQL1.8 was connected to a PIC microcontroller via its driver IC, the NSD1202 and an I2C bus. The NSD1202 was driven with a clock frequency of 20MHz. The Tracker was also connected to the I2C bus and the data was read out and displayed on an LCD display driven by the PIC microcontroller. The Period Count register was loaded with a value of 116 implying that the switching frequency to the piezo drivers was 172.4kHz. Various experiments were conducted on the motor by loading different values into the Pulse Count register and the Duty Cycle register. Loading the Pulse Count register with 2047 made the motor move forward approximately 230 encoder counts (112um), although this depended on the load on the motor. Reducing the pulse count to 256 reduced the ‘step size’ to 24 (12um) and further reducing the pulse count to 16 seemed to stop the motor (the Tracker output stopped incrementing). It was only when the register was refreshed continually that it was discovered that the motor was actually moving, but with a displacement so slight that the Tracker could not pick it up. A slight ‘click’ could be heard from the motor and for every 3 clicks the Tracker incremented its count by 1. This implied that on average the SQUIGGLE motor was moving 489nm every 3 loads of the pulse count register – an incredible 163nm average motion! The pulse count register was then reloaded with 2047 and the duty cycle register changed. With a duty cycle of 50% the motor moved approximately 7mm per second. Reducing the duty cycle register value to 12%, the motor moved on average 0.125mm per second. Conclusions The above experiments have proved that the SQUIGGLE motor’s speed and linear displacement can be easily controlled and monitored by simple commands over the I2C bus. This motion is sometimes so small it is invisible to the naked eye, but with the aid of the Tracker, can be monitored in a closed loop fashion. New SQUIGGLE Motor Products New Scale Technologies are not standing still. They have improved on the product evaluated in this article and have just released the SQL1.8RV. This runs on a reduced voltage, so the boost converter (and all its associated components) is no longer needed, thus further reducing the footprint of the solution. The controller IC (NSD2101) Page 3 of 4

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that drives the new part is similar to the NSD1202. The frequency of the motor can still be set, as can the duty cycle. However, the new controller includes a 25MHz oscillator, has the ability to fine tune the switching frequency of the piezo motor driver and also has a hybrid mode allowing the motor speed to be controlled more efficiently. The C code for driving the SQUIGGLE motor is available from the author.

Summary Mechanical components have always been the bulky part of any electronic design, often with these parts occupying far more space than the electronics that controls them. New Scale Technologies, Inc. has invented the world’s smallest linear motor (the SQUIGGLE motor) that occupies 2.77mm x 2.77mm x 6mm and has a lot of push force. This article describes how they work and how to drive them.

Biography Simon Bramble is the UK applications engineer for austriamicrosystems AG. He has been with the company since April 2006 and lives in Hampshire with his wife and daughter. He is still passionate about all aspects of both analogue and digital electronics.

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