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Article

Mechatronics in Miniature

Published in ECN.com

August 2009

By Lisa Schaertl

 

New motor and sensor technologies enable mechatronics in miniature. Closed-loop electromechanical motion systems, based on piezoelectric micro motors, measure just a few millimeters in size. These systems are being developed to meet demands for:

• more features and performance in portable consumer electronic devices. Examples include improved picture quality in phone cameras, optical image stabilization in digital still cameras and video cameras, and zoom lenses in pico projectors.
• less invasive medical procedures. Under development are miniature mechatronic systems that are small enough to fit in the head of a standard endoscope, and even robotic devices that crawl across a beating heart.
• tiny electronic locking devices that fit in the space of a standard mechanical lock cylinder.

 

This level of miniaturization brings new requirements to the mechatronic equation.

 

• First and foremost, of course, all components must be small.
• They must consume very little power, especially in portable consumer devices where battery life is a high priority.
• In most cases there is a requirement for precise positioning, meaning that position sensors and control electronics are needed for closed-loop feedback.
• Finally, the components must be robust enough to withstand the shock of being dropped, or the high temperatures of an automobile interior on a summer day.

 

Until a few years ago, the options for meeting these requirements were limited. Miniaturizing traditional motors below a diameter of about six millimeters results in an unacceptable loss of efficiency, useful force, and precision. Alternative motion technologies such as shape memory alloys, voice coil modulators and solenoids provide even less precision and force, and a limited range of motion (less than 2 mm).

 

Now with the commercialization of several types of piezoelectric motors, miniature mechatronics development is on the rise.

 

Piezo micro motors for precision motion

At the heart of these micro motors are piezoelectric ceramic elements which change shape when electrically excited. This principle has been used for decades in devices such as ink jet printers and speakers. Piezo motors employ various mechanisms to multiply or magnify this very small motion and create millimeters of travel range.

 

Various techniques have been commercialized. In the smallest linear piezo motor, the SQUIGGLE motor, four piezoelectric ceramic actuators are attached to a metal tube. The tube is threaded inside and a mating screw inserted. Applying a two-channel square wave to the actuators (figure 1) creates ultrasonic vibrations in the tube, causing it to move in an orbit similar to a person’s hips in a “Hula Hoop.” The tube engages the threaded screw and drives it along the tube in a smooth linear motion. Reversing the phase shift reverses the direction of the orbit, and hence the direction of the screw.

 

Thread friction drives the shaft and locks the screw in place when power is turned off. The highest efficiency of the mechanical coupling between the tube and the screw is achieved at the ultrasonic frequency matching the first bending resonant frequency of the tube.

 

Figure 1 - a piezoelectric motor from New Scale Technologies uses ultrasonic vibrations of the piezo elements to drive a threaded shaft

 

This motor is less than 3 x 3 x 6 mm, weighs less than 1 gram and has a push force of 30 grams force (3 N). It travels faster than 7 mm/sec and has a travel range of 6 mm.

 

In a typical mechatronic system, the motor is installed so that the screw pushes against the load to be moved. The screw must be allowed to rotate freely, therefore a light spring preload is used to maintain contact between the screw tip and the load, and provide good coupling between the threads of the screw and the nut.

 

Users can signal the motor to move in increments as small as 0.5 μm by signaling it to travel for a specific time at a known speed, where speed can be determined by the level of voltage applied to the piezoelectric elements (hence the vibration amplitude) or by the duty cycle of the vibrations.
However, motor speed varies with applied load and device friction. Therefore a closed-loop control system is needed to achieve exact positioning, repeatable positioning, or precise speed.

 

Sensors for closed-loop control

In a closed-loop motion system, a sensor detects the actual position of the load and feeds the information to the motor controller. The controller compares actual position to desired position, and moves the motor to correct any error. This allows the motor to reach a precisely controlled position. Similarly, controlled speed is achieved by adjusting the driver gain to minimize the difference between the required position and the actual position at regular time intervals.

 

Until recently it was difficult to specify a position sensor with tiny size and high precision to match the capabilities of SQUIGGLE micro motors. To fill this need, New Scale Technologies and austriamicrosystems developed a line of micro position sensors. The TRACKER sensor is a unique magnetic sensor with integrated on-chip encoder plus technology to essentially eliminate sensitivity to stray external magnetic fields. As such, it is not only the smallest sensor available, but also overcomes many of the limitations of both traditional optical encoders and magnetic sensors. It is less than 10 mm²in chip-on-board packaging, compared to 21 mm² for miniature optical encoders. Integrating the encoder directly onto the SOIC with the sensor array eliminates the need for external pulse counters, reducing the size of the supporting electronics needed.

 

A linear array of eight Hall effect sensors on the chip measures the spatially varying magnetic field produced by a multi-pole magnetic strip moving above the sensor. The magnetic field generates internal sinusoidal and phase-shifted sinusoidal signals. These signals are filtered and transformed into angular and magnitude values, representing the absolute linear position of a 2 mm long magnetic encoder strip pole pair. The position information is read via an I2C interface (figure 2).

 

Figure 2 – TRACKER position sensor includes a magnetic sensor array and integrated encoder on a single chip.

 

Automatic gain control (AGC) adjusts for DC bias in the magnetic field and provides a large dynamic input range of the magnetic field for higher immunity to external magnetic fields than is expected with normal Hall Effect sensors. It also provides an absolute magnitude of the magnetic field intensity, which can be used to detect the end of the magnetic strip and thereby serve as a built-in zero reference, eliminating the need for an external reference. This allows the system to power off or be put into sleep mode, with rapid absolute position reading on power up to minimize the time a user must wait for the system to be ready; for example the time to align a lens in a camera.

 

Because no external light source is required, this technology is smaller than optical sensors and better suited to imaging applications, where light from the optical sensor source could be a problem.

 

Control electronics

The third pillar in a micro mechatronic system is the electronic controller. The NSD-1202 is a dedicated piezo motor driver ASIC capable of driving two SQL Series SQUIGGLE motors from a single 2.8 to 5.5 VDC supply. The two motors can be controlled independently using an I²C interface.

Four half bridge drivers create pairs of phase-shifted square waves at ultrasonic frequencies as required to drive SQL Series SQUIGGLE motors. An on-chip DC-DC step-up converter and external boost circuit generates the high supply voltage (24 to 40 VDC) required by the piezoelectric elements of the SQUIGGLE motor. Total area required for the drive circuit is approximately 6 x 9 mm.

Next-generation motors now in development will operate directly from a 3V battery, eliminating the boost and reducing total drive circuit area to less than 3 x 3 mm.

The main building blocks of the system are a voltage reference, step up converter, I²C interface, registers, selectable feedback, and four (4) half bridge drivers (figure 3). Supplementary blocks such as biasing or power-on reset are not shown.

 

Figure 3: block diagram of the NSD-1202 dual piezo motor driver ASIC, and photo of the total driver circuit in a 6 x 9 mm footprint.

 

 

The step-up converter is built as a hysteretic step-up converter. The half bridge drivers operate rail to rail (VSSP to VDDH). User supplied external components C1, C2, L1 and D1 provide voltage boost and regulation. The output voltage can be programmed via the I²C interface in 0.5V steps between 24V and 40V. This voltage, along with the duty cycle (or pulse width) of the drive signal, determines the speed of the motor.

Registers define the switching frequency of the motor, which can be dynamically adjusted from 140 KHz to 180 KHz for optimum motor performance. Other registers control motor direction and the number of pulses the motor is active (correlating to distance traveled). The XPD input enables a stand-by mode.

 

Reference design

New Scale offers a reference design and developer's kit for the SQUIGGLE motor and TRACKER position sensor (figure 4). In this kit, the motor is mounted in a slide assembly that demonstrates proper loading. The controller board connects to a PC via USB interface. Control software is included that allows the user to control and evaluate the motor using the graphical user interface or a scripting interface. Components can be removed from the kit with basic tools for subsequent integration into a system.

 

Figure 4 – SQUIGGLE motor and TRACKER position sensor reference design and developer's kit.

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