Piezoelectric motors are increasingly popular for linear motion, but the design of the accompanying drivers is very different from those used by traditional electromagnetic motors. Employing the piezoelectric effect, a voltage across a specific type of crystal causes it to elongate by a small amount (typically 0.1%). The motors themselves are small (usually only 10mm long), providing movement that is measured in microns – thus making them of value in many high precision applications, from infusion pumps, microscope stages and optical positioning to instrumentation and inkjet nozzles. The technology is fast, operating in the multi-kHz range, and the motors have no bearings that need lubricating which might potentially cause contamination. The non-metallic nature of the crystal can also be an advantage in situations where strong magnetic fields operate.
These motors can be arranged in a number of configurations. For example, a pair of motors can be used as a clamp or as a piston, as with one end clamped, applying a voltage creates a piston motion. An array of multiple piezoelectric motors can even be arranged in a circle to provide rotary motion from a linear effect, and a series of motors may form the basis of a fluidic pump. Piezo-based motion can also be incorporated into inexpensive, lower-quality piezo devices utilized for audio sounders, alarms, and even small loudspeakers,
A complete piezoelectric motor assembly has three parts: The electronic drive, the electrical-mechanical transducer or motor, and the output linkage. While electromagnetic motors need high currents for the inductive load of the coils, the piezoelectric motor needs high voltages. The driver has to provide a relatively high voltage to create the electric field in a capacitive load and modulate this voltage to create the correct amount of elongation.
This voltage depends on the size of the piezoelectric crystal, the amount of elongation and the rate of motion. At the low end, the voltages may be between 20V and 30V with a current of 20mA to 30mA, but most higher-performance piezo units need at least 10V and a current that can vary from 10mA to several hundred milliamps. Some larger piezoelectric motors reach 1kV at several amps.
The piezo driver also has to remain stable despite the capacitive nature of the load, which can be as high as 1nF. As the piezo device is a floating, differential device, most applications require a differential, bipolar driver output. This leads to a number of key considerations around the physical isolation and protection from the voltages as well as the ISO and EN regulations that define the minimum creepage and clearance dimensions to protect the users of the end equipment. Any driver circuit has to adhere to these layout and placement conditions carefully as well as achieving the desired performance – especially as most of the amplifier chips are low voltage and the high voltage chips are designed to drive the IGBTs and MOSFETs in traditional motors.
A basic high voltage driver relies on a transistor with adequate voltage rating and is used in simple piezo vibrator designs (such as speakers). This will not deliver the high degrees of precision, controllability, and stability that a piezoelectric motor needs though. It will also be deficient in terms of the protection for failure modes and cannot provide a bipolar output. Several chips have now been developed for piezo drives that offer additional features – such as control of high voltage waveform slew alongside thermal, overload and short-circuit protection mechanisms.
The ADA4700-1 developed by Analog Devices has a wide operating voltage range of ±5 to ±50V to drive a piezoelectric motor. The 8-lead SOIC is designed to provide a high slew rate output of 20V/µs into capacitive loads while remaining stable. It has been tested at various voltages, loads, temperatures, distortion levels, and overshoot.
Overshoot is a critical problem for a piezoelectric motor. Ideally, the voltage should drive the crystal to a specified extension immediately, and while the ADA4700-1 has minimal overshoot when driving capacitive loads, extra compensation can be used for more accurate positioning.
By employing a small snubber network for capacitive loads up to 1nF with a combination of a 150Ohm resistor and 10nF capacitor. For larger loads up to 10nF and higher gains reaching tenfold, the resistor is decreased to 22Ohm while the capacitor increases to 100nF. The drive current level can be boosted by adding an external pair of complementary (PNP/NPN) transistors.
For driving highly capacitive loads, a simple external RC snubber circuit is added to the ADA4700-1. [Source: Analog Devices]
Texas Instruments has also developed a driver IC for piezoelectric motors. The DRV8662 integrates a boost converter with a 105V boost switch from a 3.0V to 5.5V supply. The boost voltage is set simply utilizing two external resistors, while the device gain can be set to one of four values using two I/O lines.
At 300Hz, the device can drive 100nF at 200VPP , 150nF at 150 VPP , 330nF at 100VPP and 680nF at 50VPP . The 24-lead, 4mm×4mm QFN package is simple to use with the two resistors and the I/O line setting shown in Figure 2.
The DRV8662 requires just a few external components to set basic operating parameters. [Source: Texas Instruments]
Reference design options that implement the driver technology on a board can help developers implement a piezoelectric motor quickly and easily. Microchip Technology, for example, has developed a reference design featuring all the hardware for the high voltage drivers needed by fluid micropumps. This reference design provides the flowcharts, code, schematic and layout alongside the low voltage power subsystem for the charger, microcontroller, and other components. This helps designers meet the wider regulatory requirements of creepage clearance.
The reference design has a two-stage high voltage section with the HV9150 DC/DC boost converter and HV913 drive amplifier. This provides 250VPP at a maximum frequency of 300Hz from a rechargeable battery to actuate the piezoelectric micropump. The driver has a high voltage, unipolar, push-pull output and a series of pulses are generated from the microcontroller to drive the piezoelectric element as a pump.
A two-chip approach is used by Microchip to generate 250V for a piezoelectric fluid micropump. [Source: Microchip Technology]
Piezoelectric motors can be employed for precision linear positioning in a broad range of micro-motion applications. The high voltage drive and capacitive loading means the design of drivers must be approached in a different way than would be appropriate with conventional electromagnetic motors. While standard amplifiers can be used with appropriate external voltage-boost transistors in low-end designs, most piezoelectric motor drivers have higher safety and control requirements, from thermal and other shutdown safety features to short-circuit protection. Consequently, several chip vendors provide the drivers for these applications, with simplified configuration and I/O designs through to full reference designs that include the microcontroller and software code.