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Piezo motor technology: Questions answered

By MICROMO engineers

What's a Piezo LEGS^® "walking drive" motor?
The Piezo LEGS rotational motor uses Piezo LEGS Elements to create motion. Each Piezo LEGS Elements consists of multiple bimorph actuators co-sintered into a single body with four movable legs made out of ceramic "muscles." In other words, the Piezo LEGS Element has only one part. This simple design makes Piezo LEGS Elements easy to produce cost effectively in large quantities with a high degree of precision.

Piezo LEGS Elements "muscles" create motion with no need for gears or mechanical transmission.

Applications include: optics (high-precision focusing high-precision gimbal), filters and combiners, medtech, factory automation (rotary and linear stages), and Scanning Electron Microscope (SEM) or Transmission Electron Microscope (TEM).

How does the motor work?
By applying voltage to a piezoelectric material, you can make it change its shape. Through clever design of the Piezo LEGS ceramic actuators, each of the motor "legs" can be elongated as well as bent sideways. With appropriate drive signals, you can create synchronized movement of each pair of its four legs, and it will begin to walk just like an animal would -- step by step, and stopping at any instance on a nanometer level. The direct friction coupling between legs and drive rod means Piezo LEGS will operate without the slightest backlash or mechanical play. The direct drive also gives full-force, power-off locking without any power consumption.

Piezo LEGS motion principle.

 

 

Can you count the number of steps and know your position?
Normally not. The Piezo LEGS motor has a friction coupling between internal piezo actuator legs and drive rod or rotor disc. This means that under constant load there is a certain variation between different steps (under constant load normally between 5% to 10%). With varying load and temperature, this figure is even higher. Therefore, to know your position accurately, you need a position sensor or other feedback loop from your motion system.

How slow can a Piezo LEGS motor run?
One of the great advantages with a Piezo LEGS motor is that you can run the motor extremely slow: in a closed-loop system, it is possible to achieve continuous smooth motion at speeds under single µm/s.

How fast can a Piezo LEGS motor run?
The speed of a motor is based on step length and step frequency. The linear LEGS motors will, in general, be limited to run no more than 10 to 15 mm/sec. Speed will depend on external force loads, as well as temperature; in other words, the motor will not run at constant speeds without the proper closed-loop controller.

Will Piezo LEGS give good battery life on a portable device? How energy efficient is the motor?
A Piezo LEGS motor can be very energy efficient compared to conventional motor alternatives. It does not consume any power in hold position, and the motor does not draw peak currents when starting or stopping. Power consumption is not related to inertia, meaning it will consume the same amount of power regardless of external load/torque. In point-to-point applications with low-duty cycle, the motor will give you excellent battery life.

What is the difference between holding force and stall force?
The stall force is the highest force load a running motor can dynamically hold without slipping backward. The holding force is the highest load a powered-down motor can statically hold without slipping backward. Holding force is generally about 10% higher than rated stall force.

What is the difference between full steps, reduced steps, and micro stepping?
The operating principle of the Piezo LEGS motor is very similar to walking -- the frictional grip and release of our feet as we walk on the ground. In a Piezo LEGS motor, the tip of two or more actuator legs will grip the drive rod or rotor disc, move it forward, and release as a second set of legs makes contact. Just as with humans, the motor has the ability to take long or short steps (full steps or reduced steps). A full step is also referred to as waveform-step (wfm-step), because such a step will be completed for one waveform period of the control signals.

It is possible to divide a full step into a large number of smaller increments - so-called microsteps. The smaller the incremental step, the higher the resolution of the motor. Ultimately, resolution is a factor that is determined by the drive electronics, which can give sub-nanometer microsteps (less than a billionth of a meter).

What are waveforms, and why are there different ones?
The actuators (or "legs") of the Piezo LEGS motor need to be moved in a specific sequence and in patterns that will give the optimal type of "walking" motion. The elongation and bending of the legs is controlled by waveform voltage signals - so-called waveforms. One set of legs grips the rod with friction force and moves it forward, while the other set of legs repositions and gets ready for the next step.

The movements require four electrical signals -- four phases of the waveform. Different waveforms are available that are tailored to give different performance. A waveform called Rhomb will give longer step length and higher speeds, while a waveform called Delta is better suited for smoother movement and high-precision positioning. The waveform amplitude is typically 42 Vpp, and its frequency at maximum of a few kHz. The ability to divide the waveform into discrete voltage levels is what makes it possible to take fractions of a full step (so-called microsteps).

What is the recommended operating force range?
For the best microstepping performance and in order to get long life, the motor needs to be properly aligned and should ideally be working at maximum 50% of the rated stall force. If working outside the recommended operating range, close to the rated stall force, the motor will perform with reduced step length, higher wear rate, and more non-linear microstepping.

Will the Piezo LEGS motor be damaged if it is run to a hard stop?
The motor will not be damaged if it is run to a hard stop. It is possible to manually adjust the position of the drive rod without damaging the motor. However, you need to overcome the static frictional holding force of the motor to make the drive rod slide.

Is there any backlash or mechanical play?
No, the direct-friction drive gives motion without any mechanical play or backlash. There are no gears or transmission, so changing direction of the motion will introduce no error. The Piezo LEGS motor is also extremely stiff.

What's the resolution, accuracy, and repeatability?
The motor resolution is the smallest incremental step that can be taken. In the case of using the microstepping drive methodology, the resolution is the same as the smallest possible microstep.

For a linear Piezo LEGS, the microstep resolution can be sub-nanometer (less than a billionth of a meter). To compensate for temperature drift, hysteresis, and other real-world factors, your positioning possibilities will be highly dependent on having a capable system for position control. The control system will need to resolve the position on a high level, will need to be accurate, and allow for high repeatability. Accuracy is how close a measured position is to the actual true position. Repeatability is the ability to come back to the very same position time after time. A system can be repeatable without being accurate.

Often, Piezo LEGS are used together with optical encoders with the ability to resolve single nanometers (1 nm = 0.001 µm) and accuracy better than a few microns over the complete stroke. When designing your system with a Piezo LEGS motor, have in mind that performance is highly dependent on the chosen encoder system.

What encoders can I use together with Piezo LEGS?
It depends on the drive electronics, but generally speaking, most commercially available encoders will work. Our controllers will read quadrature signals, serial SSi encoders, or even analog signal. When you design your motion system, consider what kind of position resolution and accuracy you need, and pick the encoder accordingly. Most times, the motor and controller combination will not limit you since you can easily get sub-micron microsteps (even down to single nanometer resolution). Your system accuracy will only be limited by the specifications of your encoder.

Can the force of the motor be adjusted?
The motor force is set in the factory by adjustment of internal preload springs. It should not be adjusted by the user, since the motor is carefully calibrated to meet the specified stall force and holding force, and to have optimal alignment. The motor will deliver force up to minimum the rated stall force. The motor is self-locking due to its frictional clamping, and it will hold force loads that exceed the stall force by ~10%. It is not possible to change motor output force by changing drive voltages, since output force is actually determined by the internal preload and friction characteristics of the motor.

Must I use drive electronics supplied by MICROMO/PiezoMotor?
No, but you need a drive that can create the correct waveforms for Piezo LEGS. Most OEM customers have designed their own drive electronics. All relevant information to facilitate this will be supplied to customers on request.

In what temperature range can the Piezo LEGS motor be operated?
Piezo LEGS motors are normally specified for operation within the temperature range of -20 deg C to +70 deg C. Step-length performance will be lower for lower temperatures and higher for higher temperatures. For application requirements outside the standard temperature range, please contact MICROMO/PiezoMotor.

What about using motor in vacuum?
All Piezo LEGS motors run in a vacuum environment. For low outgassing and better performance in vacuum, you should select the vacuum-rated versions of the motor. Because of changed friction characteristic when running in vacuum, motors will wear out faster, but they will remain high performing in regards to step resolution and force. Motors can also be baked at temperatures up to 125 deg C.

Are the motors completely non-magnetic?
Piezo LEGS motors don't have the windings of a DC motor and will not be a source of magnetic flux. The standard housing material is stainless steel, so the body of the motor will be ferritic by nature and will disturb an external magnetic field. Completely non-magnetic motors are also available, where the motor housing and other parts are made from non-magnetic alloys. Motors of that kind will be compatible to use even inside an MRI machine without disturbing the image. Non-magnetic motors have a magnetic flux density of less than 1 nT (sensor sensitivity in reference measurements) at a distance of 10 mm from the motor housing.

Will PiezoMotor consider making a custom version motor to fit certain requirements?
Yes, many of our OEM customers purchase custom units based on our patented Piezo LEGS motor technology. We have the engineering capabilities and expertise to supply you with tailored solutions to better fit your application. If you need integrated encoder and guiding, if you are looking for a smaller envelope, or if you could benefit from a lightweight version of our motor, don't hesitate to contact us at MICROMO with your request.

Learn more about Piezo LEGS technology and motor solutions from MICROMO at www.micromo.com/products/piezo-motors/piezo-legs-datasheets.

Published May 2019

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