A piezo stage moves the carriage using the expansion and contraction of piezo ceramics. This article answers one question: how does piezo achieve nanometer positioning?
The answer: the piezoelectric effect converts voltage directly into displacement, and a flexure guide has no friction, no backlash, and no wear, so resolution reaches the nanometer range. The limits are small travel, creep, and hysteresis.
This article covers the two piezo actuator types (stack and piezo motor), the flexure guide, and how to read the numbers (travel, self-holding when powered off, creep and hysteresis).
Figure 1 | Top: the stack, fixed anchor, flexure blades and carriage are mechanically connected. Bottom: a conceptual walking drive; the separate runner/carriage guide is omitted. Motion is exaggerated and not to scale. Click for full size.
1. Piezoelectric Effect: Stack vs Piezo Motor
Piezo materials (PZT and similar ceramics) expand and contract under voltage, called the piezoelectric effect. The expansion changes with voltage; the open-loop relation also has hysteresis and creep, with displacement in the micrometer range (for a stack a few centimeters long). The applied voltage is typically a few volts to a few hundred volts, generated by a piezo driver; higher voltage gives larger deformation, but too high a voltage damages the ceramic, so the driver’s voltage range must match the stack’s rated voltage.
There are two ways to use piezo actuation:
| Piezo stack | Stepping piezo motor | |
|---|---|---|
| Principle | Multi-layer ceramic expands directly under voltage | Periodic deformation converted to continuous motion by a stepping mechanism |
| Travel | Small (limited by layers × deformation) | Large (set by drive-rod length) |
| Self-holding when powered off | No inherent mechanical lock; holding voltage differs from power-off | Walking drives can self-lock with preload, within the rated holding force |
| Typical use | Nanometer fine adjustment, fast positioning | Long-travel nanometer positioning |
Stack: multi-layer ceramics are mechanically stacked, and each layer’s deformation accumulates. Under voltage the whole stack expands or contracts directly, moving the carriage via a flexure guide. Travel is limited by the stack length and material deformation, typically in the micrometer to hundred-micrometer range[1]. A stack’s displacement can be amplified with a mechanical lever: a flexure lever multiplies the ceramic’s few-micrometer expansion by several to several tens of times, trading resolution and load capacity for larger travel.
Stepping piezo motor: Figure 1 uses a walking drive as an example. Preloaded actuator groups alternately contact the runner, feed it and lift to reset. Repeating these cycles accumulates travel beyond a single ceramic stroke. Total travel depends on usable runner length and the guide structure; the holding force depends on the particular motor and preload[3].
2. Flexure Guide
The short-travel stack stage in Figure 1 uses elastic flexure blades to guide its carriage. The blades connect the carriage to the fixed frame and allow motion mainly along the intended axis. This describes the stack/flexure example; long-travel piezo-motor stages can use separate bearings or rails, so flexure guidance is not universal to all piezo stages.
The flexure guide’s advantage is no friction, no backlash, no wear. This is the key to a piezo stage’s nanometer-class resolution — there is no friction or backlash limiting the resolution. The limits are that travel is constrained by the elastic deformation range, and load capacity is constrained by the flexure’s strength.
Flexure guides come in two basic configurations: parallel flexure (stiff, high load, small travel) and serial flexure (large travel, low stiffness). Selection trades off stiffness, travel, and load.
3. How to Read the Numbers: Travel, Self-Holding, Creep and Hysteresis
Travel: a stack has small travel (limited by layers × deformation × amplification ratio); a stepping piezo motor has large travel (set by the drive rod). Check the required travel first when selecting.
Self-holding when powered off: a conventional stack is a capacitive actuator. Holding a voltage requires little static power, but this is not a mechanical lock; removing voltage or discharging the stack changes its extension, so its commanded extended position is not guaranteed to remain. Preloaded walking drives can hold a runner mechanically without drive power, within the particular model’s rated holding force[2][3].
Creep and hysteresis: piezo materials have creep (a slow drift after voltage is applied) and hysteresis (voltage and displacement are not a single-valued function; the rising and falling voltage paths differ). These two effects affect positioning accuracy and repeatability; high-precision applications need closed-loop compensation (a position sensor reads the actual position). Closed-loop compensation reads the carriage’s actual position with an optical scale or capacitive sensor, and the controller corrects the creep and hysteresis error — like a linear-motor stage, it reads the carriage itself, not the driver’s command.
Figure 2 | Hysteresis gives different positions at the same voltage; creep changes position while voltage is held. Feedback compares requested position r with measured carriage position x and adjusts the drive. Curves are qualitative, not measured data[4]. Click for full size.
4. Applicable Range and Limits
Piezo stages suit nanometer-class, small-travel, high-speed, friction-free applications. Their resolution reaches the nanometer range, but travel is small. For longer travel, use a stepping piezo motor or a linear motor (see the previous article in this series).
Typical applications: AFM probe positioning, optical alignment (aligning a laser to a fiber), and fine adjustment of fast scanners. These need nanometer resolution, small travel, and high speed — exactly piezo’s strengths. A ceramic actuator itself can respond quickly, but a complete stage’s bandwidth and settling time also depend on structural resonances, load, driver and controller. A component response time is not the settling time of the assembled stage.
Common Misconceptions
“Piezo is always more accurate”: Piezo has high resolution (nanometer class), but creep and hysteresis affect accuracy; without closed-loop compensation, accuracy is not necessarily better than a linear-motor stage.
“A stack and a piezo motor are the same”: They are not. A stack is direct expansion with small travel; a piezo motor is continuous motion with large travel.
“Piezo needs no power”: a stack still requires a controlled voltage for its commanded extension. Low static power does not mean power-off locking. A preloaded, self-locking walking motor may hold at rest without drive power, but movement and position sensing/control require power.
Self-Check for the Reader
Can you state the principle difference between a piezo stack and a stepping piezo motor?
Why can a flexure guide reach nanometer-class resolution?
What are creep and hysteresis, and how do they affect positioning?
Back to the opening question: how does piezo achieve nanometer positioning? The answer is that the piezoelectric effect converts voltage directly into displacement, and a flexure guide has no friction, no backlash, and no wear, so resolution reaches the nanometer range. Applicable conditions: the principles here come from PI technical documentation, and travel and resolution are qualitative descriptions. Unresolved limits: creep and hysteresis need closed-loop compensation; travel is small, so long travel needs a stepping piezo motor or a linear motor.