Piezo Motion Control Tutorial

Piezo drives are vital in today's ultra-precision motion control systems. They provide the best combination of stability, accuracy, responsiveness and resolution.

The piezoelectric effect – the conversion of electrical energy to mechanical energy and vice versa - is exhibited by natural materials, such as quartz, etc.  However, man-made polycrystalline ferroelectric ceramic materials, such as Lead Zirconate Titanate (PZT), have far advanced material properties and achieve more gain. Ferroelectric ceramics need to be poled to become piezoelectric. Charge separation between the positive and negative ions is the reason for electric dipole behavior of the piezoelectric effect. Piezoelectric ceramics come in many variations optimized for actuator or sensor applications. 

Origin of Piezo

The term “piezo” is derived from the Greek word for pressure. In 1880, Jacques and Pierre Curie discovered that an electric potential could be generated by applying pressure to quartz crystals; they named this phenomenon the “piezo effect”. Later, they ascertained that when exposed to an electric potential, piezoelectric materials change shape. This they named the “inverse piezo effect”. 

More on the fundamentals of the piezo effect, equations and background information 

Piezoelectric materials are used to convert electrical energy to mechanical energy, and vice-versa. The precise motion that results when an electric potential is applied to a piezoelectric material is of primordial importance for nanopositioning. Actuators using the piezo effect have been commercially available for 35 years and in that time have transformed the world of precision positioning and motion control. 

Piezo ceramics needs to be polarized to show the piezo effect. 

Open and Closed Loop Operation

In most applications, piezo positioners are used with a position feedback sensor and closed-loop control. 

In contrast to many other types of drive systems, some piezo actuators can be operated without servo-control for a variety of applications. With suitable controllers, closed-loop operation enables reproducibilities in the sub-nanometer range as well as elimination of the piezo hysteresis. Learn more on digital vs. analog piezo controllers for closed-loop operation 

PZT Ceramics Manufacturing Process

PI develops and manufactures its own piezo ceramic materials at its PI Ceramic factory. Multilayer and bulk piezo actuators use different manufacturing processes. Both require a number of highly specialized machines. More information on piezo manufacturing technologies at PI Ceramic. 

For a quick overview, click here 

Piezo motion devices are often divided into two groups: actuators and motors. 

Traditional piezo actuators expand analogous to the applied drive voltage. They provide short travel ranges typically under 1mm. 

Piezoelectric motors require more complex drive electronics and can provide long travel ranges (up to 100’s of mm). They typically consist of one or more of piezo elements driving a runner. 

Piezo Motors are based on different drive principles optimized for high force, high speed, minimized dimensions and cost.  All piezo motors are intrinsically non-magnetic, vacuum compatible and self-locking at rest.

Piezo Inertia motors (stick-slip) are most compact and simple with relatively low forces of 1N to 10N and speed to 10 mm/sec.

Ultrasonic resonant motors provide a wide dynamic range from microns/second and below to 100’s of mm/sec.

PiezoWalk® linear motors can achieve the highest forces along with picometer resolution, however velocity is limited from 1mm/sec for high force devices and up to 15mm/second for the new V-8 PicmaWalk drives. 

More on piezo motors

Piezo Actuators and Flexure Nano-Mechanisms

Actuators are the most basic form of piezo drive elements and provide travel ranges from a few microns to a few 100 microns (1-2 mm in the case of bimorph or lever amplified actuators). 

Actuators are divided into these basic groups:

• Piezo Stacks
  • Highest stiffness & resonant frequency, travel typically 10 to 300 microns. Preloaded for push/pull. 
• Programmable Piezo Shims
  • Programmable shims are similar to piezo stack actuators but do not require a continuous voltage source to hold a position once programmed. 
• Piezo Benders & Bimorphs
  • Low profile, long travel to 2 mm, relatively low stiffness, and resonant frequency. 
• Piezo Tubes
  • Short travel (10 micron range) multi-axis motion feasible, often used as scanners in AFM. 
• Piezo Shear Plates / Actuators
  • Lateral motion, travel 10-20 microns, high stiffness, fast response.

Lifetime / Ceramic Encapsulation of Cofired Piezo Stacks

PI's piezo flexure nanopositioning systems employ the patented, award-winning PICMA® piezo actuators, the only polymer-free actuators with co-fired ceramic encapsulation. The monolithic PICMA^® actuators are space qualified and were submitted to 100 billion cycles of life testing at NASA's JPL. Read about the testing 

The PICMA piezo technology was specifically developed by PI’s piezoceramic division to provide higher performance and lifetime in nanopositioning applications. Cofired multilayer piezo actuators are similar to ceramic capacitors and are not affected by wear and tear. PI nanopositioning systems are designed to be driven at lower voltages than most other piezo systems (100 V vs. 150 V). The research literature, PI’s own test data, and 30+ years of experience all confirm that lower average electric fields lead to longer lifetime.

Tech Article & App: Piezo Actuators on the Mars Rover

In addition, PI’s monolithic ceramic-encapsulated design provides better humidity protection than conventional polymer-film insulation. Diffusion of water molecules into the insulation layer is greatly reduced by the use of co-fired, outer ceramic encapsulation. Humidity is the main influence on the longterm reliability in low-dynamics or quasi-static operation modes, where the piezo actuator is supplied with a DC voltage to maintain a position for a long time. 

Force Generation: High Speed Valves, Dispensers, Pumps

One of the advantages of piezo ceramics is their high force generation and extremely fast response capabilities. This is why piezo actuators (mostly stacks and flexure amplified stacks) are often used to replace solenoids or other classical actuators in fast valve applications, micro-dispensers or pumps. It is helpful to understand how displacement and force correlate to get the most work out of an actuator. Internal and external spring constants should be similar. FEA simulations help optimize dynamics. 

Flexure Guided Actuators and Positioners, Mechanical Amplification

Guided piezo actuators (1 to 6 axes) are complex nanopositioners with integrated piezo drives and solid-state, friction-free linkages (flexures). They are used when requirements like the following need be met: 
 

• Extremely straight and flat motion, or multi-axis motion with accuracy requirements in the sub-nanometer or sub-microradian range


• Isolation of the actuator from external forces and torques, protection from humidity and foreign particles

With the use of flexure guides and mechanical levers, the motion of a piezo stack can be multiplied (up to 100’s of microns or even a few mm) and guided at the same time. Flexure motion is based on the elastic deformation (flexing) of a solid material. Friction and stiction are entirely eliminated, and flexures exhibit high stiffness, load capacity, and resistance to shock and vibration. Flexures are maintenance free and not subject to wear. They are vacuum compatible, operate over a wide temperature range, and require neither lubricants nor compressed air for operation. 

Left: Wire EDM-cut flexures in a piezo nanopositioning stages provide friction-free motion and extreme guiding precision. 

Center: Motion performance of a P-752.11C piezo flexure stage driven with a square wave control signalshows crisp response and true sub-nm positional stability, incremental motion and bidirectional repeatability. 

Right: Sub-µrad bidirectional trajectory repeatability means processes may be performed bidirectionally for twice the productivity.

Parallel Metrology with Parallel Kinematics Provides Higher Multi-Axis Precision and Dynamics

For the ultimate in dynamics straightness/flatness/orthogonality and for coordinate transformation situations (such as rotating about an arbitrary point in space), parallel kinematics are required. Parallel kinematics systems, if designed well, are superior to serial kinematics systems. To reap the benefits of parallel kinematics, you must have high-bandwidth, direct drivetrain-output metrology, so the workpiece can be simultaneously observed and controlled in multiple degrees of freedom. 

Since it takes more experience and more advanced controllers to produce a good parallel kinematics precision positioning system, some designers try to avoid that challenge dismissing the technology as unpredictable, uncontrollable. Everyone who has seen a modern hexapod 6-axis motion system compared to a classical stack of translation and rotation stages will immediately understand the benefits of parallel kinematics. 

Coupled and Uncoupled Motion, Static and Dynamic

Serial kinematics are sometimes referred to as uncoupled motion systems vs. coupled motion systems for parallel kinematics. When it comes the nanometer realm, uncoupled motion does not exist. By nesting or stacking a second and/or third axis onto a translation stage, there will be an influence on the first axis. Any load attached will have a further influence on all axes, even when at rest. And in positioning, things are not always at rest; actually most nanopositioning / scanning applications require very high dynamics. The seemingly uncoupled multiaxis-system then provides coupled (unwanted) motion in many degrees of freedom that cannot be detected by its internal serial metrology sensors, and hence goes partially uncontrolled. These errors may not be critical in many applications, but can be detrimental in some others. 

Direct Parallel Metrology: All Motion Inside the Servo Loop

Multi-Axis Measurements Relative to a Fixed Reference

Parallel kinematics facilitates implementation of Direct Parallel Metrology — measurement of all controlled degrees of freedom relative to ground. This is a more difficult design to build but it leads to clear performance advantages.

The parallel-kinematics / parallel metrology system “sees" motion in all controlled degrees of freedom and will respond to it. This means that all motion is inside the servo-loop, no matter which actuator (or unwanted external force / crosstalk) may have caused it, resulting in superior multi-axis precision, repeatability, and flatness. Direct parallel metrology also allows stiffer servo settings for faster response. Off-axis disturbances—external or internal, such as induced vibration caused by a fast step of one axis—can be damped by the servo.

Nano Metrology Blog

Position Feedback Sensors for Closed-Loop Control

High accuracy position feedback is essential in a good nanopositioning system, and direct motion metrology is the preferred choice. Direct metrology measures motion where it matters most to the application. Examples of high-resolution, direct metrology sensors are capacitive sensors, laser interferometers, and non-contact optical, incremental encoders. PI employs these and other sensors depending on the application requirements.

Capacitive Sensors

For travel ranges of less than 1mm, capacitive sensors have emerged as the default choice.  They are compact, high-bandwidth and absolute measuring devices providing sub-nanometer resolution.  For less demanding applications, strain gauge sensors (piezoresistive sensors) are a good alternative. 

Piezoresistive (Semiconductor) Strain Gauge Sensors

Piezoresistive strain gauge sensors (PRS) are low-cost, high-resolution, but temperature-sensitive devices that are easily integrated in positioning devices. Like most strain gauge sensors, they are glued to the flexure structure or piezo stack, but the extra layer of epoxy between the sensor and structure makes it a challenge to get sufficient long-term stability. PRS do not measure distance directly, but infer position of the moving platform from the nanoscale warping of the structure. Due to the indirect measure of position, inaccuracies can occur and orthogonality errors are unobservable. Calibration to an interferometer allows them to achieve adequate accuracies for classical microscopy applications. Since their output signal is a low voltage DC current, the derived position can be more susceptible to noise pickup and drift.

Choice of Control Interface: Digital or Analog or Both?

Analog interfacing provides high bandwidth and remains a popular way of commanding piezoelectric motion systems. It is usually the choice when the control signal in the application is provided in analog form. A key advantage of analog interfacing is its intrinsic deterministic (real-time) behavior, contrasted to the difficulty of accurately timing high-bandwidth communications on present-day multitasking PCs. 

However, when analog control signals are not available, or when a significant distance between the control signal source and the nanopositioning controller would affect signal quality, digital interfacing (which must not be confused with digital control) is the preferred choice. 

Digital signals can be transferred through copper wires, or for complete EMI immunity, through optical fibers. 

Several types of digital interfaces are typically used in piezo-nanopositioning applications: TCP/IP, parallel-port, USB, SPI, RS-232, Fiber Link, GPIB, and, with some digital controllers, direct DSP links. For dynamic, high-precision applications, the exact timing of an interface can be significantly more important than the data transfer rate.

Interface Speed: Bandwidth vs. Timing and Determinacy. When does it matter?

Piezo-driven stages can respond to a motion command on a millisecond or microsecond time scale. 

That is why synchronization of motion commands and data acquisition have a high impact on the quality of many applications, like imaging or micromachining. Often it is not about interfacing speed, but determinacy; USB and TCP/IP are indeterminate, even though they are good at transmitting huge amounts of data. USB was designed to transfer large blocks of data at high speeds, but exact timing was not a big concern. While insignificant in less responsive positioning systems, this kind of nondeterministic behavior may not be tolerable in high-speed tracking or scanning applications. Each motion command—comprising just a few bytes—must be transferred instantaneously and without latency. A lowerbandwidth bus with higher timing accuracy may perform better in many applications. There are several factors that affect the response of a digital interface: 
 

• the timing accuracy of the operating system on the controlling computer
• the bus timing protocol
• the bandwidth of the bus
• the time it takes the digital interface (in the piezo controller) to process each command

Parallel-port interfaces do not require command parsing and offer the best combination of throughput and timing accuracy. 

In addition to the interface properties, the bandwidth of the nanopositioning system (mechanics and servo) matters. A slow system (e.g., 100 Hz resonant frequency) will not benefit from a responsive interface as much as a high-speed mechanism. 

Advanced Servo Algorithms: APC and Digital Dynamic Linearization (DDL)

Conventional piezo controllers cannot completely eliminate phaseshift and tracking errors in applications with rapid, periodic motion. This is due in part to the non-linear nature of the piezoelectric material, the finite control bandwidth, and the inherent limitations of P-I-D (proportional integral derivative) servo-control, which cannot react before a position error is detected. 

The DDL firmware upgrade option, available with most digital piezo controllers, solves this problem. This technology, developed by PI, reduces the error between the current and desired position to imperceptible values. The dynamic linearity and effectively usable bandwidth are thus improved by up to three orders of magnitude (1000-fold). DDL is of benefit to single- and multi-axis applications where motion follows a given trajectory repeatedly.

Video: Hybrid Positioning Technology: Two drives, one position sensor

Piezo actuation can also be used to enhance the performance of classical motors and drive systems, forming a coarse/fine positioning system. Traditional coarse/fine mechanisms were run with independent control loops, hindering overall system repeatability and accuracy. The advent of higher-performance motion-control chips and linear encoders with nanometer-scale resolution has allowed construction of hybrid mechanisms in which one feedback sensor provides information to both the coarse- and fine-control loops. 

The construction of such a hybrid servo is significantly more complex (analogous to controlling an internal combustion engine and electric motor in a hybrid vehicle at the same time) and the result is the combination of piezo-class dynamics, repeatability, and high stiffness with many centimeters of travel. Requirements for motion devices like these come from the astronomy community, where the next generation of multimirror optical telescopes need long travel, extremely high forces, and motion uniformity with tracking accuracy of less than 2 nm and short-term interpolation errors of less than 0.5 nm. 

Piezo Motors

Piezo motors are used where long travel ranges in the millimeter to 100’s of mm range are required. Piezo motors can be divided into three main categories: 

Piezo Walking Drives

Inertia Piezo Motors (Stick-Slip Motors)

Piezo motors use different types of controllers, and typically, incremental feedback sensors. Force and displacement are not interdependent. 

Piezo Stepping Motors (PiezoWalk), such as PI NEXLINE^® and NEXACT^® Drive

Features

• Virtually unlimited motion range
• Based on accumulation of small highly controllable steps
• Picometer resolution by means of direct piezo actuation (linear mode, dither mode)
• High forces to 800 N
• Speed 1 to 10 mm/s
• Fast response (< 1 m/sec feasible)
• Very high stiffness
• Drift-free position-hold when powered down

Piezo Inertial Motors (Stick-Slip Motors) — very compact, low cost

Features

• Most economical and compact design
• Forces 3-10 N
• Speed 5-10 mm/s
• Open and closed loop