Precision Motion Control

Closed-loop PI piezo controllers feature:■ High linearity■ Positioning with sub-nanometer accuracy■ Excellent long-term stability■ Noise approx. 1 mV (RMS value)■ Low power consumption■ Notch filter for higher bandwidth■ Output voltages adapted to various piezo

actuators and piezo drives■ Analog interfaces for fast direct command-OEM piezo controller card. The piezo control voltages
are generated on-board, operation only requires a stabilized voltage between 12 and 24 V

Flexible Controllers to Match the Mechanics
PI offers the world’s largest portfolio of precision motion technologies for positioning in theaccuracy range from one micrometer down tobelow one nanometer. Fast settling or extremely smooth low speed motion, high positional
stability, high resolution and high dynamics –the requirements placed on piezo mechanisms
vary greatly and need drivers and controllers
with a high degree of flexibility. PI provides abroad spectrum of piezo electronics from versatile general purpose controllers to highly
specialized devices. Units come in different
levels of integration from customized OEMboards and a plug & play bench-top devices tomodular controllers to scalable to almost any
number of axes.

ing in real time
■ Short-circuit strength

Piezo mechanisms directly respond to thesmallest change in the drive voltage with achange in displacement. Response times of afew microseconds are possible, depending on
the mechanical design and the performance ofthe piezo controller.
In static operation, i.e. when a certain positionis held, the stability of the power supply is alsodecisive because piezo actuators react even tothe minutest change in voltage with a motion.
Therefore, noise or drifting must be avoided asmuch as possible.

This high-performance piezo controller delivers
peak power of up to 6 kW. A digital interface
module offers extended functionalities, such asdata reco
recorder and function generator

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Overflow

Slew Rate

Servo OFFON+

The performance of a piezo mechanism not
only depends on the mechanical design but
also largely on the capabilities of its controller. PI's low-noise, drift-free piezo amplifiers
ensure optimum stability and resolution. Highbandwidth allows for rapid response times andhigh scanning frequencies.
Closed-loop position control compares thetarget position with the information provided
by the position feedback sensor (actual value)
and automatically compensates for nonlinearity, hysteresis and drift.
The servo-control part of most analog piezo
controllers manufactured by PI is identical: A
proportional integral control loop specifically
optimized for piezo operation. One or moreadjustable notch filters considerably improveusable bandwidth and dynamics because
resonances are suppressed before they canaffect the system stability. In digital controllers,
optimized control algorithms further minimize
settling times and increase bandwidth and stability. High-end closed-loop piezo positioning
systems can achieve a repeatability down to
the sub-nanometer range and bandwidths to10 kHz.

Proportional Integral
Term
Term

PZT
Out

PZT
Monitor

DCOffset

Optimized Control Design Improves
System Properties

PZT
Amplifier

NotchFilter

PZT
Sensor–SensorMonitorSensorGain

SensorRange
Bandwidth Adjustment

ZeroAdjust

Block diagram of a typical closed-loop piezo controller

Resolution in Closed-Loop and Open-LoopControl
Closed-loop piezo systems guarantee higher linearity and repeatability than open-loopsystems. The position resolution of piezo
actuators and flexure-based piezo mechanisms
is not limited by friction but influenced by electrical noise at the sub-nanometer level.
Because of the additional sensor and the servo
circuit, the noise is slightly higher in closedloop operation compared to open-loop control,
where only the piezo amplifier contributes toelectrical noise. If high-quality components areused, sub-nanometer positional noise levels
are possible in closed-loop operation. Capacitive position sensors achieve the best resolution, linearity and stability.

Advantages and Disadvantages of Position Control
Most precision positioning applications greatly benefit from closed-loop control.
When maximum bandwidth is crucial, open-loop may be worth a consideration:
A closed-loop controller always operates in the linear range of voltages and currents. Since
the peak current is limited in time and is therefore nonlinear, it cannot be used for a stable selection of control parameters. As a result, position control limits the bandwidth anddoes not allow for pulse-mode operation. In switching applications, it may not be possible
to attain the necessary positional stability and linearity with position control. Open-loop
operation may be a better choice here with linearization obtained by means of chargecontrolled amplifiers or by numerical correction methods.
In closed-loop operation, the maximum safe operating frequency is also limited by the phaseand amplitude response of the system. Rule of thumb: The higher the resonant frequency ofthe mechanical system, the higher the control bandwidth can be set. The sensor bandwidthand performance of the servo (digital or analog, filter and controller type, bandwidth) also
limit the operating bandwidth of the positioning system.

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InStandardized Measurements Logs:
The Good Feeling When Your Expectations Are Met
Nanopositioning systems are an essential but
costly component in applications. PI therefore individually tests and optimizes the static and dynamic parameters of every system.
The measurement log is delivered with thesystem. The customer can thus retrace theperformance of the system at delivery andwhich system components belong together at
any time.
PI continually invests in improving the testing
methods and testing equipment in order to beable to supply systems of even higher quality. Closed-loop nanopositioning systems aretested exclusively with high-quality calibrated interferometers. The test laboratories are
insulated against seismic, electromagnetic andthermal effects, temperature stability is better
than 0.25°C in 24 hours. PI thus sets the standard in the testing and specification of nanopositioning products.

Controller Tuning
To optimize the system performance, information about the application is required, suchas the desired operating frequency, step-andsettle, the size and weight of the payload, orthe spring constant of a preload in relation towhich the piezo actuator is operated.

Highly dynamic, closed-loop nanopositioning system: A piezo scanner achieves
the full travel range of 100 μm in only a few milliseconds

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TEC68 Precision Motion Control 10/2017 Subject to change without notice. © Physik Instrumente (PI) GmbH & Co. KG 2017

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