Here are some guidelines and information on the piezoelectric actuators from Physik Instrumente (PI). Users should also read the following manuals:

  • E-802 Servo-Controller Submodule (PZ 150E User Manual),
  • E-509 Position Servo-Control Module (specifically section 4 E-509 Calibration Routines),
  • E-801 Sensor Submodule (PZ 117E User Manual, specifically the section on E801B1008)

General terminology#

  1. The terms “piezo,” “actuator,” “piezoelectric actuator,” and “piezo stack,” all refer to a stack of piezoelectric elements encased in a metal tube. The term “actuator” is preferred by the PI engineers.
  2. All of the piezoelectric actuators we have are “preloaded” meaning there is a static load applied such that they are able to be bidirectionally moved.
  3. The “probe” is anything that we move with an actuator.
  4. The piezoelectric actuators are either open or closed loop. All of the open loop piezos have just one thin grey cable that is for charging the stack to drive motion. The closed loop piezos have two cables, where the second is part of a feedback system.
  5. All piezos, both closed and open loop require an amplifier to convert our command voltages (e.g. generated in matlab) to a high voltage / current signal that will charge the piezoelectric stack and thereby drive the actuator. The amplifiers are all identical.
  6. Importantly, each closed loop piezo is paired with a “controller”. These controllers contain servos that allow for both precise positional control and a readout of current position. Controllers are factory calibrated (in a way that we cannot recreate in lab) to ensure linearity in control and accurate positional information. See below for more guidelines / rules about these.
  7. PI sells both large and small chassis that house several controllers and amplifiers. These distribute power to the amplifier and controller, and have some logic for additional, optional boards. Some amplifiers and controllers have multiple channels. It is not always straightforward which amplifier slot links to which controller channel.

Rules for using PI piezos#

  1. When adding or removing a probe (small, front adapter): apply even pressure and use a small, 5mm on the flat to prevent movement. Failure to do so may cause a crack in the ceramic element and require repair and re-calibration. Alternatively, the stack may become misaligned and the feedback signal will be misleading.
  2. Be mindful when mounting actuators. Do so using either the rear threads or with an even clamp along the body length; do not over tighten and do not bend the cable exit. For example, do not use a mount that will apply focal pressure and cause damage to the housing. This may compromise the calibration and introduce artifacts not seen in the sensor signal.
  3. Never swap a controller and actuator, keep them paired. They are paired when factory calibrated. For multi-channel servos, the correct channel is indicated. Swapping can result in feedback signals off by large margins, or ringing in step responses that may not be visible in the feedback signal. In the worst case it will irreversibly damage a piezo and require factory repair (e.g. if over-voltages are constantly applied).
  4. Always use extension cables with the closed-loop actuators. Do not EVER cut the cables. Extension cables are only strictly required for the short travel actuator, it hasbeen calibrated with the cable by our request. Cutting cables results in a need to both re-calibrate and re-connectorize.
  5. Be careful when calibrating actuator step responses. Do not EVER change the potentiometers associated with static calibration when changing the step response (see below). This will result in a need to factory re-calibrate.

Delivering mechanical stimuli#

  1. Displacement (voltage charging) is bandlimited by the capacitive load of the stack and the current delivery capabilities of the amplifier. Charts by load are in PI documentation. This is responsible for hard limits on different frequency amplities.
  2. When changing the step response, familiarize yourself with all the potentiometers, and take measures prevent yourself from touching those that influence the static calibration in particular. An excellent practice is placing a piece of clean room tape over top of all the potentiometers except those used in the step response.
  3. Servo feedback is bandlimited, and so linearity is only guaranteed below this point. Check the model for specifications.
  4. Resonance listed in PI documentation is only axial resonance for an unloaded piezo. The cantilever resonance on an attached probe will be much lower and can only be measured with a separate sensor (e.g. a laser Doppler vibrometer positioned at ~90 degrees to the displacement). Also, if adding a load this will modestly reduce the axial resonance.
  5. Splitting an analog signal going to both the piezo amplifier and a DAQ board will typically result in “sampling noise” that exists in the piezo displacement. This is sometimes hard to detect but can be bypassed by sending a separate copy on a different analog output channel.
  6. There is a small voltage offset on all analog output channels on DAQ boards. Not accounting for this will result in “jumps” typically at physiological scale at the beginning and end of stimuli. An initial calibration routine using the servo feedback (or direct measurement of the channel) can null out this effect.

Good habits#

  1. To avoid temperature fluctuations, it is OK to leave amplifiers on, however it is not required.
  2. Before use, “exercise” the piezo with the servo off by turning the DC-OFFSET knob all the way in each direction a few times.
  3. Perform a “zero-point adjustment” (See E-509 Position Servo-Control Module User Manual PZ 77E) every week or so, or write a matlab routine that helps check if it is required.
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