Fiber Positioner Circuits (A2089)

© 2019, Kevan Hashemi, Brandeis University HEP

Contents

Description
Static Piezo Driver
Piezo Amplifier
Piezo Driver

Description

[03-JUL-19] We use assembly number A2089 for a family of medium-voltage power supplies and amplifiers that drive piezo-electric (PZ) actuators. In particular, we are focused on tube actuators that bend when we apply voltages to four longitudinal quadtrant electrodes. Our plan is to mount a rigid support pipe in the end of the PZ tube, run an optical fiber through the tube and the pipe, and so use the PZ tube to move the fiber tip around. With such fiber positioners arranged in the focal plane of a telescope, we can hope to align a large number of fibers with separate galaxy images, and so obtain their spectra simultaneously.


Figure: Direct Fiber Positioner. Here we show a PI Ceramic PT230.94 under maximum deflection, holding the pipe and fiber above a rigid base.

The PT230.94 has a 500-μm wall thickness. Four quadrant electrodes are distributed around the outer radius, and one electrode on the inner radius. The permitted drive voltage for each outer electrode with respect to the inner electrode is ±250 V. The above figure shows the deflection that results from +250 V applied to one side and −250 V applied to the other.

Static Piezo Driver

[17-JUL-19] The A2089A is a static piezo-electric driver consisting of a ±250 V power supply and two hand-operated multi-turn potentiometers to set each of the two output drive voltages to a value −250-+250V.

The circuit is powered by 24 V delivered through two bananna jacks. Two VG1524S250 isolated DC-DC converters produce +250V and −250V from the 24-V input.

The ±250-V voltages are higher than we are used to working with in an open-frame circuit. We touched the +250-V and −250-V power supplies with one finger, while holding a ground terminal with the other hand, and felt only a slight sensation. To make sure that the outputs are safe for use without special precautions, they are presented to bananna sockets through 100-kΩ resistors. The resistors limit the current through the humn body to less than 2.5 mA. A typical hand-to-hand resistance in the body is 100 kΩ. Such a body resistance would pass no more than 1.2 mA from the R or L outputs, dissipating roughly 100 mW of power.


Figure: The Static Piezo-Electric Driver (A2089A).

The A2089A allows us to connect ±250 V to two opposite electrodes on a PZ tube and so cause static displacement of a fiber pipe glued into the tube.

Piezo Amplifier

[18-JUL-19] The A2089B provides two inverting amplifiers with output dynamic range ±250 V. Each amplifier has gain −25. We connect two function generator signals to LIN and RIN and so obtain two amplified signals a LOUT and ROUT respectively. Signals are brougt to and from the board with vertical BNC sockets. Power for the amplifier comes from a 24-V power adaptor with 5.5-mm center-positive plug.

The ±250-V power supply is produced by two VG1524S250. Amplification is provided by the PA95U high-voltage power operational amplifier. The exposed metal backs of the two amplifiers U1 and U2 are connected to their output ports, so avoid touching these when the output signal is large. The A2089B draws roughly 200 mA from its 24-V power supply.


Figure: The Piezo Amplifier (A2089B). Shown here with power applied to the 5.5-mm power jack, signal applied to RIN, and an oscilloscope probe plugged into ROUT. The 250-V power supplies are mounted on the bottom side.

The A208901A printed circuit board has two errors. One is an inversion of the footprint for L1 and L2, which we get around by mounting the two converters on the bottom side of the board. The other is a reversal of the connections to P1. We correct this reversal by cutting tracks and adding wire links.


Figure: The Piezo Amplifier Modifications (A2089B). We must cut three tracks and connect 0V to P1-3 (outer contact) on the bottom side. On the top side we connect +24V to P1-1 (center contact). The bottom-side mounted converter can is visible in the lower right corner of the photograph.

We apply a 1-Vpp sinusoid and a 10-Vpp sinusoid to RIN and observe the amplitude of ROUT. For the 10-Vpp input, we see slew-rate limiting of the output starting at around 20 kHz. The PA95U has a typical slew rate of 30 V/μs with a 4.7-pF compensation capacitor. We are using 10 pF and we see a 20 V/μs maximum slew rate.


Figure: Gain versus Frequency for the A2089B Amplifier. We plot for small signal (1 Vpp input) and large signal (10 Vpp input).

[14-AUG-19] The A2089B2 is a modification of the A2089B where R1 = 130 kΩ so that the gain of the U1 amplifier from P2 to P3 is −1. We connect a signal to RIN. We connect ROUT to LIN. Now we see RIN multilied by +25 at LOUT and −25 at ROUT. Thus the A2089B2 provides complimentary drive output for two PZ electrodes using one signal.


Figure: The Piezo Amplifier (A2089B2) Producing Complimentary Outputs.

We use a BNC-T on the ROUT socket so as to share it between the PZ electrode and LIN. We connect ×10 oscilloscope probes to the output resistors of both the left and right channels so as to view the output signals.

Piezo Driver

[12-AUG-19] The A2089C provides two complimentary ±250-V outputs generated from one ±10-V input. The OUT+ signal is +25 times the input, the OUT&minus: signal is −25 times the input. The driver allows us to generate the complimentatry East-West or North-South drive voltages for a PZ tube, such as the PT230.94, from a single input signal.

The A2089C printed circuit board (the A208901B) provides an extension with six footprints designed for solder-mounting of six PT230.94. Each footprint is supplied with the same east, west, north, and south drive voltages from four BNC sockets.


Figure: The Piezo Driver (A2089C). Circuit Board Rendering

The A2089C amplifiers are identical to those of the A2089B, except one is connected with gain −1 to the output of the first amplifier, thus producing the gain of +25 to compliment the first amplifier's gain of −25.