Creating a Reliable, Cost-Effective Virtual Camshaft for Solid State Hydraulic Pump Using NI LabVIEW and PXI

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"Using LabVIEW FPGA Module, we created a reliable “virtual camshaft” in just one week at minimal cost, allowing Active Signal to conduct tests culminating years of effort spent researching, designing, and building the pump."

- Eric I. Lyness, Mink Hollow Systems, Inc.

The Challenge:
Coupling the angular velocity and phase of a rotary valve with the expansion and contraction of a “smart material” to create a solid state hydraulic pump that moves small amounts of hydraulic fluid thousands of times per second.

The Solution:
Using the National Instruments LabVIEW FPGA Module and the NI PXI-R Series reconfigurable I/O module to tightly synchronize the high-speed valve operation of the pump with the oscillation of the magnetic field generating the motion in the smart material.

Author(s):
Eric I. Lyness - Mink Hollow Systems, Inc.
Arthur Cooke - Active Signal Technologies
Dennis Kohlhafer - Active Signal Technologies
Keith Bridger - Active Signal Technologies

Electro Hydrostatic Actuator Testing

Control surfaces on aircraft are typically actuated by hydraulically driven rams. The source of the hydraulic pressure most often comes from an electric motor-driven pump. Traditionally the pump is centrally located powering a network of hydraulic tubes. However, in the recently developed “fly-by-wire/power-by-wire” systems, the pump is collocated with the actuator at the control surface. This pump-actuator combination is called an electro hydrostatic actuator (EHA), and can be fabricated in a suitably compact form for manned aircraft. However, the smaller unmanned military aircraft of the future will require actuators significantly smaller than the current EHAs. Recognizing that a new technology would be required, the Defense Advanced Research Projects Agency (DARPA) focused on developing electromechanical actuators that take advantage of the high energy density of smart material transduction elements.

Active Signal Technologies spent the last two years developing a prototype smart material powered electro hydrostatic actuator that uses the magnetostrictive material Terfenol as the pump driver. The Terfenol is placed within a cylinder that is essentially an electromagnet. When the magnet is on, the Terfenol expands in proportion to the strength of the field. Because the expansion is very small, the field must be oscillated thousands of times per second to produce useful power. A rotary valve containing multiple orifices needs to be synchronized with the high-frequency excitation of the Terfenol to allow for the input and output of hydraulic fluid thus rectifying the flow.

To accomplish the tight coupling required, NI Silver Alliance Partner Mink Hollow recommended the PXI-R Series reconfigurable I/O module combined with the NI LabVIEW FPGA Module. With the high speed and deterministic behavior of LabVIEW FPGA Module running on this module, we could read all nine bits of the encoder and provide an analog output waveform directly linked to the absolute position of the rotary valve. Further, should the encoder connection be temporarily interrupted, the system would immediately resume correct operation when the connection re-established.

Closing the Loop

For the pump to function, control hardware needed tight synchronization between the valve location and the waveform sent to the Terfenol piston. A motor rotates a rotary valve with six evenly spaced openings at about 30,000 rpm. Each time a valve is aligned with the piston, the piston must be at or near full expansion or flow is inefficient or nonexistent. The feedback from the motor comes from a 9-bit absolute encoder with 0.7 degrees of resolution per step. The speed of the motor is controlled by a GPIB-based power supply. To close this loop, we used the PXI-R Series reconfigurable I/O module.

The FPGA feedback works by reading the absolute encoder position, and thus the location of the rotary valve, and outputting the corresponding voltage to the Terfenol piston. In essence, we had to create a virtual “camshaft” to couple the rotation of the motor with the excitation of the Terfenol – outputting sinusoidal waveforms at 3,000 Hz. At this rate, FPGA was the only viable option. Additionally, like the spark advance of a car engine, we also needed to tune the system so that each opening aligns before, after, or right at the peak expansion (“top dead center”) of the Terfenol piston.

Creating Virtual Resolution

The software consists of two components: a host module and an FPGA module. The host module provides a user interface for operators to set the phase offset, amplitude and attributes of the output waveform. As much as possible, the host also performs floating-point calculations for the FPGA. The FPGA module performs the I/O to implement the virtual camshaft.

The FPGA module presented challenges that required a new way of thinking. The FPGA does not allow floating point math so it cannot perform a simple sine function. It can use a lookup table, but this introduces a new problem affecting phase resolution. The 512 steps of the encoder provide angular resolution of 0.7 degrees per step. However, because the output waveform cycles six times per revolution, this provided only 4.2 degrees of phase resolution per cycle. A resolution of about one degree per cycle was desired for pump efficiency.

The 44 MHz loop rate of the FPGA provided a robust means to create virtual resolution using a technique similar to a sigma-delta ADC. By over sampling the encoder and timing bit transitions, we increased the feedback resolution from 4.2 degrees to 1.05 degrees. Physically at the top speed of 30,000 rpm, the 9-bit encoder changes from one step to the next every 3.9 microseconds. A single cycle loop running at 44 MHz on the FPGA iterates 171 times in 3.9 microseconds. By counting the cycles between encoder steps, the software interpolates the angular position of the motor during the very next step of the encoder.

For instance, if the software counts 200 cycles between steps, it calculates that on the next step the position will be approximately 1.05 degrees every 50 cycles. Because the angular rotation between any two steps of the encoder is very small, the interpolation is relatively accurate even as the motor accelerates.

Successful Virtual Camshaft

We used the FPGA camshaft module within a larger data acquisition application to perform crucial experiments on the pump. We automated a sweep across a range of phase offsets and frequencies to find the optimal performance characteristics.

Two years of effort in developing the solid-state pump depended on creating software that could control it. Using LabVIEW FPGA Module, we created a reliable “virtual camshaft” in just one week at minimal cost, allowing Active Signal to conduct tests culminating years of effort spent researching, designing, and building the pump.

Author Information:
EricI. Lyness
Mink Hollow Systems, Inc.
120 Ashton Road
Ashton, MD 20861
United States
Tel: 301-260-1821
Fax: 240-342-2045
elyness@minkhollowsystems.com

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