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Signed and sealed: David Maidman of Scitek Consultants outlines how his company built a test rig for polymeric seals.

Polymeric seals are used in difficult environments with ever-increasing demands on service life: several years' continuous operation without replacement. In most cases, service life is proved through extensive endurance testing. This requires advanced test rigs capable of continuous operation for several years while testing the seal under representative temperature, pressure, and movement.

The project involved developing a rig to test the performance of the seals. We made the frame out of a stiffened steel box section and used an air-cooled shaker to produce small, high-frequency movements (0.1 mm at up to 200 Hz) under closed-loop control and a servo motor lead screw assembly to produce large axial movements of +/-6 mm.

We housed piston, cylinder, and seals under test in an environmental chamber that could maintain setpoint test temperatures of -50[degrees]C to 300[degrees]C. With liquid nitrogen, we achieved temperatures below ambient.

The system we developed can cycle seals within the range of the mechanical and thermal envelope, which represents real-life seal operating conditions. To replicate conditions, we used a programmable pressure regulator to vary pressure inside the seals. With a solenoid-operated ball valve, the seals were kept under pressure while we did a leak test to record pressure decay as a function of time. We used CompactRio to control automated pressure decay tests repeated at set intervals to see the level of seal performance degradation.

The test sequence was controlled by a PC-based human machine interface (HMI) connected to an NI cRIO-9074 control system via ethernet. The PC also sent commands to the servo motor controller via a USB interface. The temperature controller of the environmental chamber was connected to CompactRio via RS232 using the Modbus communication protocol, so CompactRio provided basic supervisory temperature control while concentrating processing power on more advanced control and acquisition tasks.

We also equipped the system with an analog input module to acquire pressure and seal position. We used an eddy-current-based, noncontact displacement sensor to record small movements induced by the shaker and provide feedback for the closed-loop control done by the FPGA.

The system measured large movements of the seal piston using a linear magnetic encoder connected to a digital input. The FPGA processed the encoder signals.

We used an analog output module to set the programmable pressure regulator and a thermocouple module to take temperature from eight Type K sensors. All control settings and instrument calibration factors were stored on the cRIO-9074 so we could replace the PC and not affect the system.

Although the HMI provided top-level test-sequence management, the cRIO-9074 controlled the rig and data acquisition. This device was essentially two separate systems in a single chassis, including a 400MHz real-time processor and an FPGA connected to the processor via PCI bus. We used the LabView FPGA Module to rapidly develop the deterministic FPGA code that performed hardware-level I/O tasks, including data acquisition and control.

Sensors connected to NI C Series input modules on the FPGA acquired the data and transferred it to the real-time controller by direct memory access first-in-first-out memory buffers. We created a customisable front end with the C Series modules. CompactRio acquired and processed the data and sent it to the HMI for display and logging. The system acquired and logged summary data at user-defined rates in the range of 0.1 to 10Hz, while waveform data was acquired and logged at 20kHz.

We used the FPGA to implement a proportional integral derivative (PIP) algorithm for closed-loop shaker control. The shaker setpoint was supplied by a setpoint generator loop operating in parallel on the FPGA. In practice, we could achieve adequate control using PI control only.

Implementing closed control on the FPGA gave us the high loop rate and deterministic control we needed for the shaker. Additionally, we implemented highspeed adaptive control algorithms on the real-time system, including gain scheduling of control parameters and amplitude/offset adjustment, in addition to the PI control running on the FPGA.

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We implemented adaptive control of the amplitude/offset by processing each complete cycle waveform and using an iterative control algorithm to update the FPGA setpoint generator to achieve the desired setpoints. For gain scheduling of the control parameters, we used the setpoint test frequency to select the required PI settings from a lookup table, updating FPGA control settings as needed.

The result: using in-house expertise in rig design, frequency, finite element analysis, and vibration with off-the-shelf CompactRio hardware and LabView software, we designed and built a powerful piston seal test rig for continuous running for long periods. The rig was commissioned and is now at work.

David Maidman was a finalist in the UK Graphical System Design Achievement Awards
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Title Annotation:Control
Author:Maidman, David
Publication:Environmental Engineering
Geographic Code:4EUUK
Date:Dec 1, 2012
Words:777
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