Product category:
Programmable Logic Devices
News Release from: Anadigm | Subject: Conradt Measurement and Control
Edited by the Electronicstalk Editorial
Team on 24 January 2002
Analogue array solves DVD performance
problems
The Conradt Measurement and Control development company has designed an innovative analogue lens control solution which dramatically improves the write speed of DVD peripherals.
The Conradt Measurement and Control development company has designed an innovative lens control solution which dramatically improves the write speed of DVD peripherals - and their resistance to vibration - by implementing the control loops using analogue technology in place of established digital, DSP-based approaches Based on an Anadigm AN10E40 field-programmable analogue array (FPAA), the design provides a cost-effective means of increasing control loop performance way beyond the levels offered by today's DVD pick-up designs
This article was originally published on Electronicstalk on 4 Jan 2001 at 8.00am (UK)
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Anadigm has added filter synthesis to its field-programmable analogue array (FPAA) configuration software.
It avoids expensive increases in the performance of digital control circuitry, while simultaneously overcoming problems of pick-up oscillation that DVD manufacturers face as write speeds increase.
Analogue-based control was chosen over digital because the target increases in control speeds would have required prohibitively-expensive high resolution A/D conversion, along with increases in DSP speed.
The 'programmable analogue' FPAA solution was then preferred over discrete components, primarily because of the way it allows manufacturers to program the DVD device during production for individual laser focus assemblies or batches - something which would be expensive and time consuming to achieve using discrete components.
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Moreover, the FPAA's programmability allows DVD manufacturers to auto-adapt the circuit configuration throughout its life, to optimise the control loops for temperature-related problems associated with the mechanical damping of the lens positioning assembly.
The solution is based on two PID loops to control the x and z axial movements of the lens (which is actuated via coils), according to error inputs from a photodiode sensor array.
The complete circuit is implemented inside a single FPAA chip, using 'drag-and-drop' summing amplifier, integrator, differentiator, amplifier and filter circuit elements from Anadigm's library.
The complete dual-PID circuit requires only 70% of the FPAA's configurable analogue blocks.
The control loops execute a proprietary PID (proportional-integral-derivative) algorithm developed by Conradt.
This is targeted at oscillating systems, especially harmonic oscillating systems such as a DVD pick-up, and it offers the unusual ability to make settling time user-definable.
Using this algorithm the damping control is now able to react so fast that the physical barriers to DVD write speed now revert to other areas including the crystallization rates of the disk substrate, the speed with which the lens actuators are able to work, and the data-rates of the DVD controller.
In practice, this means that an increase in write speed from today's 1x or 2x to at least 10x is easily possible, combined with greatly improved resistance to vibration - helping DVD manufacturers to make improved products for portable equipment such as notebook computers.
It also has the benefit of better control of laser focus for writing data, providing the high resolution required for repeatable accuracy of 40nm over the 0.5mm movement of the DVD read/write head - a feat which would normally necessitate a fast 14bit ADC in digitally based systems.
The same circuit has also been tested on a CD-ROM drive, and achieves a 200x rate without problem.
The FPAA design can be implemented as a fixed configuration, in just the same way as typical DVD controllers today.
The analogue circuit configuration is loaded at boot-up from a serial EPROM device, which may be programmed during production to account for the characteristics of individual lens control assemblies if required.
However, Conradt has also prototyped a microcontroller-hosted FPAA design which allows the PID loops inside the FPAA to dynamically auto-adapt.
This approach provides a solution to the problem of handling the temperature-dependent characteristics of the polymer-based damping mechanisms on lens control assemblies, which can cause read errors.
In fact, this is a key problem holding up speed DVD enhancements, as weakly damped pick up mechanisms (which are required for faster operation) are in general more prone it is to temperature fluctuations, and oscillation - which can cause the equipment to fail.
To implement this design, the microcontroller simply requires an input from a temperature sensor, which it uses to reconfigure the FPAA as appropriate, by loading a new configuration bitstream - an operation taking approximately 125us.
As digital technologies begin to pervade electronics design, so increased performance typically relies on huge increases in sampling rates, which are only available at the top-ends of DSP product ranges - which can be exorbitantly expensive.
If the DSP element is used in IP form, embodied inside some ASSP or SoC, then it almost invariably lags the state of the art in DSP performance, providing a fundamental barrier to progress.
Reverting to analogue control, especially in the form of one of the new generation of 'programmable analogue' components, provides a simple solution, and one which meets the needs for high-frequency control loops across a very wide range of applications, such as in the control systems for the flying surfaces of today's fighter aircraft.
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