Level Measurement & Control

More questions than answers at the speed of light.

Feb 2000 Level Measurement & Control

In the 1980 the South African government supported the development of many advanced technologies ranging from the benign coal-to-fuel process at Sasaol to the Orwellian monster at Pellindaba.Many of these technologies have been sold,diluted or lost to present day South Africans whilst others are trying to establish a commercial foothold with the help of a small number of enthusiasts.One of these emerging technologies is ,laser range-finding and distance ,measurement,which forms the focus of this article.

The design process of futuristic products is a convoluted path that relies more on the determination of the human spirit than it does on good planning. When engineers at NASA set out to put a man on the moon they faced innumerable engineering challenges that (even by today's standards) were daunting at best and impossible at worst. Notwithstanding these barriers the collective knowledge and experience of many people was harnessed and focussed on one objective with unprecedented success. An important lesson in this is that knowledge must somehow be harvested, stored and transferred between people and generations if it is to have any significant value to society. Unfortunately, the demands of capitalism are such that information is seen as a valuable commodity, which must be hoarded and traded, rather than as the heritage of a nation.

It is in this context that certain technologies and ideas are presented in this article with the aim of offering insight into the challenges that need to be faced in designing a laser distance measuring instrument. Hopefully, aspirant laser engineers will find useful hints on how to design their own systems whilst the ravenous spectre of capitalism will be satisfied that not too much proprietary information has been revealed.

The product worthy of our attention is called the Pixie. It is a pulsed laser instrument, which measures the distance to any surface up to 10 m away. The design principle is simple: time how long it takes for a flash of light to travel from a laser, to a surface directly ahead and back again. Multiplying this time by the speed of light gives the distance between the laser and the target material.

This method of measuring distance is well documented and a number of suppliers offer various components and sub-systems that can be integrated into a laser distance-measuring instrument. Unfortunately, the final product built this way would be extremely expensive, large and power hungry.

In contrast, the design of the Pixie was motivated by a vision of what laser instruments would be like ten years from now. This vision suggested that small size, low cost, low power consumption, short manufacturing times and easy installation would be critical success factors. It is perhaps an indictment of our restrained, adult curiosity that a vision such as this can suddenly give rise to a burst of innovation. Why we are unable to push forward at a much faster rate under normal circumstances is a question for the psychologists to answer.

The Pixie uses a high power, semiconductor laser that needs a 2 kW electrical pulse to produce the laser light. Heat dissipation would normally cause the laser to melt down after a few milliseconds so the pulse has to be restricted to an on time of 20 ns. Those readers with experience in high power circuits will appreciate the difficulty in turning on a heavy current circuit in 2 ns, holding it on for 16 ns and then turning it off again.

To make life more interesting, the laser firing circuit has to be designed with very low electrical noise levels so that the rest of the electronics can operate normally. Most laser products incorporate heavy electromagnetic shielding around the laser whilst the Pixie has a new type of 'virtual shield' which absorbs polarised RF emissions along the most critical axes.

In line with the directives of our futuristic vision of small size, the Pixie's laser and firing circuit occupy less than one cubic centimetre. If this is not a staggering enough achievement, a new type of 'energy conserving' power supply has also been developed which allows the peak power requirements of 2 kW to be generated from a 12 V supply whilst consuming an average current of less than 1 mA.

The informed reader will realise that numbers such as these cannot be achieved using conventional ways of thinking but be assured that no laws of energy conservation have been violated. The breakthrough in design came from tackling the problem as one of 'the transfer of energy from one point to another' rather than the atavistic approach of building power supplies and control circuits as separate electronic modules.

Having launched the laser light through a collimating optical system and bounced it off some obscure target material it is now up to the optical detection system to tell us when the light has returned. Unfortunately for the unwary designer, a few ugly problems arise. Firstly, how can an optical detector tell the difference between a laser pulse and background light? Secondly, given that the laser pulse is so fast, how can it be amplified enough to be used by a timing circuit? Thirdly, how can the laser pulse be amplified without introducing an extra time delay into the round trip path?

The first teaser is tackled by the Pixie using a combination of optical and electronic filters. It turns out that the characteristics of narrow, optical bandwidth of the laser light combined with its lack of significant, low frequency electrical harmonics produces a high signal to noise ratio when appropriately filtered. However, a Fourier analysis of this low duty cycle, pulsed system will immediately reveal that neither conventional, narrow band, electrical filters nor phased locked loop techniques can be used to extract the signal. Standard components are not much help here and a certain brute force philosophy provides the most cost-effective solutions.

The trade-off between speed and power in electronic circuits means that sufficient amplification to reach ECL or TTL logic levels places heavy demands on the amplifier and power supply design. In particular, gain-bandwidth products in the tens of gigahertz range require a sophisticated circuit layout combined with highly stable, high current supplies. In its ignorance (or arrogance) the Pixie chooses not to use power supplies at all. Again, the idea of working with energy transfer rather than normal, circuit modules provides a clue as to how to design a circuit with almost unlimited power capacity and near perfect regulation.

Overcoming the final, detector pitfall of introducing electronic delays into the path of the laser light is normally solved by using some automatic zeroing technique. This works by measuring both the return signal and the outgoing signal and finding their time difference mathematically. These automatic, self-zeroing systems often tend to have stability problems. This is due to the disproportionately large time added to the laser path by the amplifiers compared to the actual flight time of the laser pulse. For example, the laser takes 67 ns to travel to a target 10 m away and back again. In comparison, even a fairly fast amplifier system can add as much as 100 ns extra delay to this path, making a total of 167 ns of which 40% represents the distance to be measured. These numbers do not seem to present a problem until the issue of resolution is considered. If the 10 m distance is to be measured with a resolution of 1 cm then the path length of the laser light must be stable to within 67 ps. The total time is now 167 000 ps of which the last 0,04% is the 1 cm section we are interested in. Any delays introduced by the amplifiers are highly significant and small variations in these delays can render an instrument useless. Suffice it to say that the Pixie does not use this type of system.

Having successfully launched our laser pulse and detected its return, there remains the thorny issue of timing the trip. There are no small, off-the-shelf timers that have a resolution of the order of a few tens of picoseconds. Many laser instruments use a complex combination of averaging and multiple, high frequency crystal circuits. The futuristic Pixie not only achieves this difficult timing feat but also combines the timing mechanism with a trick called 'last pulse detection'. This allows the timer to discriminate between successive return pulses whilst ignoring those which are caused by spurious obstacles such as an intervening glass window.

From a purely philosophical point of view it is not possible to build a timing circuit which 'knows' that a particular pulse is the last one coming back and that there is no chance of any others returning at a later time. For this to happen the timer must somehow look into the future and 'see' that a pulse that is about to arrive is not being followed by any other pulse. Some laser products use a form of 'range gating' superimposed on their timing system to accomplish this type of detection. This solution tends to be complex and requires that a whole array of measurements be taken before any answer is forthcoming.

Fortunately, the designers of the Pixie were not hampered by the burdens of philosophical logic and so developed a remarkable system called a 'pre-emptive' circuit. Simply stated, this circuit knows what is coming even before it arrives. One day I hope to be able tell someone how this works but for now this falls into the category of 'capitalistic, trade secret'.

Having the essential technology embedded in a product is a necessary but not sufficient condition to guarantee the usefulness of that product to a customer. It is a common failing of engineers to leave their high technology products flying in the clouds rather than bringing them down to earth for use by mere mortals. For every hour of design spent on the high technology aspects of the Pixie, two hours were spent on the production methods and usability of the final product.

Today the Pixie is a modular product which goes beyond the basic ability to measure distance and offers its customers various standard interfacing options, different mounting arrangements, corrosion resistance, small size, light weight and so on. Its development has spawned a whole range of new products, which have taken their inspiration from the compact solutions that the Pixie offers. A particularly exciting derivative has been the TRI-XY which is a three dimensional scanner for surface, volume and mass analysis of stockpiles. But that is another story.


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