Tuesday, 25 March 2014

Putting the ghost in the machine

In January 2014 the space probe Rosetta resumed communication after a 31 month sleep period. You may have seen it on TV, a bunch of engineers cheering because a machine that had been launched way back in 2004 had activated itself and was communicating with its creators. No wonder the engineers were so delighted, Rosetta's 'boot up' was pretty complicated, it had to power up, stop itself spinning, orientate its solar panels towards the Sun and align its antenna with the Earth. 

This diagram shows the very complex trajectory of Rosetta. Because there was no rocket powerful enough to inject it directly into its final orbit Rosetta first made two close encounters with Earth and one with Mars in order to build up sufficient velocity to match orbit with Comet  67P/Churyumov–Gerasimenko.

The German movie, Die Andere Heimat (Home from Home) is set in the year 1840. This film, which depicts a rural community mostly living in extreme poverty, touches on the very start of the technological age. A blacksmith and his two sons shoe horses, make tools and repair cart wheels. This is hard, relentless work. Yet, compared to the local farm workers, the blacksmith's family are middle class. And even the lord of the manor is dependent  on the blacksmith when he needs his cart repaired. 

At this time most of the work of the local community is done by men and horses.  The blacksmith decides that he will try and build a steam engine. He's never seen one but somehow he's heard of the efforts of the British in this field. James Watt's first steam engines were up and running about 60 years earlier.

The effort to produce such a machine, in the marginal economy of the time, would have been immense. And yet the machine is produced and it runs. The moments of first starting of the machine and the joy of success is one of the great moments of the movie. For the nascent engineers of  Die Andere Heimat  the moment would have felt at least as momentous as the instant those signals came in from Rosetta. 

However, it is not an immediate total success. In this case the machine actually runs so fast that it shakes itself to pieces. Eventually, however, one of the sons recognises the need for a governor, to limit and control the speed of the machine. 

With a governor, as depicted above, two metal balls spin has the machine turns. The faster they spin the further out they move. This movement is linked to the steam control valve  which reduces the flow of steam as the desired speed is achieved. 

The diagram above represents the governor in a more general way. At the left is the reference value, the desired speed. Subtracted from this is the output of a sensor that measures the actual speed. In the case of the governor the speed, the measured output, is the movement outwards of the spinning balls. The difference between desired speed and actual speed is the measured error and it is this 'error' that is used to open or close the steam valve and thus alter the speed of the machine.

These days such a control system can even be found in a washing machine. A controller will 'demand' different motor speeds for washing and spinning. A sensor measures the speed of the motor and increases or decreases power to ensure that the speed stays close to that required, regardless of how the load on the system varies with more or less washing and water. 

But back to the Age of Steam. After James Watt had perfected his steam engine James Clark Maxwell carried out a mathematical analyses of the behaviour of Governors and presented it to the Royal Society in 1868.  Maxwell on Govenors  Presenting such a theory to the Royal Society brought it to a wide audience. Eventually, the topic of Control Theory, and the exploration of stability in natural and manmade systems, would be examined by economists,  biologists and sociologists. 

And within engineering, the technology of sensors and computational devices continued to improve. The 20th century brought electronics and Control Theory now had a better means of implementation than through the mechanical devices of the Industrial Revolution. First analogue computers, then digital computers gave Control Theory more applications and better technology. The Governor was the great grandfather of the much more sophisticated control systems which in modern times control aircraft, will soon be at the heart of completely automatic cars and, of course, control the space craft Rosetta.

In the movie Die Andere Heimat the great moment of triumph came when those early engineers added the governor and finally got their engine under control. Now it was no longer a toy, it could do useful work. That was when those early engineers had the same kind of triumph as the engineers in the control room of the Rosetta probe. That magic moment where the machine 'boots' up and starts to carry out its intended purpose. 
This is the instant when a device made by men becomes something with a goal and the ghost takes hold inside the machine. 

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