20.01.2013 Views

The Art of the Helicopter John Watkinson - Karatunov.net

The Art of the Helicopter John Watkinson - Karatunov.net

The Art of the Helicopter John Watkinson - Karatunov.net

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

Fig. 7.28 In PCM or digital signalling, <strong>the</strong> position <strong>of</strong> a control is sampled periodically and <strong>the</strong> sample is<br />

expressed as a discrete number.<br />

Those who are not familiar with digital signalling may worry that sampling takes away<br />

something from a signal because it is not taking notice <strong>of</strong> what happened between <strong>the</strong><br />

samples. This would be true in a system having infinite response rate, but no real control<br />

system can be like this. All control signals from joysticks, actuators, gyros etc., have a<br />

distinct limit on how fast <strong>the</strong>y can change. If <strong>the</strong> sampling rate is high enough, a control<br />

waveform can only change between samples in one way. It is <strong>the</strong>n only necessary to<br />

carry <strong>the</strong> samples and <strong>the</strong> original waveform can be reconstructed from <strong>the</strong>m.<br />

Figure 7.28 also shows that each sample is also discrete, or represented in a stepwise<br />

manner. <strong>The</strong> value <strong>of</strong> <strong>the</strong> sample, which will be proportional to <strong>the</strong> control parameter,<br />

is represented by a whole number. This process is known as quantizing and results in an<br />

approximation, but <strong>the</strong> size <strong>of</strong> <strong>the</strong> error can be controlled until it is negligible. If, for<br />

example, we were to measure <strong>the</strong> height <strong>of</strong> humans only to <strong>the</strong> nearest metre, virtually<br />

all adults would register 2 metres high and obvious difficulties would result. <strong>The</strong>se are<br />

generally overcome by measuring height to <strong>the</strong> nearest centimetre. Clearly <strong>the</strong>re is no<br />

advantage in going fur<strong>the</strong>r and expressing our height in a whole number <strong>of</strong> millimetres<br />

or even micrometres.<br />

<strong>The</strong> point is that an appropriate resolution can also be found for a control system, and<br />

a higher figure is not beneficial. If a conventional pushrod control system is considered,<br />

each bearing and joint in <strong>the</strong> system has some backlash and <strong>the</strong> cyclic stick on a typical<br />

helicopter can always move some small distance before any motion is detected at <strong>the</strong><br />

swashplate. If <strong>the</strong> movement step size <strong>of</strong> a digital system is <strong>of</strong> <strong>the</strong> same order as <strong>the</strong><br />

backlash <strong>of</strong> a mechanical system, both systems have <strong>the</strong> same accuracy.<br />

<strong>The</strong> advantage <strong>of</strong> using whole numbers is that <strong>the</strong>y are not prone to drift. If a whole<br />

number can be carried from one place to ano<strong>the</strong>r without numerical error, it has not<br />

changed at all. By describing control parameters numerically, <strong>the</strong> original information<br />

has been expressed in a way that is better able to resist unwanted changes. <strong>The</strong> amount<br />

<strong>of</strong> backlash in a mechanical system increases with <strong>the</strong> number <strong>of</strong> ball joints and bell<br />

cranks in a control run. <strong>The</strong> accuracy <strong>of</strong> a mechanical signalling system would decline as<br />

Control 293

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!