Autopilots & Rate Gyros

[AlanPound]

Thinking about fitting a new a/p. Some of the newer types sport rate gyros... presumably these have advantages over just a fluxgate compass. Are these advantages real?

Have had Simrad AP22 recommended to me by dealer (on price/performance grounds). There is the option of a standard fluxgate, or a fluxgate with rate gyro... Simrad do not make a big thing of the advantages of the rate gyro (which is more £s), but say that it is beneficial on 'difficult' boats (steel, fast, whatever)...

So will the Simrad with the rate gyro option perform as well as pilots where the rate gyro is mandatory (ie. is it making full use of the rate information)?

 

[Chris_Stannard]

I beleive the main advantages are in two areas, firstly your autopilot may perform better. seocndly if you have fitted radar and want to have a stablised picture, either North up or on ships head, then you may need the better heading input, since flux gate compasses do not seem to produce the heading data often enough to give good picture stabalisation.

Chris Stannard

 

[ccscott49]

The fluxgate compasses we use out here, have continuous output. We use rate gyros aswell, but only in areas of known magnetic interference, such as inside the steel casings. The only time I've seen them used apart from that is in steel ships and aircraft inertial navigation systems. When they are normally used to stabilise accelerometer platforms.

 

[jimg]

Chris is right I beleive, I have the Simrad autopilot and Raymarine radar and chartplotter. I was told told that to get a perfect North up especially on a fast boat that it was necessary, also essential for chart overlay on radar image. This is more fun to play with in harbour than real practical use.( IMHO )

 

[yachtbits]

It is not the refresh rate of a gyro that is the key point. I actual fact stand alone gyro's are often configured to 10hz NMEA output, the same as many fluxgate systems. 10Hz in electronic terms is slow, but in boat helm response is fast enough.

The benefits noticed on the radar reflect the benefits your autopilot will see. Basically it is more stable and more accurate which is why they are more money!

can't comment on whether a simrad is better than a raymarine as I've not had the chance to test both side by side yet. but the technology will be similar. If you can afford it, buy it, they are better, but dozens of people have been around the world without.

kev
yes, we sell both





<A target="_blank" HREF=http://www.yachtbits.co.uk>http://www.yachtbits.co.uk</A>
sales@yachtbits.co.uk
marine electrics & electronics

 

[Piers]

I was testing the new rate gyro AP for Raymarine for a year (together with MBM's Calm Voyager and some other boats) before it was released onto the market.

Without doubt, it is superb. Far better than a fluxgate AP, especially over the N/S steering which is always a bugbear for APs.

Try speaking with Raymarine's AP main engineer for the product - Dave Wellcome on 02392 693611 and dial 1614 when the automated operator answers. Say Pier suggested you call him.

Piers du Pré
www.playdeau.com

 

[AlanPound]

Ok, well answering my own post here (partly)... I just spent a morning searching the web - and in particular looking at the NKE site, and talking to the NKE disti...

So it seems that some of the new-generation pilots (NKE may have been the first), use the rate gyro angular acceleration info as the primary source of heading (and heading change) input, and the fluxgate info (heavily averaged) separately as a way of determining North. This direct use of rate info makes them much more accurate.

Older systems that only use fluxgate info have to average the directional data over a period of time, due to the short-term effects of pitching and rolling, and this makes the information rather less than real-time.

The Simrad (it would seem), when using a gyro-stabilised fluxgate, would get rather better (more timely) heading data, but is not able to directly use the angular acceleration info that it would have if it read the gyro directly. (This opinion is down to some assumptions I have made about the wiring configuration, as a result of reading the somewhat inadequate data sheets).

So the Simrad is probably less wonderful than the NKE (and possibly other makes that use rate data directly). NKE (French) is widely used by mini-Transats, right up to open 60s...

 

[tome]

Rate gyros have large and unpredictable angular drifts so need to be stabilised with a heading reference. However, they are very good at tracking rapid changes in direction so ideal for autopilots.

They need to be coupled with a compass to be useful. Electronic compasses are available in a variety of flavours. The simple ones fitted to tiller steering APs etc use a rotating polarised filter and a second filter aligned to the required heading with a couple of light cells to detect angular misplacement. Better ones use fluxgate compasses with output rates from 1 Hz at the cheap end to 10Hz plus at the performance end. 5Hz is about the minimum required to stabilise a North-up radar screen.

Fluxgate compasses have to compensate for the vertical component in the earths magnetic field whereas a rate gyro can have it's axis aligned with the boat. Fluxgates need to be damped and so the combination is ideal. I believe this is what KVH have been using for years to stabilise their Inmarsat antennas.