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This article has been put together by our Pete Thomas. Having put a lot of effort in to building a really good looking model of a HDML (see our gallery page), the very poor manoeuvrability of the model with two channel control using a single speed controller working both shafts was a source of considerable disappointment. Not a scale steering regatta candidate, certainly! The question was, how to improve this without going to the considerable expense of another speed controller, plus four or more channel radio gear (i.e. a cost of £100 plus). Inspiration came from an article in Model Boats by Roger W. Sutherland on his model of Lady Woes, the Mobile Marine Model's classic steam tug, which used micro switches driven by a cam operated by a slave servo from the rudder servo. Unfortunately, the article did not give much information on the set up of this scheme. The way the system works is that as the rudder servo moves through approx. 15 degrees, the motor on the inside of the turn stops. If the rudder carries on to approx. 33 degrees of throw, the topped motor goes in to reverse, enabling a small turning circle to be achieved. The wiring diagram for this set up is shown below. It should be self explanatory, but here is a brief outline of how it is connected.
What has to be designed very carefully is the shape of the cam and the positioning of the switches. It is most important that motor one remains switched off by switch one whilst it is reversed by switch two. Similarly, it is most important that motor two remains switched off by switch four whilst it is reversed by switch three. A rough guide to the shape of the cam and the positioning of the switches is shown below.
There are three options regarding the positioning and operation of the cam. The cheapest option is to mount the cam on the rudder servo. You will however need a lot of space to accommodate the cam and the switches, plus good access to allow the inevitable minor adjustments to the set-up. This is not always an easy matter at the stern of a model. The next cheapest option is to move the cam and switches forward of the rudder servo and have the cam operated by push rode from the rudder servo. Again, however, this can be a problem if not enough space is available with a straight, unimpeded access run for the push rods from the rudder servo and with good access for adjustment. A good anchor point for the cam pivot is required as well, requiring cross hull beams. Probably the easiest set-up, but also the most expensive, is to use another servo driven via a 'Y' lead from the same channel of the receiver as the rudder servo, so that it operates as a 'slave' of the rudder servo. Depending on the available length of 'Y' lead, this slave servo can be placed in any convenient position in the boat with good access, etc. In Peter's HDML the latter solution was applied, and with two channel radio this model can now literally spin on a sixpence (whoops, now who's showing their age!) |
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Copyright © South Manchester Model Boat Club 2002 This site created and maintained by Steve Booty Or address snail mail to our Secretary: Mrs J Booty, 26 Amberwood Drive, Baguley, Manchester M23 9ND Or telephone 0 161 998 1204
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SOUTH
MANCHESTER MODEL BOAT CLUB
