Nicks workshop mutterings

Various railway orientated modelling projects in my new workshop/railway room.

Servos-analogue or digital

Three signals and two points on Tinworthy are driven by servos. When first installed two signals and a point had digital servos and the third signal and one point had analogue servos. All the servos are driven by Heathcote servo driver boards via relays switched by the MERG CBUS system.

I found that the two analogue servos ‘twitched’ when a loco was driven past but not the digital ones. According to the Heathcote web site, they recommend digital servos as they do not twitch at startup.

I am not an electrical expert but I understand that the amount the servo moves is determined by the control pulse coming from the control board which has controls to determine the start and end points of travel of the servo. However there appears to be a degree of overthrow before the servo cuts out and the position is then maintained by the gearing in the motor. This is fine when working a signal but with a point the end point needs to be set so that the point blade is hard up against the stock rail (which is why in the prototype there is a facing point lock) and therefore the digital servo is continually trying to reach the cut off position and the result is ‘chattering’ of the servo. This doesn’t happen with analogue ones. If the throw of the servo is adjusted so the point blade only just reaches the stock rail then the chatter is eliminated but there is the risk of trains derailing because the blade is not ‘locked’ in place.

So do I twitch or do I chatter!

Recent postings on the forum of MERG suggested that by using omega loops or ‘Z’ loops the chattering of a digital servo can be eliminated and the loop keeps up the pressure of the point blade on the stock rail. I have tried this and it has solved the problem.

So now I don’t twitch or chatter!!

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