All Torque No Action?

...or maybe when too little torque is, well, not enough, perhaps? No, not really.

I'm trying to control 2 points with one servo and it just proved too hard.

I did an experiment today inspired by a friend of mine - Mr Merrigan. I have a number of points for switching lines which really can only move together (unless you enjoy derailment).

Chris suggested trying to make thes emove usnig a single servo. Well, I had a red hot go at this on Saturday and a good part of Sunday morning.

The idea it to mount the motor roughly in the mid-point of the meeting switches and have a beam going out across both switches with rods hooking into the cross-bars of the switches. One motor, two moving switches in synchronicity.

You can see the servo above and the spar above that with a nice mount point for the motor.

Just to make this a bit more obvious I have mounted the spar on top of the tracks so you can see how it would work. I've also put a videoon YouTube where I show the movement by hand.

Now this looks like a great idea but in fact it does not work well for a lot of reasons:

• You need very careful alignment to make this work and that is not insurmountable but is hard.

• The motion of the corss bar is straight across but this is radial and so puts more strain on the motor at the extremes of the switch positions. That's not a problem - just use a big motor. But that means a $20 motor instead of 2 x $4 = $8 :-(
  
• The long central spar (15cm) means that even a small turn of the motor (1 deg)  results in a big movement, so your points no longer switch nice and smoothly.

I tried and tried but no luck. When I thought through the logic I realise it was a dead loss really.

For all of these reasons, I'm moving back to one cheap servo per point. Sorry Chirs, nice idea though.

I've got my servo mounting system down to about 10 minutes per point.

Of course, there are other ways to do the mechanism so 1 servo will drive 2 points but it then takes more than 10 minutes per point to mount!

How about 8 points per Arduino?

So I have constructed my nice point motor controller using servo motors and an Arduino - very nice, thank you.

However, now I have a layout that needs to control 8 points, not 4. I also have limited I/O lines on the main controller so dedicating 8 to driving the point controller is just too much.

I decided to control all 8 points from a single Arduino and use the serial port to control the position and calibration of the points. I still use a pot to control the calibration process because it's just so intuitive.

This posed quite a few challenges. Firstly, don't try doing this with an Arduino Diecimila - you need a newer system than that - I used a Freetronics TwentyTen (discontinued) but the Eleven will be fine as well.

The other consideration is POWER. Yes, sorry but it's not going to work from USB and it's not even going to work well from a PC power supply without a little help. Yes, I'm talking about large capacitors and even a diode for return current.

Have a look at the photos below...