Variable Speed Drives

The heart of this model is a pair of identical Variable Speed Drives (VSD) one is used for the Vertical Landing Module motion, and the other is used for the Horizontal Landscape motion.

The idea is to simulate momentum and air friction, by providing a delayed and gradual change in speed following any thrust demand.

This caters for a number of simulation factors as follows -

Due to "momentum", it takes time for full thrust to take effect and accelerate the Lander Module (LM) to maximum speed in the desired direction.

When thrust demand is removed, the LM will continue to move, again due to "momentum" but "air friction" will cause a gradual deceleration back to zero.

This also means that if there is a sudden demand for a change in direction, before that can happen, the LM motion continues in the original direction as it slows to zero, before then responding to the new demand direction.

To achieve these effects, the final output drive comes from a fixed pickup wheel which is driven by a moveable high-speed friction wheel (with a rubber disc for a positive drive). When the pick-up wheel sits in the centre of the friction wheel, there is no output. But as the Friction wheel is moved left or right, the pick-up wheel is now driven in one direction or the other. Maximum speed is reached at the periphery of the friction wheel.

The high input speed, combined with rubber-on-rubber contact, and high reduction gear at the output, produce a reassuringly powerful drive with an infinitely variable ratio.

The friction wheel and motor is mounted on a sliding carriage which is translated by a crank arrangement. This crank is driven by the demand motor via a differential gear. An opposing input also feeds into the differential to counteract the drive demand. This opposing drive comes directly from the output shaft itself, so it automatically re-centres and stops when the output speed is zero.

I've attached a schematic Virtual-Mec drawing to illustrate how this works, as well as a few photos of the VSD final design. This final design has evolved following numerous failed design attempts, some of which are also shown here for interest.

I experimented with different disc sizes and carriage drive systems. Earlier versions also had a third motor to carry out the "centring" function. This caused all sorts of problems with regard to sensing and maintaining the centred position. It used microswitches to stop the motor at dead centre, and reverse the motor as and when required. These switches were frustratingly difficult to set up and adjust - nigh on impossible in fact. My eureka moment was when I realised that I could use the output shaft to perform this function flawlessly, doing away with the extra motor and the troublesome limit switches.

Earlier versions still, employed a larger drive disc (the thinking here was to provide a wider speed range and therefore better speed control). There's a trade-off here between disc size and speed, and it's worth experimenting to get the best of both. Smaller discs give a more compact size, but probably require higher speeds which is likely to cause more vibration and noise. The key question is, what final output speed and power is required?