Input-Input!

Redesigned Spade Grip

Introduction

We’ve been hard at work to research the very best simulator control options. Our selection criteria was based on a balance of the following attributes:

  • the best accuracy
  • the easiest installation and setup
  • low cost

There are just such a plethora of options available that it became important to understand what the benefits of each are and work from there. The control options were categorised along the following lines:

  • Inputs – the sensors providing information to the control devices
  • Control Devices – the controller hardware which receives the Input information
  • Control Software – the software that is required for the Control Devices in order to interpret the Inputs and provides the simulator with information it can understand.

As indicated in various previous posts, there is no need to read information from the simulator to control external monitors, gauges and the like. Given that our simulator is built around VR interaction, our controls only need to provide a one direction feed to the simulator. In other words, we are creating a giant joystick interface.

The following is a summary of our findings on means of input. We will post separately on the control devices and control software.

For inputs we differentiate between Digital (as in switching) and Analog.

Digital Inputs

The Digital inputs can be:

  • Physical –
    • Toggle switches (Lever and Rocker),
    • Pushbuttons (momentary, latching and interlocked latched) and finally
    • Rotary switches.
    • In our design we use both Toggle switches and Pushbuttons but not Rotary.
  • Hall Effect – This is switched by the proximity of a magnet. They can be:
    • Latching (switch stays in position even when the magnet is removed, and can be either
      • pole sensitive where you need say a South Pole to activate and deactivate the switch, or
      • Non-pole sensitive, where any pole will do.)
    • Non-latching (switch flips as soon as the magnet is removed). Again, these can be:
      • Pole sensitive – examples are the Allegro A1101/2/3/4/6, which switch on South Pole only
      • Non-pole sensitive – example the UTC UH8104
    • In our design we use exclusively the UTC UH8104 non-latching, non-pole sensitive switches for the proximity sensing of the following:
      • switch cover is open or closed
      • fuel cut-off position
      • gunsight dimmer slider position
      • chassis control lever position
      • (the incorporation of these are currently undergoing redesign)
    • Finally it should be noted that Hall Effect Sensors come with through-the-hole (SIP3) wiring or as SMD (Surface Mounted). The latter makes it a pain to connect so we have stuck with units that are available in through-the-hole.
  • Encoders – These send on/off signals or pulses through two channels. The signals between the two channels are offset in a certain pattern depending on which way the encoder is being turned. It is then up to a controller to determine the direction and speed and provide that information to the simulator. It really provides multiple button or key presses and they are ideal for controls where there is no set number of rotations. An example would be setting a clock or altimeter. We have settled on a Bourns PEC11R which has a 12mm threaded through panel mount. They provide between 12 to 24 pulses per 360 degrees.
Linear Hall Effect Sensor for Brake

Analog Inputs

The Analog inputs serve axes of movement, such as trim, rudder, elevator, throttle etc. They can be linear, i.e. slide forward and backward, or rotary/angle sensors, measuring number of degrees turned. Most often the role for these were fulfilled by mechanical potentiometers, which slid across a resistive material thereby providing an indication of its position. These are being largely replaced by solid state Hall Effect sensors.

The advantage of Hall Effect sensors is that they provide a very steady, accurate signal, whereas potentiometers very often provide a bit of a quivering signal. A potential problem with Hall Effect sensors is that they send data in serial bitstream, which adds complexity and increases the real-time processing requirement to interpret these signals.

In the last few years however these have become more and more friendly, some providing an analog output based on a variation in voltage, typically from 2,5V to 5V.ย  These can then be simply used as straight-up replacements for potentiometers.

So for our design we have decided to keep it simple yet very accurate by replacing all rotary potentiometers with the Melexis Tri-Axis MLX90371 (Gen III). These are tri-axis – they provide linear and rotary on-axis or off-axis. They also come in SIP3 Through Hole and provide an analog output. It should be noted that they measure 360 degrees, so for instance for the elevator trim where 4 turns are required end to end, we will be designing printed gears.

There are a few applications where a linear Hall Effect sensor will be used, eg. the brake lever. Replacing the original design, which had a very expensive Bourns 3046 linear pot, has simplified matters greatly through the use of an Allegro A1324 linear Hall Effect Sensor, again with analog output.

Tri-Axis sensor at the bottom of the Mk.II Gunsight

All under control..

This week we have been auditing all the different controls and came up with the following statistics:

Pushbuttons: 30

Toggle switches: 32

Incremental Rotary Encoders: 7

Potentiometers: 5

Absolute Rotary Encoders: 4
Extract of all the control elements

We have also been assessing control options and intend providing guidelines as to how various different open source programs may be applied in conjunction with variousย  low cost controllers such as the Arduino.

As far as possible we will be looking at substituting the potentiometers for absolute rotary encoders for simplicity, robustness and accuracy. While slightly more expensive than potentiometers they provide a lower life cycle cost and better performance in the long run.

Thus work continues with the refinement of these options.

SOME DESIGN DETAILS

We can share some of the last control elements which were finalised over the last two weeks.

Windscreen De-Ice System

The windscreen de-icing system consists of a reservoir containing a 50/50 mix of distilled water and glycerine glycol, which can be manually pumped to a perforated distribution tube positioned in front of the windscreen. A small regulator valve next to the pump regulates the amount of flow while a cut-off valve returns the fluid to the reservoir when placed in the OFF position. All in all the associated brass tubing gives a wonderful steam locomotive effect to the Spitfire cockpit, which is quite unlike any of the more modern layouts seen in say the Mustang or Focke Wolf 190.

 

Identification Friend or Foe

The Spitfire Mk.IX was fitted with a radio transmitter/responder (transponder) sending coded signals when interrogated. The advent of radar made it essential to identify whether aircraft were friendlies (the system is unable to confirm whether aircraft are unfriendly). Early in WWII there were a number of incidents on both the British and German sides where forces were attacked by their own. The depicted Mk.III transponders were designed by engineer Freddie Williams to cope with the new radar technology introduced after 1940. It responds to specific ‘interrogators’, rather than replying directly to received radar signals.

The model also has two pushbuttons which, when pressed simultaneously, will destroy the transmitter to prevent it from falling into enemy hands.

We were also able to modify a modern NKK rocker switch to resemble the Air Ministry standard toggle switch and will be applying these to other functions as appropriate. They are 3D printed units from SLS Nylon and will also be available with all the other printed items from Shapeways when the designs have been finalised.

 

Other Controls

Oxygen Valve

Carburettor Air Filter Control

 

Linkage system for Engine Hand Control

End of the beginning…

This Sunday, the seventh, will be a year to the day when this project was started. With it, a milestone has been reached. The basic designs are complete.

The words of Winston Churchill come to mind.

“Now this is not the end. It is not even the beginning of the end. But it is, perhaps, the end of the beginning.”

So what comes next?

The next six months will be dedicated to the building of the prototype and refining of the designs. The target is to have the Spitfire Mk.IX Simulator design pack and components ready for sale by the end of 2018.

But what will it all cost?

Difficult to say what the actual build will cost at the moment, but I am targeting an overall material and component cost of below $5000 for the full simulator. That excludes the computer and VR hardware which could add an additional $2000 odd for a very high end rig. The DCS World simulator is free for personal use with an additional $49 or so for the Spitfire Mk.IX module and similar amounts for editions of various terrains etc. Your labour in assembly and building is assumed to be free ๐Ÿ™‚

The full plans set with cutting and bending patterns, assembly instructions etc. will be sold for under $300.