A good base for simulation

If it wasn’t for my huge ego, my experience at Motionbase might have been enough to make me give up messing with anything technical ever again. My only consolation was that Dr Richard Danbury and his team were generous enough to conceal their mirth and understandable derision.

What was it that I was failing miserably at? Flying a helicopter of course.

Not a real helicopter a blessing considering the number of times I backed into mountains, had head-on collisions with the ground and went so far off course that the game gave up on me and left me in helicopter limbo but a helicopter simulator designed and developed by Motionbase.

Motionbase, an off shoot company of Optimised Control, was set up with the intent of developing a system which would demonstrate the strengths of the Optimised Control’s automation and motion control products. It set about designing a motion platform. Optimised Control specialises in synchronising two or more electric motors to provide the required motion.

When Optimised Control was sold to the US 18 months ago, Motionbase become a company in its own right. The motion platform developed is an electric Stewart platform offering six degrees of freedom.

The motion platform, Maxcue, generates all the physical sensations of speed, banking, swooping and crashing has six axes of freedom. Surge, sway, heave and pitch, roll and yaw are generated simultaneously for a realistic ride. These characteristics make for an exciting leisure product but in the more serious areas of professional training they are absolutely essential. For not only is the Maxcue the foundation of that fiendish helicopter simulator, it can also be employed in a number of other less frivolous applications.

Traditionally simulator platforms have been hydraulically actuated: commercial airline simulators are perfect reproductions of the aircraft with all conditions capable of being simulated at all times. They are, therefore, very heavy. Only a huge hydraulic base with its associated problems could manage the weight. The trend now is for part task training so the complete craft does not need to be simulated and defence and commercial payloads are in the region of 1tonne. The lower weight can adequately be supported by a motion platform driven by electric motors.

Hydraulically actuated platforms need a considerable infrastructure, are tough to install and can generate considerable mess through leakage, noise and heat. The electric alternative is cool, clean, easy to install and portable which is important in military applications.

An extremely high level of platform performance is needed in a simulator. After all, if the cockpit doesn’t respond immediately to the controls and in complete synchronisation with the graphical images you could be left feeling not a little unwell, and in training situations a false impression might be created.

A high efficiency electrical actuator unit was designed to meet the performance and safety requirements of Motionbase’s Maxcue. This unit comprises a precision ball screw directly driven by an in-line brushless DC servomotor. The actuator has sub-micron position resolution and a high peak thrust of 13kN per actuator. The actuator is very smooth in operation reversal bump is well below human perceptible levels at 0.02g. It is also very dynamic with a bandwidth in excess of 25Hz (most hydraulic systems are 5Hz to10Hz maximum).

Traditionally an actuator of this type would require an encoder for positioning and to determine the speed at which the ball screw moves. In addition, a sensor is required to provide commutation signals to the servo motor. Motionbase decided this was a solution lacking in elegance, requiring too much wiring, taking too much room and susceptible to lag. What the company needed was a sensor that could control speed, position and commutation in one unit.

Richard Danbury started his search and was rewarded by discovering that sensor manufacturer Stegmann was working on just such a sensor.

Stegmann specialises in two product types: small motor and gearbox assemblies, and encoders, in which the company has 25 years experience.

The development of the Dicoder Motor Feedback System and the Maxcue moved forward hand-in-hand. Stegmann benefited from having a real application to work on and Motionbase from a sensor that really met its requirements.

The resultant CDD 50 Dicoder is easy to install in the rear end of the motor shaft and has just one RS 422 port to connect for both incremental and commutation signals. It is of a remarkably small height adding very little to the overall length of the actuator. As Motionbase was also working on keeping the overall cost of the motion platform as low as possible, it was satisfied with the cost of Stegmann’s Dicoders.

Motion control of the platform is provided by a PC ISA bus motion controller card based on a 32bit floating point digital signal processor, with a benchmark performance of 40 million floating point operations per second. This dedicated processor interfaces with a flight or vehicle model on the PC and performs generation of motion cues and kinematic transforms. The PC card commands each actuator motor via a servo amplifier with current control bandwidth of over 2.5kHz.

The combination of a high performance motion controller and a high bandwidth servo amplifier provides rapid onset cue in response to changes in demanded acceleration. Transport delay the time taken to process vehicle module output and generate the respective motion cue is just 16ms. Transport delay is a key factor in simulator sickness and disorientation.

The platform can be controlled in two modes: direct control of each degree of freedom suitable for leisure rides that run predefined motion synchronised with a film image or real time motion cue generation for training or interactive leisure rides.