Drive unit
of the ornithopter model EV8
Ball screw drive
For transforming a rotating motion into a linear motion a
ball screw is used for the
EV8. Paul
MacCready has used a corresponding technology for his pterodactyl
replica QN
in 1985 (For details
about his drive principle please take a look at the
articles
(in German) about his project).
For the hereby chosen dimensions a revolution reduction of 38:1 is included - based on one flapping cycle. The efficiency for both conversion directions constitutes of about 0.9. These excellent properties were the crucial factor to renew the drive development.
But therefore two motor reversals of rotation are necessary in every flapping cycle.
Note:
Instead of the ball screw drive a ball reverser actuator may
also be used. Then, the electric motor must not change the
rotating direction (Please take a look at
external link 1).
Spring for Compensation
Instead of the inflatable pneumatic spring with a rolling-diaphragm usually used in the EV-models (please look at drive mechanism of the EV4), an industrial gas spring is used in the EV8. Unfortunately, its spring force is not adjustable.
With great spring forces in connection with a small spring rate gas springs offer advantages to steel springs. They have a smaller design and lower weight - but also higher losses.
With an effective energy recirculation via the ball screw drive and the
speed control in the battery packs this so-called compensation spring
can later be omitted.
Motor
At the beginning of the EV8 development a brush less motor was planed - at that time still with sensors - because of its relatively small mass moment of inertia. But after model completion there was no speed control with a sufficient short delay at reversal of rotation. Therefore a motor with carbon brushes had to be used instead.
Today, there are also speed controls for brush less motors with a sufficiently fast reversal of rotation.
Electronic
To regulate the speed control with exact wing positioning in glide position, speed modification, reversal of rotation etc. is very laborious.
In the course of testing the shown test setup the decision was made to accomplish it with a programmable logical modul (also called PLD or microcontroller). Lacking sufficient programming knowledge I could only substitute about half of this hardware with it at first.
DownloadBASIC program
(zip 8 KB)
But in the meantime microcontrollers have become so capable, that even with a simple BASIC programming language all can be stored in only one chip. With the pictured storage-programmable control device one can control drives for flapping frequencies up to 8 Hz.
Single components of the driving mechanism
In front, one can see the swing
(frame of plywood)
usually used in EV-models for
load balancing to the wing adapter rollers. Behind this,
there's the chassis of the mechanism first conceived for
the EV8.
Chassis
with screw drive, springs and guideway, but without the motor.
It made the installation of the drive as a whole in the fuselage
and its removal possible.
The gas spring is used as guideway for the steel spring.
Drive
Front view
Drive
Back view
Bottom view
The brake mechanism as shown above have already been changed during the tests of gliding flight. The braking forces have been too small for high starts.
At the same time the connected digital rotation counter has been replaced by an analog position encoder (poti).
Complete driving mechanism, date 1997
The drive unit can be mounted relatively simply in and out of
the fuselage as a complete unit.
Weight 440 g [15 oz] (without wing adapter rollers)
The whole electrical equipment is placed in the front part of the fuselage.
Design
The whole design of the EV8 was made with CAD. Thereby, a very compact construction could be obtained with passable effort.
External link
- Product data sheet
Ball Reverser Actuator
by the company Norco Inc. (USA): http://www.motiontech.com.au/assets/pdf/Ball%20Reverser.pdf (0,2 MB)