A BRIEF LOOK AT AUTONOMOUS FLIGHT

One of the most interesting developments in R/C modelling in recent years has been the to trend towards stability augmentation and on to the next step, full autonomous flight.

With the successful crossings of the Atlantic, first by Aerosonde and later by Maynard Hill and his team, it has been demonstrated quite forcefully that the technology for long range, autonomous flights by model aircraft is now well and truly within the reach of the advanced modeller.

SECURITY AND SAFETY CONSIDERATIONS

Setting up a small UAV is quite simple and relatively inexpensive using some of the currently available off the shelf modules. For example the autonomous control system used in the 80" span Silvertone Weightlifter shown below cost approximately AU$2000.00.

One of the major difficulties facing the budding UAV operator is convincing the suppliers of equipment and relevant governing authorities of two things:

(1) That you are not a terrorist intending to use this equipment for nefarious purposes.

(2) That you are a responsible operator who will not fly your autonomous aircraft in a manner dangerous.

Here at Silvertone, in the 56 years that we have been in business, we have had several instances of our equipment being used for illegal purposes. These incidents involved us in some messy entanglements with police and other agencies. So please bear with us if some of the questions we ask when first embarking on UAV business arrangements seem a little pointed.

One of these incidents arose out of a series of anonymous letters being sent to the NSW Police Anti-Terrorist squad. These letters claimed that one of our customers was going to use some of our equipment to bomb the Sydney Olympics in the year 2000. It was all nonsense of course, but the authorities must take these threats seriously.

The dangers of this type equipment being misused are quite real. But then, that is true of all technology. Mobile phones are used to detonate explosives at precise times and locations in public places. Airliners are used to knock down buildings. Alongside an Airliner, a 10kg model is insignificant. However for some reason, government authorities are particularly fearful of technology capable of delivering a payload, however small, to a point anywhere on Earth. Therefore please respect our need for caution and concern when embarking on new business ventures.

With regard to point No 2 above, many Model Aircraft Governing bodies virtually ban autonomous flight by placing these types of models outside their insurance specifications. Such is the case here in Australia. The MAAA has ruled that models must be flown manually in full sight of the operator to meet their insurance requirements.

Obviously then becoming a UAV operator is a task not to be undertaken lightly. It is also obvious that we share the concerns of all authorities and we will involve them, and co-operate with them, to the best of our abilities if we become suspicious of any customer’s intentions. In the case of the Olympic incident we immediately dismantled the model in question and hid key components in various locations until after the Olympics. This was at the request of the Police and as a precaution that the model was not stolen and used without our knowledge.

UAVs are a serious business and it is most important that everybody understands clearly from the outset, what is involved and where we all stand in regards to security and safety.

With that little bit of unpleasantness behind us, let us now move on to the business at hand. Namely how does one make these little marvels of technology work?

SILVERTONE WEIGHTLIFTER

A typical example of a small UAV is the Silvertone Weightlifter shown below. The following paper is intended to provide some background information on setting up such an aircraft.

Silvertone Weightlifter Mini UAV. Fitted with GPS navigation, optical roll/pitch stabilisation, Altitude hold and TV downlink. This aircraft flies at 18lb.

View from rear showing large area flaps. Military versions of this aircraft have flown at 45lb AUW with larger engine and strengthened U/C.

The aircraft discussed in this paper is one of a batch of Silvertone Weightlifters built in the mid-1970’s. Of these only two are left flying. One is with the DSTO in South Australia and other with Silvertone in Sydney, Australia. The Weightlifter is an 80” WS monoplane fitted with an OS80RV twin glow plug motor. All up weight is 18lb of which 6lb is payload.

The Weightlifter is an ideal UAV trainer. Stable and easy to fly, the Weightlifter fits easily into any small UAV project. Designed to fly at 18lb, some examples fitted with larger engines and strengthened undercarriages, have flown at 40lbs. Landing speed at 18lb is approx 17mph with lift-off occurring at approx 19mph with take-off flap. Cruising speed is approx 50mph with the OS80RV.

A batch of Weightlifters awaiting delivery circa 1975

The Weightlifter is fitted with a series of modules, which together comprise the autonomous flight system. The GPS steering unit, the altitude hold module, an FMA IR attitude hold module (or alternatively a Silvertone UV AH), throttle Fail-Safe and serving as the downlink, a real-time video system fitted with a video overlay. The overlay displays groundspeed, altitude, compass heading, GMT time and GPS location. The GPS receiver, the two autonomous control units and the attitude hold when combined, form the autopilot and are designed to plug into a standard R/C airborne system between the receiver and servos. The overlay is switched in or out between the camera and the video transmitter.

Block diagram of the Weightlifter featured showing disposition of control units.

Since these modules function as control rather than augmentation (assist) units, they are either in complete control or by-passed. None of the units are designed to provide "mixing" of manual and automatic control.

Each module has an enable channel input so that the operator can bypass the automatic operation and operate the servo directly by normal, manual command. This enable channel can be plugged into any spare channel in the system, preferably a switched channel. Several of the units and an attitude controller (wing-levelling optical or gyroscopic unit) can all have their enables "Y'd" together using the same enable channel for all functions. Therefore with the flick of a single switch, all units can be enabled simultaneously. With some radios (especially the modern receivers featuring long-lead filtering) a buffer may be required between the receiver and the enable inputs. This is especially true if the enable inputs are "Y"ed together. A single 4050 hex non-inverting buffer will supply all of the buffering required. In practise, it is a good idea to use separate channels for each of the enable inputs if there are sufficient channels available, as it makes initial set-up and trimming much more simple. Trying to sort out an aircraft with three fully interactive automatic control devices operative can be a daunting task. Better to tackle one at a time and gradually build up to the full system.

View showing twin glow plug OS80RV, pipe, FMA sensor and GPS bay cover	GPS cover removed showing Garmin III GPS receiver with antenna and separate battery pack.

The Altitude hold module operates as a set and hold unit. Thus to program this unit, the model is flown manually to the altitude required and then the unit is enabled. The current altitude is then stored in memory and remains as the default (fail-safe) setting. To re-program the altitude, the ATH is disabled and the aircraft is then flown to the new altitude. Enabling the module then stores the new setting. Upon loss of signal, either accidental or deliberate, the module will default to the last setting.

To get the modules to automatically take control when the R/C radio loses command signal, you need to use an R/C system such as PCM that comes with a built-in fail-safe (preset) feature. The modules can also be used with standard AM or FM (PPM) R/C radios but to get the units to enable automatically, you will need to add a "missing pulse detector" (P.O.D) type fail-safe accessory. The type of encoding (PCM, PPM) is not relevant, only the fact that the radio has a built-in fail-safe feature. All units can be enabled with a single POD fail-safe module when used with PPM systems. This module is plugged into the enable channel; between the receiver and the auto-flight module enable input. In the case of a PCM receiver, the enable channel is programmed for fail-safe enable.

One important point here in regards to the type of PPM receiver, It is absolutely essential that the receiver be fitted with a reliable audio muting circuit otherwise noise from the IF stage with no carrier present will pass on to the decoder and prevent the missing pulse detector from moving into the fail-safe mode. PPM receivers such as the Silvertone Mark 22 or an IPD receiver are quite satisfactory in this regard.

Above: Weightlifter with Silvertone Mark 22-MIL multi mission transmitter. Note variety of antennas fitted. Internal telescopic, 1/4 whip and BNC fitting for remote 50ohm antennas. Right: Close-up of Navigation control group. Gain (sensitivity) knob for U/V autopilot flanked by enable switches for the GPS steering module and altitude hold.

Note: Separate switches simplifies initial trimming.

The altitude hold module is a boon to pupils and instructors during ab-initio training. Flying an aircraft fitted with altitude hold and attitude hold is like taxying the model on top of a glass ceiling. It is quite an eerie feeling for someone who learned the hard way and remembers chasing the model all over the sky when learning. When steering with rudder only the model feels completely crash proof. It is the solid feel of the model that is so remarkable and it is very easy to visualise a sheet of glass under the wheels.

Using the GPS module in conjunction with a GPS receiver, a wing levelling unit (optical or gyroscopic) and an altitude hold makes it possible for an aircraft to be sent off on a fully automatically controlled mission to any point within range of the aircraft. Manual control via the transmitter is only required for take-off and landing. The transmitter may be switched off for the rest of the flight.

Payload for a simple UAV. From top to bottom, left to right. GPS receiver, battery pack (6V), Optical (UV) roll/pitch Attitude hold, GPS steering interface module, Fail-safe module, Altitude hold module and Silvertone Mark 22-SM receiver

Underside of Weightlifter showing the mushroom shaped U/V sensor head for the optical roll/pitch stabiliser on the left and the 2.4GHz TV antenna on the right. The U/V sensor looks between the tuned pipe muffler and the fuselage, which shades it from the Sun. This sensor must not be mounted above the fuselage.

The microprocessor based GPS steering module receives output data from a handheld GPS receiver and converts it to an R/C servo position command. Your GPS receiver performs the navigation calculations and manages waypoints and routes. Simply connect the handheld's PC data cable to the GPS module and it will translate the track/bearing error into a servo position command. This module also corrects for cross-track error so it will stay on course for long distance navigation. It has an enable input for transparent pass-through control, and a Gain adjustment from the transmitter. The GPS module and a handheld GPS receiver all that are needed to steer a boat, ground vehicle or stable aircraft to a waypoint. Add an attitude hold and an altitude hold and you have a complete aircraft control system at a fraction of the cost of a traditional autopilot. The GPS module is designed to be a functional component of an unmanned guidance system and its low cost makes it ideal for expendables.

VIDEO DOWNLINK

The type of system outlined above is ideal for extended range missions, such as aerial photography, fire detection, agricultural survey etc. In keeping with the requirements for these missions, the Weightlifter is fitted with a real time video downlink. The block diagram below shows the basic layout of the various components.

Block diagram of the real-time video downlink installed in the Weightlifter.

The heart of the surveillance system is the video camera and we recommend the use of the best that can be justified under the project budget. All of the frame grabs shown on this web site are taken with a 380-line simple security camera and are of quite poor quality as a result. We are about to take delivery of a 620-line high-resolution camera not much larger that the 380-line model currently used but quite a deal more expensive. This should improve picture quality dramatically. Vegetation suffers very badly with low-resolution cameras as the photo below shows quite clearly.

By far the best quality video is obtained with a digital video camera on-board and using a low-resolution real time mini camera as an aiming guide. There is always the risk of course of the loss of a very expensive camera but that has to be balanced out by the results obtained. The photos below vividly illustrate the difference in results.

Real time frame grab taken while taxying to the landing strip at the UMAC field near Erskine Park, a suburb of Sydney. The new camera is a mini Hi Res 620 line unit measuring just 38 x 38mm. This result is starting to compare favourably with an on board DV camera.

Frame grab taken from an on-board digital video camera. Note the big improvement in picture quality. This clearly shows a lost model floating in the pond. This model was found accidentally while on a routine survey mission. Photo courtesy of Dave Jones, AUAV.

Referring now to the video block diagram the system begins with the TV camera. The video output is fed into a relay switching module, which either routes the video directly to the TV transmitter or through a video overlay unit. This relay is controlled from a separate video control transmitter, along with the signals to control the pan and tilt servos for the camera. The cameraman who is seated in front of the video monitor operates this transmitter. The camera is able to pan through 170 degrees in the horizontal and 100 degrees in the vertical.

View showing latest camera location	Close-up of camera installation

To prevent excessive panning speeds, a dual servo slow unit is fitted between the fail-safes and the pan and tilt servos. This unit holds panning speeds to an acceptable level. Fast panning speeds give a very jerky look to the finished video and can make the viewer quite nauseous after prolonged viewing.

The fail-safes are fitted to serve as set-locks. If the transmitter is switched of in flight the camera will move to the pre-set position and sit absolutely still in order to further enhance the quality of the finished video. All of these refinements are fitted to give maximum flexibility combined with a rock solid finished video. Excessive movement in the finished video is annoying and detracts from the quality of the finished product.

View of CCMAC field Toukley NSW from 850’ AGL	Air to air shot at Malabar NSW 239’ASL
Looking south from the CCMAC field	Looking West from the CCMAC field

A variety of video grabs from various locations. Overlay figures from top clockwise. Compass heading, altitude in feet ASL, GPS coordinates, Time and date GMT, groundspeed in mph plotted from GPS coordinates. All figures derived form the GPS receiver data.

Finally a word on the video receiver antenna. Here only the best will do. As the airborne video transmitter is by law a low power unit a very good antenna is required on the video receiver. We are currently using a 17db yagi hand-held and pointed at the aircraft by an assistant. Both the Yagi and dish antennas are very directional and must be pointed directly at the model. This is a tedious and somewhat boring task for the assistant and their mind often tend to wander. As a result there are blocks of scrambled video in the middle of the clip. A better arrangement would be an auto tracking antenna or possibly an omni directional antenna such as a high gain collinear antenna.

We are playing with new antennas and this one shows great promise in early tests. It is a Vagi or “V” type Yagi. Early testing shows a markedly reduced drop-out rate with very little ghosting. A simple low cost antenna.

Shot taken from 350’ AGL with Hi-Res camera, 12mm lens and Vagi in Vertical Polarisation mode. Excellent clarity and resolution with no evidence of ghosting. The Vagi in photo opposite is in horizontal polarization mode.

We hope this paper has help clarify some of the mysteries of autonomous flight. Be sure to keep an eye on updates of this page.

Here is a preview of some of the items soon to be discussed.

The Observatine project. A web based “Virtual Tourism” project designed to combine technology and tourism in a supremely artistic manner, capturing images highlighting and combining the beauty of nature and flight. Observatine is displayed above.	Head on view of the camera housing under the nose of Observatine. This is the real-time targeting camera used to direct Observatine and the on-board high-resolution camera to the target. Turret features 180^opan and 110^o tilt.
The new Silvertone 16 channel expansion module (top). Simply plugs into our standard 8 channel encoder to deliver a staggering 24 channels with servo reversing, dual rate, ATV all may be programmed for switched or proportional mode.	3D presentation of the proposed Silvertone Flamingo high efficiency long range cruise UAV. Wing span 4 metres with pusher prop, drop-off U/C, wet fuselage, underwing hard points and tip tank attachments.

Stay responsible and keep yourself and others safe.

Happy flying.

FLYING FIELD SAFETY. IT IS PRICELESS.