Servo Actuated Auto Throttle

Quick links: Throttle Mods | EPIC code for AT

I use a Thrustmaster® Cougar HOTAS Throttle, modified with a Hall sensor. The throttle is a faithful copy of the Block-50 throttle used in the F-16. It is not what you would find on a B737, but I like it for its ergonomy and implementation of the HOTAS concept (Hands On Throttle And Stick). And it fits beautifully into my F-16 sized cockpit.

The movement of the throttle lever in auto throttle mode is achieved with a powerful electric motor and a reduction gearbox, connected to a shaft-mounted electromagnetic clutch. When the clutch is engaged, the throttle lever is driven by the motor. If the clutch is disengaged, the lever can be moved manually. In manual mode the thrust lever only moves two gearwheels and the armature of the clutch at a reduction ratio of 1:2. Since all gearwheels are mounted on ball bearings, they do not produce much resistance when the throttle is moved manually. They do however convey the rather authentic feeling of something mechanical moving inside.

All gearwheels are Module 1, made either from nylon or steel. The following photos and annotations describe the design and building process in chronological order.


First design study. The motor has a 1:810 reduction gearbox and runs at 3v for smoth (and controlable) throttle lever movement. A clutch with a rubber inlay will be mounted between motor and electromagnetic clutch to provide smoother starting and stopping.

One of the early designs. It took me some time to get a feeling for how the gearwheels should be grouped. Finally, I went for a 1:2 reduction between electromagnetic cluch and throttle lever. This increases the max. torque at the throttle to 3,6 Nm (The EM-clutch is rated at 1,8 Nm). The 40-teeth gearwheel in the middle only acts as a spacer to bridge the distance between throttle and cluch gear wheels.

The Excel sheets contain calculations of RpM's and deg/sec variations at different voltage settings. I also used these tables to figgure out which geargbox to use with the motor and to try out different gear wheel combinations.

The same thing, some days later...

The motor, mounted vertically, is outlined in red.

... seen from the other side. The gray tube in the throttle housing is the axis of the Thrustmaster TQS, with potentiometer attached.


...the 3rd perspective, a view from above.

The components are color coded for clarity's sake, but I am aware of the fact that it might be a bit difficult for the casual observer to figgure out how all the pieces are supposed to fit together.

I was amazed to find out how difficult it was (for me!) to design even a simple gearbox. Quite a number of things can - and will - go wrong during assembly, with pieces not fitting and being in each other's way. I tried to think of everything during in the designing phase - and failed!

So, after disassembling the box for the 5th time to fix yet another problem, patience became an essential virtue...

Modification of the throttle lever, first step.

There is a plastic washer attached to the end of the axix (close to the stock pot). It serves as a guide to limit lateral movement of the lever.

The washer is attached to the axle with a small screw. Because of this, I had to use a spacer disk that provides an even surface for mounting the gear wheel at the end of the lever. The stock pot of the Cougar throttle was in the way, so I had to remove it (together with its metal base). Not a big deal, since it was to be replaced with a Hall sensor anyway.

A close-up of the spacer and gear wheel that will be fixed to the end of the throttle axle. The upper half of the gear wheel would not fit inside the throttle case, so I had to remove the teeth in that part to reduce the diameter.

All parts are made from Nylon.

The throttle lever with the 30 teeth gearwheel attached. I used a custom made bolt to center the gear wheel on the throttle axle while drilling the holes for the four M3 bolts.



Same thing as above, from a different angel.

On the left is the armature of the EM clutch, allready connected to a gear wheel. I used a 60/15 reduction gearwheel and cut the 60-teeth part off in order to match the size of the metal armature, i.e. 43 mm. With the help of two ball bearings it rotates on the same axle as the the clutch stator. Between the metal armature and the plastic gear wheel is a prestressed disk spring which allows for movement of the armature when the magnet of the clutch is powered, thus connecting the two parts of the cluch for torque transmission. This is a difficult part to manufacture, since the tolerance for centering is only 0,03 mm.

Below left is a prototype of the nylon pieces that hold the ball bearings in place on the gearbox walls (outlined in green in the drawings above).

One of the gearbox walls, made from 2mm aluminum. All axles turn on ball bearings. These are held in place by nylon spacers. The spacers are only pressed into openings in the aluminum wall.

The position of gearwheels and the clutch assembly on the axles is fixed either with safety clips (that's what the grooves on the axles are for) or - in places with enough space - with safety rings.

The walls of the gearbox are connected to each other with L-shaped brass or aluminum profiles.

Close-up of the gearbox. The lower gear wheel is connected to the armature of the clutch. Note the ball bearings at the center of the armature. They allow that piece to rotate freely on the axle. Without this, manual movement of the throttle would not be possible (nor would the clutch itself make any sense).The rest of the clutch is not yet installed.

As the building advanced, I started to realize that the whole construction is probably an overkill in strength and sturdiness. But I wanted to get things right at the first attempt, so I applied the principle: Better save than sorry!

The small diameter axle above the clutch holds the anti-rotation tag of the stator in place and thus prevents rotation of this part of the clutch.

The aluminum profiles sticking out on either side of the gearbox will be fixed to the wooden support of the throttle case with three M3 bolts each. Exact alignment is of utmost importance to ensure flawless connection between the gear wheels on the throttle axle and the gearbox.

For Information on how to get rid of EM interferences from DC motors click here.

The Cougar is a USB-based system with no link to EPIC. Neither gearbox nor throttle contain a dedicated position sensor. So where does information on the actual throttle position come from?

As it happens so often in Cockpit building, Pete Dowson's FSUIPC comes to the rescue: It provides an offset that allows to read the value of the throttle pot during (and independent from) Project Magenta's MCP auto throttle operation. It just provides raw position data with values between -16843 and +16843. My EPL code compares these values to the AT value commanded by the MCP (yet another offset), and the lever is moved accordingly.


The base plate of the throttle housing was cut open to allow connection to the gearbox / clutch assembly below. Note the gear wheel inside.

The wooden base with the servo assembly attached underneath. The white gearwheel on the right drives the throttle lever. The throttle housing is connected to this base with four M6 bolts.




The relay card. It controls the servos and clutches for both Auto throttle and speed brake. For reversing the direction of the servos, I used double switching relays. The motor is turned on and off by another relay (see switching diagram). A close-up of the relay card - both sides - can be found here.

For easier disassembly in case of repairs, all connections are done with jacks. The flat cable with EPIC related wires (data leads, ModRows and 32-point-output leads) connects to the Mechanics panel. From there, a cable harness with DB-25 connector goes to the distribution cards.

The Auto Throttle, attached to it's wooden base.

The clutch is engaged only during lever movement. The lever can be repositioned manually, as soon as the motor stops. But as opposed to the real B737 autothrottle, my system has no manual override. Manual movement of the throttle lever during AT operation has no effect on the engines. Instead, the control code will immediately engage the clutch and reposition the lever to the position commanded by the MCP auto throttle system.

The EPIC code for the auto throttle lever movement can be found here.

Click on the image to the left to see a video that shows the motorized throttle in action (B737 take-off from CYUL). Don't pay attention to the outside views. They appear heavily distorted because of the position and wide angle setting of the camera used to shoot the clip.

Duration: 4:43,Size: 24,5MB, Format: wmv