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  Electrical Engine Throttle and Shift Controls

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Author Topic:   Electrical Engine Throttle and Shift Controls
jimh posted 08-07-2008 08:48 AM ET (US)   Profile for jimh   Send Email to jimh  
Control of an engine's speed and its associated gearcase or transmission has historically been accomplished with mechanical devices, and, despite a lot of excitement lately about "fly-by-wire" controls, in the main mechanical linkages are still being used for engine controls. We investigate more in detail:

Engine speed is typically determined by a throttle plate setting. The rotary motion of the throttle plate shaft is the primary control input for engine speed. On modern engines an electronic sensor (called a throttle position sensor or TPS) is mechanically linked to the throttle plate shaft. The TPS provides an electronic input to the engine management module (EMM) or engine control module (ECM) to communicate to the processor where the throttle has been set. However, the control input which determines engine speed is completely mechanical. Typically a linear motion of a control cable is converted to a rotary motion to actuate the throttle shaft.

Traditionally this linear mechanical motion was provided through a cable which was in turn linked to a remote throttle control level. The motion was again converted from linear to rotary motion, and the input control was provided as a throttle lever. Engine speed has been controlled in this way for many decades. The system looks like this:

Mechanical rotary motion --> mechanical linear motion --> mechanical linkage--> mechanical linear motion --> mechanical rotary motion

As far back as the 1960's, electrical devices were introduced into this system. The principal reason for adding electrical devices is to replace the mechanical linkage between remote controls and the engine. The electrical devices worked in two ways. At the remote controls, mechanical input was converted to an electrical signal. The electrical signal was sent by electrical cables from the remote to the engine. At the engine another device converted the electrical signal back to mechanical motion. The control system now looks like this:

Mechanical rotary motion --> conversion to electrical signal --> electrical linkage--> conversion to mechanical rotary motion

The advantage gained in this electro-mechanical system is the elimination of the mechanical cable linkage. Instead, the remote controls could be linked with just electrical cables. The disadvantages of the new control system are its dependence on electrical power for operation and the inclusion of two new devices, the convertors at each end which change mechanical motion into an electrical signal or vice versa. Instead of simple cams or levers as in the all-mechanical system, the electro-mechanical system requires electrical servos or actuators.

When the control system passes from the mechanical realm to an electrical realm, it becomes possible to implement some features or new controls into the system in an electrical manner. For example, it might be possible to regulate a control setting in an electrical manner. Feedback from the engine could be obtained electrically, and a control loop created to maintain or manage certain functions. For example, engine speed could be regulated electrically. An engine could be made to run at a certain speed, perhaps matching a fixed control voltage or even chasing a variable control voltage such as that from another engine. With the control in an electrical realm, all sorts of added features are possible.

With the invention of modern digital processors, movement of the electrical controls from an analogue system to a digital system became possible. Processing of digital controls is easily accomplished by modern micro-computer chips. This opens up a whole new realm of possibilities. Such a system looks like this:

Mechanical rotary motion --> conversion to electrical signal --> conversion to digital data--> digital communication link--> conversion to electrical signal-->conversion to mechanical rotary motion

If we want to add more processing into this system, we just add a digital processor into the system. It accepts digital input from the control surfaces, processes it, then outputs digital data to the engine control. Now our system looks like this:

Mechanical rotary motion --> conversion to electrical signal --> conversion to digital data--> digital communication link--> digital processor --> conversion to electrical signal-->conversion to mechanical rotary motion

Now that our electrical signals are in the digital realm, we can add a layer of networking to them as well. This will allow integration of other control surfaces, for example a second set of engine controls on a fly bridge. Now our system looks like this:

Mechanical rotary motion --> conversion to electrical signal --> conversion to digital data--> control's digital processor -> digital data network --> engine's digital processor ->conversion to electrical signal-->conversion to mechanical rotary motion

Because engine control is very important, system designers usually provide redundant systems. For linking the digital data, redundant networks are often used. Now our system looks like this:

Mechanical rotary motion --> conversion to electrical signal --> conversion to digital data--> control's digital processor -> primary digital data network IN PARALLEL secondary digital data network--> engine's digital processor ->conversion to electrical signal-->conversion to mechanical rotary motion

When we have multiple engines or controls, we will need to add some supervision to the system in order to maintain order and control. On the control side of the link we will need some supervisory process to determine which set of controls will be allowed to be active. At the engine side of the system we will need some sort of supervisory process to manage which engine is going to be the dominant engine in terms of speed matching or other functions where one engine is supposed to follow another. The system is getting rather complex:

Mechanical rotary motion --> conversion to electrical signal --> conversion to digital data--> primary control's digital processor ->

AND

Mechanical rotary motion --> conversion to electrical signal --> conversion to digital data--> secondary control's digital processor ->

primary digital data network IN PARALLEL secondary digital data network--> supervisory engine control processor -->

primary engine's digital processor ->conversion to electrical signal-->conversion to mechanical rotary motion

AND

secondary engine's digital processor ->conversion to electrical signal-->conversion to mechanical rotary motion

jimh posted 08-07-2008 09:05 AM ET (US)     Profile for jimh  Send Email to jimh     
However, note that in all of the control systems, the initial input and the final output remain in the same form: rotary mechanical motion. Between those two points there is a varying amount of added complexity in the various systems.

In addition, there is likely a great deal of further redundancy in some of the electrical control systems which is not shown. For example, the input devices which convert rotary mechanical motion to an electrical signal are typically rotary potentiometers. To insure reliable operation these devices are sometimes provided in triplicate and a local processor system monitors all three. By monitoring of all three potentiometers, the local control system can compare them and detect any failures. This complexity is hidden from the user who only sees a single throttle handle to use for control.

The great advantage of all the added complexity of the digital control system with embedded multiple processors and redundant data networks occurs when there is sophisticated software installed in those processors to oversee and supervise operation of the engine. For example, control software could prevent an engine from being rapidly moved from forward to reverse when the engine speed was too high for a safe shift of gears.

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