What is the difference between open-loop throttle position control and closed-loop speed control?
In open loop only the throttle lever position is driven; in closed-loop speed control the engine speed is measured and the throttle command corrected to it.
The difference lies in what the system measures. In open-loop throttle control the kit only drives the position of the throttle lever; how many revolutions the engine is turning at is not measured. In closed-loop speed control there is a sensor that reads engine speed and the throttle command is corrected against that measurement. The GS740 and the GS740-3P drive position; the closed loop on the GS740-3P is over actuator position as well, not over engine speed.
Position control and speed control are not the same thing
- Position control: "hold the throttle lever at this angle". The GS740-3P closes that loop with an AS5600 encoder; that is, the actuator stays at the commanded angle despite the resistance of the cable. The quantity measured is angle.
- Speed control: "hold the engine at this rpm". That calls for a separate sensor reading engine speed. When the load rises the throttle opens automatically, and when it falls it closes.
The wrong expectation forms at exactly this point: a system being "closed loop" does not on its own mean the speed will be held constant. The question to ask is this: what is the quantity being fed back? The AS5600 magnetic encoder reads the angle of the actuator; a speed feedback that reads the speed of the engine is an entirely separate sensor.
How the two systems behave when the load changes
In open loop the lever stays at a fixed angle. When the load rises the speed falls somewhat; that is not a fault but the natural behaviour of the engine's own governor. A speed drop under load is the expected result, and the operator makes up for it by opening the lever a little further. In closed-loop speed control that compensation is made by the system rather than by the operator: the speed reference is held constant and the throttle position shifts of its own accord to suit.
Which is enough for which application
| Application | Load behaviour | What is enough |
|---|---|---|
| Water and irrigation pump | Almost constant | Throttle position control |
| Mud pump in drilling | Slowly changing | Throttle position control |
| Generator needing mains quality | Sudden and variable | Speed feedback |
In practice the distinction rests on this: the relation between speed, flow and pressure on a mud pump changes slowly and the operator can keep up with correcting the setting by hand. On a generator that needs frequency stability the change is faster than the reaction time of the operator; there, throttle position control on its own is not enough.
When neither of them is enough
Neither open-loop nor closed-loop position control takes the place of the engine's own governor; the kit does not take the place of the governor. Overspeed protection, fuel shut-off and the risk of engine runaway remain the responsibility of the engine's own equipment. For the GS740, speed feedback is provided by adding a sensor that reads engine speed together with the optional active feedback speed holding module. The standard kit does not include it.
What to do
Watch how variable your load is first: how far does the speed drift on its own over a shift, and how fast does that drift happen? If it is slow enough for the operator to correct by hand, throttle position control does the job. If the speed has to stay within a narrow band without operator intervention, include the speed sensor and the feedback module in the system from the outset when you ask for a quotation.
The full topic: Electronic speed and throttle control on diesel engines