What is the difference between open-loop throttle position control and closed-loop speed control?
In open-loop throttle control the system only drives the position of the throttle lever; it does not measure at what speed the engine is turning. In closed-loop speed control a speed sensor reads the rpm and the throttle command is corrected according to that measurement. The GS740 and GS740-3P drive position; the closed loop of the GS740-3P is also on actuator position, not on engine speed.
Two concepts need to be separated:
- Position control: "hold the throttle lever at this angle". The GS740-3P does this as a closed loop with the AS5600 encoder; that is, the actuator stays at the commanded angle despite cable resistance. The quantity measured is angle.
- Speed control: "hold the engine at this rpm". This requires a sensor that reads engine speed. When the load increases the throttle opens automatically, when it decreases it closes.
The practical result: on a pump working at constant load, open-loop throttle position is more than sufficient. In applications where the load changes continuously and the speed must stay constant (for example a generator that requires grid quality), speed feedback is mandatory. For the GS740 this 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.
Full topic: Electronic speed and throttle control on diesel engines