ACU300 and solenoids

[Derek - Raymarine - Moderator]

Some hydraulic steering systems require a higher flow rate, to operate the steering cylinders at the correct speed to steer the vessel correctly, than can be provided by the standard reversing pumps.

A standard reversing pump moves the steering cylinders to and fro by reversing the direction of flow of oil to the cylinder by simply reversing the direction of rotation of the electric motor. This will only work satisfactorily up to a certain cubic capacity of hydraulic cylinder. Above this capacity, the flow rate required to achieve the correct hardover time would require a large electric motor and the power required would be too large for a reversing electric motor. You can read more about reversing hydraulic pumps here
For more information about hardover time click here

In this case, constant running hydraulic pumps can be used. These can be in two forms, either an independent electrically driven constant running pump, which has a constant supply of high pressure oil always available and the flow is directed by the solenoids which are operated by the ACU300, or there could be an engine driven hydraulic pump, which operates when the engine is running and it too provides a constant source of high pressure oil, which can be directed by solenoids.

The ACU300 can be used to control the solenoids fitted to constant running pumps manufactured by other suppliers, or to control solenoids fitted to a hydraulic manifold pressurised by an engine driven pump.

In each case, the key essential for autopilot operation is that the flow rate is correctly adjusted to provide the correct hardover time for the vessel and the steering system.

However, it is important that the solenoids in each type of installation conform to the following specification

The pull in Voltage - greater than 8V (16v for 24v systems)
The drop out Voltage - less than 2V
The operating Current - less than 5A
The Solenoid Voltage must be the same as the course computer.
The response time of the solenoid valve is less than 150 to 180 milliseconds

The pull in and drop out voltages mean that the solenoid should effectively turn off or de-power once the voltage is below 2v - thus closing the valve. Also the solenoid will not activate until the voltage reaches 8V, (16v for 24v systems), thus opening the valve, this allows to voltage hysteresis in the solenoids when opening and closing.

Please also ensure that 'flyback' diodes need to be fitted in parallel with the solenoid coils

These diodes prevent damage to the ACU300 caused by the back emf voltage from the solenoid as the coil is energised and then de-energised.

There is a very detailed explanation of flyback diodes if you want to read up more on this.

Note the oil viscosity, flow and pressure all effect the response time of the solenoids.



RAYFAQ#