electrohydraulic circuits

ELECTROHYDRAULIC CIRCUITS

CONTROL OF A HYDRAULIC CYLINDER USING A SINGLE LIMIT SWITCH

1. Circuits and Construction

An electrohydraulic system which uses a single solenoid valve and a single limit switch to control a double-acting hydraulic cylinder is illustrated in Fig. 14.10.

Fig.14.10(a) shows the hydraulic circuit diagram and Fig.14.10(b) shows the electrical circuit diagram, also known as the ladder diagram.

The hydraulic circuit [Fig.14.10(a)] consists of a single solenoid valve (which is labeled as SOL A) and a single limit switch (which is labeled as 1-LS). The ladder diagram [Fig.14.10(b)] use one relay with a coil (designated as 1-CR) and two separate, normally open sets of contacts [labeled as 1-CR (NO)]. Also two push-button switches, ofte normally closed (labelled STOP) and one normally opened (labelled START) are included.

2. Operation

The sequence of operation is as follows :

Extension stroke

1. When the START push-button switch is pressed momentarily, the relay coil 1-CR gets energized which closes both sets of contacts of 1-CR.

2. The upper 1-CR set of contacts serves to keep coil 1-CR energized even though the START push-button switch is released.

3. The lower 1-CR set of contacts closes to energize solenoid 'A to extend the cylinder.

Retraction stroke

4. When the extension stroke of the piston reaches its end, the piston rod cam actuates 1-LS (NC). Now the 1-LS opens to de-energize coil 1-CR.

5. The de-energisation of coil 1-CR reopens the contacts of 1-CR to deenergize solenoid A. This returns the valve to its spring offset mode and the cylinder rétracts.

Cycle of operation

6. The retraction of cylinder again closes the contacts of 1-LS but coil 1-CR is not energized because the START push-button switch and upper 1-CR contacts have returned back to their normally open position. Thus the cylinder stops at the end of the retraction stroke.

7. The cycle of operation can be repeated by pressing the START push-button switch. manually. The STOP button is actually an emergency button and whenever it is pressed, it will immediately stop the extension stroke and fully retracts the cylinder back.

Note 

The above circuit is designed using oil as working medium.