Boom cylinder control circuit for construction machine

1. A boom cylinder control circuit for use with a construction machine, comprising:

a boom cylinder which has an ascending-side chamber and a descending-side chamber;

a boom control unit which is configured to provide a working fluid to the boom cylinder;

a boom operation part which is operable to drive the boom cylinder by providing a pilot working fluid to the boom control unit;

a first floating valve which is configured to allow the descending-side chamber and the ascending-side chamber to selectively communicate with or be shut off from a first drain line;

a second floating valve which is in a flow path between the descending-side chamber, which is connected with the first drain line via the first floating valve, and the first drain line, and is configured to:

allow the descending-side chamber to communicate with the first drain line, or shut off discharge of the working fluid from the descending-side chamber to the first drain line, and

allow a reverse flow;

a floating selection operation part which is configured to provide an operation signal so that the second floating valve is switched in a direction in which the first floating valve and the second floating valve are communicated or shut off;

first and second input ports, which are connected to the descending-side chamber and the ascending-side chamber, respectively, and are at one side of the first floating valve;

a first output port, which is connected with the second floating valve; and

a second output port, which is connected with the first drain line, wherein the first output port and the second output port are at the other side of the first floating valve.

2. The boom cylinder control circuit of claim 1 , further comprising:

a floating selection valve which is connected to a boom descending signal line of the boom operation part,

wherein the floating selection valve connects the boom descending signal line to a descending pressure receiving part of the boom control unit, and connects a pressure receiving part of the first floating valve to a second drain line at a first spool position,

wherein the floating selection valve connects the boom descending signal line to the pressure receiving part of the first floating valve, and connects the descending pressure receiving part of the boom control unit to the second drain line at a second spool position, and

wherein the floating selection valve is selectively switched to the first and second spool positions based on the operation signal of the floating selection operation part.

3. The boom cylinder control circuit of claim 2 , further comprising: a floating release operation part which provides a signal prior to the signal of the floating selection operation part so as to selectively switch the floating selection valve to the first spool position or the second spool position.

4. The boom cylinder control circuit of claim 3 , wherein any one spring chamber of a first spring chamber of the first floating valve, a second spring chamber of the second floating valve, and a third spring chamber of the floating selection valve is connected to the second drain line.

5. The boom cylinder control circuit of claim 2 , wherein any one spring chamber of a first spring chamber of the first floating valve, a second spring chamber of the second floating valve, and a third spring chamber of the floating selection valve is connected to the second drain line.

6. The boom cylinder control circuit of claim 1 , wherein any one spring chamber of a first spring chamber of the first floating valve and a second spring chamber of the second floating valve is connected to a second drain line.