Since the "Exhaust Fan" pulse oscillation accounted fortunate than is charged by the capacitor discharge time difference formed when RP boom is moved downward, the charging resistance is increased, while the resistance of a discharge resistor the same numerical value decreases. The result is prolonged charging the capacitor; and discharge time shorter, the duty ratio of the pulse oscillation becomes large. Conversely, when the boom up-regulation of RP, capacitor charging time; and discharge time were extended, longer pulse oscillation duty ratio becomes smaller.