HP 1050c Service Manual - Page 293

Functional Overview Air Pressurization System (APS) The APS is the system that provides and controls the pressurization of the ink in the Ink Cartridges. The key "mission" of this system is to ensure the minimum required ink pressure at the inlet to each Printhead respectively at the required print rates. Note that only a minimum pressure needs to be controlled since the purpose of this pressure is simply to refill the Printhead fast enough to maintain the Printhead's internal pressure within its limits needed for dropweight control. The APS is also used to provide pressure for tube purge and as part of the blow prime system operation. The mechanism used is a low-pressure pneumatic system with feedback control of the air pressure. The air circuit includes the Ink Cartridges, flexible air tubing, manifold connectors for the tubing, a DC motor driven air pump, a 2-way solenoid valve, and an analogue pressure sensor. Connection is made to each Ink Cartridge respectively by the same needle / septum system used for the ink; there is also a "quick connect" between the tubing and manifold that remains attached to the Tubes System and the APS Module. The APS module is a service replaceable module that houses the pump, valve and sensor in one chassis which also doubles as an ink catcher: this assembly clips under the ISS Housing. The APS pressurizes the ink in the cartridge bags by pressurizing the air around them. The printer controls the pressure of this air using the pump with feedback from the sensor; when required the valve is opened to de-pressurize the air circuit. This air pressure thereby controls the ink pressure at the inlet to the printheads. The system pressure for printing is set to ensure the defined Printhead inlet pressure for the highest possible flow rate. Ink Level Sense (ILS) The Printer includes a subsystem to monitor the amount of ink left in an off-axis ink supply, called Ink Level Sensing (ILS). The ink supplies used in the Printer consist of a collapsible ink bag inside a pressure shell. Two coils are attached to the external side of the bag and aligned with each other so that they form a variable transformer. The coupling coefficient of the transformer is a function of the distance between both coils, that is, it depends on the amount of HP DesignJets 1050C and 1055CM Printers Service Manual 10-7