Tighter control, a sleeker appearance, and reduced cost—these were the three key benefits derived when Bell-Mark replaced a rotary encoder with the BML magnetic linear encoder from Balluff on its InteliJet line of ink-jet coders.
In a typical InteliJet application, an ink-jet print head traverses across flexible film on which variable information must be printed. The rotary encoder used formerly—an electromechanical device that caused the print head to stroke across the film along an air slide each time a code needed to be printed—was fine from a functionality standpoint. But it occupied a lot of space, says Bell-Mark engineer Dave Miller.
“We constantly try to make everything as compact as we can,” says Miller. He says at least one other linear encoder was considered, but “the Balluff encoder is one third the size.” Miller says he was introduced to it by local distributor RG Group. As for cost, the Balluff magnetic linear encoder is about 25% less than the rotary encoder it replaced, says Miller.
The fundamental parts of the BML encoder are a precision-encoded magnetic band or strip affixed to an air slide and a sensing head that floats above the magnetic strip. The sensing head is affixed to a carriage on which the ink-jet print head is mounted. So as the sensing head moves across the air slide, the print head moves with it.
The sensing head receives incremental position references from the magnetic strip. A solenoid valve and pneumatic air supply complete the package. When a print signal arrives from the Bell-Mark system’s PLC, the solenoid valve drives air into one side of the air slide, causing the head to stroke across the film. The Balluff encoder synchronizes the speed at which the print head moves with the rate of ink flow sprayed from the print head so that size and legibility of characters printed is always precise.
The BML encoder provides non-contact, wear-free position tracking with no signal degradation over time. A basic concept underlying the encoder is direct load measurement. Whereas the accuracy of a rotary encoder hinges on the specific actuator technology deployed—ball screw, synchronous belt, rack and pinion—the magnetic linear encoder monitors linear load position independent of actuator technology. This direct load measurement also eliminates incorrect position information that might be caused by backlash or “slop” in the linear drive system. Consequently, Bell-Mark can choose a less-expensive drive solution without sacrificing tracking accuracy. There’s no need to use more costly “pre-load” actuators just to improve the accuracy of the rotary encoder at the motor shaft. Additionally, since the BML monitors the load position directly, the system is immune to catastrophic position errors caused by drive belt slippage or breakage that can affect motor-shaft-mounted rotary encoders.