products

NetCon, MicroNet, and MicroNet Plus control systems with multiple 2 CH Actuator modules (part numbers 5501-428, -429, -430, -431, -432), have a potential for a ‘beat’ frequency to be created around 3000 Hz. This signal may create noise within the chassis and has been known to cause fluctuations in low amplitude signals such as RTDs and thermocouples. It may cause increased noise on other analog signals as well. The source of the problem is that each actuator module produces a feedback (either LVDT or RVDT) excitation signal that is independent and asynchronous to the other actuator modules generating this same output. Since these signals are likely to be slightly offset in frequency and amplitude, it is possible that a corresponding beat frequency can develop on the chassis backplane and develop on the analog common line. In 1997, Woodward created a small DIN-rail-mountable filter that is specifically designed to eliminate noise created by the actuator excitation within a tight frequency band (notch) around 3000 Hz. The unit requires about 1 inch (25 mm) of DIN rail space under the actuator FTM, and has two wire connections. One wire is connected from TB 1 to the actuator excitation (–), which on Woodward FTM 5437-672 is terminal TB 6. The second wire is connected from TB 4 to ground. Woodward’s engineering services group recommends the use of one Notch Filter per chassis for all chassis that utilize two or more actuator modules. In the case of a redundant system, two filters can be installed to ensure that this protection is available during all running conditions. If a control system has multiple chassis, each chassis meeting these criteria should have a filter.