If an actuator is required to control a level, flow or pressure in a system, then it may be required to move frequently. Modulating or positioning control can be achieved using the same 4-20 milliamps signal. However, the signal would change as frequently as the process required. If very high rates of modulation are required then special modulating control valve actuators are needed that can accommodate the frequent starts required for such duty.

Figure 5: Digital communication systems

       Where there are many actuators on a process, the capital cost of installation can be reduced by utilizing digital communication over a communicating loop that passes from one actuator to another. A digital communication loop can deliver commands and collect actuator status rapidly and cost effectively. There are many types of digital communication such as Foundation Fieldbus, Profibus, DeviceNet, Hart, as well as proprietary communication systems custom designed for valve actuator use such as Pakscan. Digital communication systems have many advantages over and above the saving in capital cost. They are able to collect a lot of data about the condition of the valve, and as such can be used for predictive maintenance programs.