Control in the field: How Foundation Fieldbus lets you maintain control without a host system
June 26, 2009 by Ian Verhappen
One of the key differentiators of Foundation Fieldbus is that with its PID Function Block, it supports control in the field and, as a result, single loop integrity. This means that with Foundation Fieldbus, it is possible to maintain control at the last set point without a host system.
Implementing control in the field enables certain applications to be more efficient than with conventional instruments or control in the host. But, like all things, this also means some items need to be considered during the design process. Let’s start with those enablers first.
The most important item to enable control in the field is that, like pneumatic control loops, all the devices in the control calculation/loop must be on the same segment (air line for pneumatic). It is for this reason that it is recommended that all FF designs be based on the principle that all elements of a loop are on the same segment so that even if you do not wish to implement field control now, it will be possible in the future without having to make any physical changes in the plant. Once this requirement is met, the rest is “just” software, engineering and configuration.
The key consideration to optimize control in the field is the assignment of the PID block and to which device it is installed. The preferred option is to place the control algorithm block (PID) in the analog output (AO) device or final control element.
Why? Remember, in basic control theory, the PID block requires the following information to perform its calculations: set point, error and output. The set point is the target or objective, output is where the control element is actually positioned and error is the difference between where we are (output) and where we should be (set point). Also remember from past columns that we do not need to use network bandwidth when using an internal device communication, since there is no need to transmit a message on the network when transferring data from one place in the device’s memory to another. Therefore, having the PID block and AO block in the output device reduces the number of pieces of data on the network by two; the output as calculated by the PID and sent to the AO block as its objective, and the return error from the AO block to the PID block for the next calculation. By the way, because the set point message is sent only periodically, it is communicated during the acyclic communications part of the macrocycle and, therefore, has no real impact on the overall required macrocycle time.
Now we’ll look at what was, until recently, the most commonly used method of implementing Foundation Fieldbus control in the host. This requires the maximum number of compel data, or network communication messages. This is because we need to send the analog input signal to the host so it can calculate the PID block (first compel data). The result of this calculation, the AO target, needs to be communicated to the final control element (second compel data), and then the AO block needs to tell the host the error as an input to the next calculation (third compel data) message. Each compel data message takes between 15 and 30 milliseconds, depending on the host system. This example also assumes that the host calculations are perfectly synchronized with the segment macrocycle. If they are not, we will get additional delays and potential error in the system as a result of using data from an earlier cycle as the input to the present calculation.
The final element is the communications required if the PID block is placed in the analog input or field sensor device. We now need two compel data messages; one from the PID block to the final control element to tell it where to go, and the return error message for the next calculation. The PID algorithm is often placed in the same device as the AI block if you are implementing cascade control in the field. The PID block is placed in the AI device for the slave loop of the cascaded loops.
Now that we can see how control in the field can have an impact on the speed of response of a control loop, several possibilities result, not the least of which is a reduction in the variability of any process because the ability of the process to more rapidly respond to changes in equilibrium will minimize overall process deviation.
The way you implement a control system in your facility can have a big impact on its overall availability and profits, so be sure to do the little things right so you can capture the big benefits that result.
Ian Verhappen, P.Eng is an ISA Fellow, ISA Certified Automation Professional, and director, industrial networks at MTL Instruments, a global firm specializing in fieldbus and industrial networking technologies. E-mail him at firstname.lastname@example.org.