Fluid dispensing and Metering applications can benefit from drives embedded with PID loop control.

Sometimes, dispensing problems or metering application are hard to solve. One way to get to the bottom of the dilemma can be to examine the drives you're using. "When you're designing systems whose function is to perform precise metering, transferring, and mixing of multiple chemicals, drives are obviously an intrinsically crucial component," says Henry Glick, vice president of operations at Chem-Flow, which specializes in making equipment for fluid dispensing and metering applications.

Add the fact that the ingredients are often quite expensive and sometimes can deteriorate quickly, the need for error-free drive operation becomes even more important. A malfunctioning drive can shut down a production line; an inaccurate drive will ruin a batch of product; a hard-to-operate drive affects efficiency, and the list can go on.

Drives embedded with PID loop control from A C Technology help provide precise operation for fluid dispensing machinery.

According to Glick, some drives offer more than others. For example, his company has been using drives from AC Technology Corp. One reason is that they have a PID (proportional integral and derivative) loop. "The PID lets our customers keep a tight rein on what's being pumped," he explains.

What Is A PID Controller?

PID controllers allow a drive to hold the desired setpoint based on feedback from the process. Variables such as pressure levels, liquid flow rate, or liquid level, are detected by a transmitter, which sends the to the variable frequency drive, for comparison to the setpoint.

Process systems often require a system-controlled parameter, such as motor speed, to be able to react to variable situations in order to keep another system attribute – pressure, flow, or temperature, for example – constant.

A simple example is a metering and dispensing system that has multiple discharge valves. For flow to be repeatable at each valve, the pressure in the supply manifold must be held constant. If the pump supplying this system is powered by a drive, the drive speed will need to increase as valves are opened, and the drive speed will need to be reduced as valves are closed, in order to maintain a constant pressure in the manifold.

A means to meet this requirement is to use a "setpoint controller", where the pressure in the manifold is measured with a pressure sensor and this value is compared with a "setpoint" indicating the value that you want the pressure to be. A setpoint controller compares the setpoint value to the actual value and generates a speed command to the drive to correct the variance or error. Some drives, such as the MC3000, have this setpoint controller function built-in.

PID Algorithms

One of the most common types of setpoint controllers uses a PID algorithm. This stands for the three types of adjustments (referred to as "gains") that are used to correct for the error: proportional, integral, and derivative adjustments. The proportional gain is the most basic adjustment, where the speed command is directly proportional to the error. If proportional gain is used alone, however, there will always be an error in the system. If proportional gain is set too low, system response will be quite sluggish. If it's set too high, the system will oscillate or grow unstable.

To eliminate the error, the integral gain is used. The integral adjustment will continue to increase the output speed command based upon the accumulated error over time (or decrease the speed in the event of a negative error.) Small amounts of integral gain can have a significant effect on the setpoint controller's performance. If set too high the system will overshoot the setpoint, especially when large step changes occur in the error.

The derivative gain is used to enhance performance. It basically looks at the rate of change in the error and forces a more dramatic change to the speed command than the one achieved with just proportional and integral (PI) alone. While this can be very useful in position control systems, for example, because it can shorten the time required for a drive to respond to a change in the error, it can also lead to a system that overshoots the setpoint or even creates an unstable system. In most cases, the derivative gain is set to zero or to some very low value to prevent this from happening.

Direct And Reverse Action

Most setpoint controllers are "direct" acting. That is, an increase in the motor speed causes an increase in the process variable you want to change. This is the case on a pump system where pressure is the process variable. Increasing the motor speed increases the system pressure.

But in some systems an increase in motor speed creates a decrease in the process variable you want to control. Take the case of a fan blowing air over a heat exchanger, and the temperature of the fluid within the heat exchanger is the process variable to be to changed. As the motor speed increases, the temperature of the fluid will decrease. In this case, the user would need to use a "reverse-acting" controller in order to achieve the desired change.