PRINCIPLES OF PROPORTIONAL CONTROL
By James W. Dennis

Manufacturers demand consistent quality in their manufacturing processes because their customers expect high quality. So it is no surprise that controlling process values and the parameters which effect them are a high priority.

Proportional, Integral, and derivative (PID) controllers used in a closed loop system are the most common method used to achieve consistent results desired. In a typical closed loop system using a furnace as an example, the controller reads the temperature inside the furnace through a signal generated by a thermocouple and generates an output to a gas control valve, contactor, or SCR to bring the actual temperature into coincidence with the set point.

Given the importance of process control, one might be surprised to learn that many people struggle to understand how these controllers work and how to make them work for their process. But in today's competitive climate and never ending technological advances, is it any wonder that people have trouble keeping up. Many of the manuals provided with PID controllers are overly technical and well just downright frustrating. With that in mind lets try to cover the basic concepts which are critical to improving process control.

Lets first contrast PID control with something that's a bit more straight forward, "ON/OFF" control. (Reference Fig.1)

ON/OFF control systems are as the name suggests either 100% ON or 100% off. These types of systems use a "deadband" or "Hysteresis" to determine which state the controller is in. This type of system dictates that the process value will always oscillate around the set point value. If the deadband is too narrow the controller will chatter between the on and off positions. In a process where precise control is not a critical factor an ON/OFF control system may be an option. And in general these types of control systems are less expensive than a PID control system.

PID control as stated before consists of three components: Proportional, integral, and derivative. Each component has a different effect on the control loop. Here is a brief outline of each component.

Time Proportioning:

1.) More precise control by applying a corrective action
proportional to the deviation between the process variable, PV, and the set point,(SP). (Reference Fig. 2).
2.) A proportional band determines when the controller applies this corrective action.
3.) Outside the proportional band the controller behaves like an
"on/off" controller.
4.) Within the proportional band the controller is either on or off as established by a "cycle time".
5.) The deviation between the PV and SP determines what
percentage of the cycle time the controller is on.
6.) Required for more precise control of process temperature.
7.) Generally more expensive than on/off control.

Integral and derivative terms

1.) Integral (reset) is incorporated to compensate for temperature droop. (Reference Fig.3)
2.) Manual reset controllers require the operator to make an adjustment for each specific SP.
3.) Automatic reset controllers compensate for temperature droop automatically. (Reference Fig.4)
4.) Automatic reset only functions when the PV is inside the proportional band.
5.) Rate (derivative) prevents large overshoots during system startup or disturbances. (Reference Fig.5)
6.) Rate measures the rate of increase of the PV an accelerates the proportional action to slow the increase.
7.) Rate is proportional to the rate of change of the PV.

Applications:

Now that we've outlined the basics of how PID control works, we can look at a typical application.

UNIQUE/PERENY USA-Series
Gas Fired Kiln

This type of kiln uses two burners in a single zone configuration. One PID controller uses a thermocouple input signal to measure the temperature inside the kiln chamber and a variable current output signal to control the volume of gas to the burners. The volume of air introduced into the kiln is fixed. The current output signal from the controller drives a motor which in turn positions the gas valve between 0 and 100% open. As in typical heating applications the controller operates in a "reverse action" mode. This means that the controller output increases as the process value, PV, decreases in relation to the set point, SP, and conversely the controller output decreases as the PV increases in relation to the SP.

When adjusting the PID controller, the operator must consider the dynamics of the process. For example, If the process requires opening and closing of the kiln door at operating temperature, then the controllers PID terms, specifically the derivative, should be adjusted to compensate for this type of system upset.

Once adjusted for the particular process the PID controller should operate consistently. A sudden change in the behavior of the controller is a good indication that something in the process has changed.

UNIQUE/PERENY Pro-Cast
Tape Casting Machine

This type of machine used in the production of thin ceramic tapes and films employs three different PID control loops during operation. These are used to control the heaters, the belt speed, and the level of product.

1.) Heaters - A Pro-Cast tape casting machine may have multiple zones of heat. In a typical control system, each control zone uses a single loop controller (ie-Honeywell 200 series) with a type-J thermocouple input and a 4-20ma output. The 4-20ma signal connects to a Silicone Control Rectifier (SCR) and varies the voltage output to the heaters.

2.) Conveyor Belt - A Pro-Cast tape casting machine with a mylar carrier unwinds a spool of mylar at the entrance end and winds up the mylar at the exit end. An SCR DC voltage drive is used in conjunction with a tachometer feedback to guarantee constant speed. This closed loop system allows the SCR DC drive to correct itself for any disturbances to the system.

3.) Product level control - Pro-Cast tape casting machines use a closed loop control system to control the level (amount) of slurry (product) being introduced into the machine. Here a single loop controller (ie-Honeywell 200 series) with a 4-20ma signal input and a 4-20ma signal output reads the level of slurry via a proximity sensor and then opens or closes a valve to control to a specific set point.