What aspects of process control instrumentation should I consider?

Introduction

Instrumentation and automatic control systems are advancing very fast. The success of automatic control depends on understanding and quantifying process mechanisms and the ability to measure relevant parameters accurately. Unfortunately, there are so many mechanisms involved in making a range of different biscuit products that very few completely automatic manufacturing plants are yet in existence.

Traditionally process control has been achieved by operators making regular measurements and then adjusting controls. Developments in electronic in-line instrumentation running continuously have largely replaced the need for manual measurements. The instruments are often monitors with alarms to prompt action by operators.

Please refer firstly to the text in The Principles of Process Control. and Summary of typical Process Control Points. You may also find it useful to consider the some of the problems addressed in List of Process Control questions and problems.

General considerations

  • Both variation in a parameter and the cause of this variation are needed to be know if process control is to be effective. Data logging from in-line instruments should be time based so that relationships can be easily compared.
  • It is sometimes difficult to have an instrument that measures in absolute units in an in-line situation. Monitors, designed to show deviations from a norm, are very useful here and require less calibration.
  • Certain machines need to have calibration checks made at frequent intervals. For example correct functioning of metal detectors and every packet checkweighers. Decide how and who is to be responsible for these checks.

Specific sites for instruments

  • The productivity of a plant is determined by speeds of the primary machines like the cutter/rotary moulder/wire cutter etc. Monitoring of accurate speeds of these machines is very important.
  • Of fundamental importance to productivity is the weight of baked product. As yet accurate in-line weighers at the oven exit are not reliable. Watch for developments in this area.
  • For economic reasons in-line weighing is needed also for chocolate pick up, oil spray pick up, cream sandwiching, etc.
  • Instruments designed to monitor oven conditions should be considered critically. The heat transfer to the dough pieces is what is really important. Unfortunately heat transfer is much more difficult to measure than temperature. Heat transfer is affected by radiation and air turbulence. Indications of oven temperature are useful mostly for basic oven setting not for critical process control.
  • Vision instruments are useful for monitoring the size, shape and colour of biscuits at the oven exit. Check that such instruments can tolerate the adverse temperature conditions at the oven exit.
  • Biscuit moisture content is an important parameter for process control. Electronic measurement of ground biscuit using infra red absorption offers a very rapid assessment. The instrument needs calibration for different types of biscuits.