You've been assigned the task of solving a product quality problem. The plant has six production lines, each for different flavors of a baked good. The process on each line begins with a mixing vat controlled by PLC. The dough drops onto a conveyor and passes through an oven, and out the end comes the finished product.

The products sometimes are a bit gooey, sometimes a bit burnt. To begin solving this problem, what should you do?

The standard response would be to look at the PLC programming and "try some things" (i.e., adjusting the ingredient mix or the conveyor speed). There are two flaws in this approach.

First, it's unlikely there's a problem with the recipe (and thus the values loaded into the PLC). Leave any recipe troubleshooting to the product design team, broaching that topic only after troubleshooting the equipment.

Second, the problem is inconsistency, not stoppage. You may need to investigate the PLC later, but you can be sure it's not randomly changing its own values. Every minute you spend paging through the PLC programming is that much longer you delay fixing the problem.

If only one line exhibited this behavior, you would be looking for recurring cause. For example, perhaps a valve is sticking at the input vat and not allowing the correct amount of oil to enter. However, because all six lines are doing this, there's a root cause that you must find. To do so, divide the process into its subsystems as follows:

  • Mixing vat. Put a power analyzer on the motor input, and look for anomalies that may vary mixing speed.
  • Conveyor. An uneven feed rate will cause uneven baking. What may cause speed variations?
  • Oven. If it's electric, use a power analyzer on the supply and a megohmeter on the connections.