Manufacturing needs machines. Machines, broadly speaking, transform energy into work. Manufacturing requires machines to do the work that processes existing materials into more expensive products. There are two kinds of processes, characterized by the state of the primary material being processed. Fluid processes deal mainly with material that is shaped by its containing pipes or vessels. Discrete processes use machines to shape the materials and to carry them from place to place. Fluid processes are mostly concerned with flow rates, while discrete processes are mostly concerned with dimensions of the parts produced. Batch processes assemble or separate materials according to procedures. Continuous processes may use procedures to start and stop, but production is relatively continuous compared to batch processes.
Machines Need Operators
Machines require operators because, despite marketing claims, there are no intelligent devices. The only intelligence exists in human brains. Humans can program equipment to do interesting things, but the equipment is completely unable to deal with anything the programmers didn't think of or couldn't afford.
Operators use different skills for different processes. What they have in common is a desire to control equipment and a sense of satisfaction from doing that well. The job has long periods when all goes well punctuated by great activity to keep an accident from happening.
The people who tended steam engines were the first operators. They had to know how to take care of the engine, particularly lubrication, and know the engine's limits, as well as how to start and stop it. They also had to know how to operate the associated boiler. See How to Boot a Steam Locomotive by Phil Jern, available at several places on the web.
Operators Need Training
The first controllers were closely tied to their sensors, which meant that indicators and controllers were mounted on a panel close to the controlled equipment. Operators had to walk through the process, checking on the equipment. Standard signals allowed instruments and controllers to be mounted on large panel boards in a central control room. Operators were then divided into board and field classes.
New operators started in the field with experienced operators. Humans are very good at retaining practices that they have been shown and given a chance to repeat. With sufficient practice, an operator could make the transition from the actual equipment to the abstraction at the central control panel. Further job advancement could put an operator back in the field looking for trouble before it got out of hand.
Operators Follow Procedures
Training must be followed by practice, or the training will be lost over time spent doing other things. Simulations are useful to maintain practice if the process is being operated by advanced automation. Process control simulations are much easier than aircraft flight simulations because the operator gets all the process knowledge from computer displays, and the control room is firmly attached to the ground.
An operator learns the details of the machinery and what the controls do, and then learns what to do with the equipment. There are procedures to follow for all sorts of potential events, including unwanted events. Operators also develop detailed mental maps of the locations of process equipment and the tag names that identify them.
Operators Make Mistakes
Equipment may break or fail in many ways, but humans make mistakes or errors. Operators make mistakes when they lose track of what the process has done, is doing and will do next. An alarm flood can confuse an operator about these things.
Procedural control can confuse an operator with no alarms at all if operators can't tell where they are in the procedure. An operator who performs procedures manually has little doubt about the present and future states of the process. A backup operator has to watch every step the active operator takes in order to stay up to speed. When automation performs procedures, the operator becomes a backup operator.
Accidents happen. The worst accidents seldom have a single cause. Trevor Kletz analyzed many accidents before writing Learning from Accidents. He found that when human error is suspected, the operator involved had not had adequate training or hadn't been trained recently. Another excellent book is Inviting Disaster by James R. Chiles.
Automation Replaces Operators
Automation introduces a machine or system to do some portion of the work of an operator. In discrete processes such as weaving and packing, machines were designed and built to make the motions that a human operator would make to produce a product. Fluid processes require devices that could hold a measurement to a target setpoint. This led to the development of PID control.
Digital computers make it possible for equipment to follow programmed procedures. Batch processes can have computers that provide procedural instructions to simpler computers in equipment that performs those functions that do not change with the product being made.
Perhaps the most advanced automation is found in aircraft, both military and commercial. People in that field talk about situational awareness and opaque automation when discussing controlled flight into terrain. Bad things happen when pilots either are unaware of what the automation is doing or are aware and can't understand what is going on, distracting them from the real situation.