I recently flew in a small single-engine airplane, equipped with an autopilot. As we cruised along at 17,000 feet, I had a chance to find out exactly how the autopilot worked.

I had heard that most auto-pilots were simple proportional-only controllers. But I could see right away that this auto-pilot must have some integral action as well.

I could see this because the controller managed to get the altitude exactly right. There was never an “offset”. You may recall from your control theory that a P-only controller will always be vexed by “offset” – a permanent error between SP and PV, except at one setpoint value.

There is no Derivative action in the autopilot, for fairly obvious reasons. (imagine how it would react to turbulence!)

The excellent control of altitude (and heading) was cool enough. But what really caught my attention was the mechanism for changing controller setpoints.

A small LCD input device allowed the pilot to provide a new setpoint for altitude AND a vertical speed setpoint…essentially setting a setpoint ramp rate. What a simple interface for some fairly complex control.

Keep in mind that ramping between two setpoints is an EXCELLENT technique to reduce control variation. When you make sudden setpoint changes, the process responds jerkily, and downstream operations may be affected. When you ramp between setpoints, the transition can be as gradual as you want it to be.

Most DCS systems can be configured to allow setpoint ramping. However, it may not always be easy to configure or to operate.

The auto pilot also used “PV Tracking”. This means that while the controller was in MANUAL, the setpoint was continually adjusted to match the PV. This provides for a “bumpless transfer” when the controller returns to AUTO. The downside is that you must be vigilant about putting the setpoint back to where you want it when you go to AUTO.

The controller had more cool features. You could pull the PV information from a variety of sources (altimeter, GPS), and, of course, there were many ways to quickly disable or override the autopilot.

The point of all of this is…Always be on the lookout for more ways to become a better process control guru. You may find examples all around you.

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