An Analysis of Automatic Closed-Loop Control of Rotary Drive Engines
Authors: Young F.S. , Sampson T.J. 
Published: December 2007

With this automatic control system for use on rigs equipped with a mechanically driven rotary table, it is possible to eliminate rotary speed variation caused by torque in the drill pipe. When closed-loop control is used, low-frequency variations in rotary speed can be completely eliminated, and high-frequency variations can be significantly damped, depending upon the power available.

Introduction
In applying technology to a drilling operation, rotary speed is of prime importance. For example, the d exponent, formation drillability, roller-bit-bearing and tooth wear, as well as many other drilling relationships are functions of rotary speed. If rotary speed is to be used in any equations related to these, it must be measurable and fairly constant over the time interval of interest. The conventional method of controlling rotary speed on a rig with an independent and mechanically driven rotary table is to use a pneumatic regulator at the drawworks. The regulator pneumatic regulator at the drawworks. The regulator supplies air pressure to a diaphragm, which mechanically positions the engine throttle. This method does not positions the engine throttle. This method does not provide a constant rotary speed. Variations in rotary provide a constant rotary speed. Variations in rotary torque present a constantly changing load to the engine. Consequently, if the throttle is held constant, the angular speed of the rotary table will vary considerably. This effect is illustrated in Fig. 1 which is an actual recording of torque and rotary speed on a rig where the rotary speed was controlled in the manner described above. Note that the average value of rotary speed varied inversely with torque until the driller finally changed the throttle setting to bring the rotary speed back to its initial value. To keep rotary speed constant, the throttle would have to be continuously repositioned to compensate for the load changes. An automatic closed-loop control system can perform this operation. Such a control system has been designed and successfully used on several rigs to control rotary speed remotely. Our purpose is to describe that closed-loop control system and the results of simulation and field testing.

Basic Control Theory
A closed-loop control system is one that controls a variable by measuring it, comparing it with a desired value and correcting it if it is not equal to the desired value. It follows then that the system must perform two equally important functions: measurement and control. A simple illustration of this would be a man controlling the temperature of an oven by looking at a thermometer, noting that it reads lower than the desired temperature. and throwing a switch that causes an increase in heat flow to the oven. In the system to be discussed here the control is automatic. There are many theories on how to study the performance of closed-loop control systems. The most performance of closed-loop control systems. The most common method, and the one that will be discussed here, uses simplified block diagrams and mathematical models. Terms that will be used throughout the paper are defined here: Input A signal in engineering units, going into the control loop or any of its components. Output A signal in engineering units, coming outof the control loop or any of its components.
JPT
p. 1305