IFCS is comprised of many subsystems that work together to provide a pilot with spacecraft stability and control. These include:

Propulsion and Attitude Control (PAC) – PAC includes, typically, the full set of thrusters, which provide both translational and rotational action, and a backup Control Moment Gyro (CMG) unit which provides supplemental attitude control. It also includes the circuitry and control software that drive these units.
Primary Control System (PCS) – The PCS provides an interface between the pilot and IFCS. It translates a pilot’s commands into control actions that are applied to a virtual control frame which represents the ideal goal action of the pilot. The virtual control frame consists of a goal velocity along any combination of axes, goal rotation rates about any combination of axes, as well as a reference attitude. This virtual frame represents the ideal state of the craft under perfect control, and all pilot input is applied relative to this virtual frame, thereby limiting the effect of external error on pilot control.
Reaction Control System (RCS) – The physical state of the PCS virtual frame is controlled by the predicted thruster and CMG output in response to pilot control. Under ideal conditions, the PCS frame attitude will be perfectly synchronized with the actual attitude of the spacecraft. However, factors such as sub-optimal thruster response or failure, external forces such as weapons fire, missile explosions, etc., can cause the real attitude of the craft to deviate from the virtual attitude. When this happens, it is the job of the Reaction Control System to drive the error between the two attitudes to zero. It attempts to do so using both thrusters and the Control Moment Gyros. If it fails to synchronize the attitude of the real and virtual frames within a reasonable time, it may reset the virtual frame attitude to that of the real spacecraft in order to avoid pilot disorientation.
Anti-gravity System (AGS) – The AGS detects and compensates for gravity, and, in general, any other continuous external force, allowing the spacecraft to maintain its position relative to the field’s source.
Turn Control System (TCS) – TCS assists the pilot in achieving stable turns. At high speed, a spacecraft’s thrusters may not provide enough force to hold a stable turn, causing the ship to slide, often resulting in a collision. A pilot will normally decrease his or her speed when turning, but TCS can manage the throttle for you by automatically setting the forward velocity to match the desired turn rate given the level of turning thrust currently available. The system takes into account the optimal banking thrust in calculating the sustainable turning velocity.
G-force Control Mode (GCM) – GCM is a safety mode that attempts to limit a pilot’s exposure to potentially dangerous levels of g-force. The primary danger for a fully constrained pilot is prolonged exposure to vertical g-forces which can cause blackout, greyout, redout, disorientation, loss of consciousness, and, if not corrected, even death. Horizontal g-forces of an extreme nature are also avoided, as they can cause both physical harm to a pilot and structural damage to the spacecraft.
In addition to these standard subsystems, other functionality may be implemented for more advanced systems.

IFCS takes as input a pilot’s commands, which may include a variety of operations, but are ultimately translated into 3 degrees of translation and 3 degrees of rotation. Additionally, other pilot inputs may be used as parameters in various phases of the IFCS control system.

Once the input values are modified by IFCS modes such as Turn Control and G-force Control, speed limits are imposed, etc., the modified inputs are passed into the Primary Control System, which includes both a linear and angular velocity PID controller. These control functions calculate the optimal force and torque which, if applied at the ship’s center of mass, will provide the motion requested by the pilot.

Simultaneously, attitude readings are passed into the Reaction Control System where a positional PID controller is used to drive the ship’s real attitude toward a goal reference attitude provided by the PCS. The control function outputs a torque that will optimally decrease the attitude error over the next time step.

Finally, a reading of persistent force fields, typically gravity, is passed into the Anti-gravity System which calculates the necessary counter-force.

Once the desired forces and torques have been calculated, propulsion resources are allocated to them in order from highest to lowest priority. AGS force is allocated first, as failure to generate sufficient counter-propulsion could be catastrophic. Next, RCS torque is allocated, drawn from primary propulsion first, then falling back on CMG torque if insufficient propulsion is available. Next, PCS rotation control is allocated, again drawing upon primary propulsion first, CMG torque next. And finally, at the lowest priority, translational control is allocated.

After a short time, once the propulsion system has acted on the IFCS commands, sensors read the ship’s actual state, which may vary from the expected state because of propulsion malfunctions, uncompensated external forces, etc., then feeds the results back in to the IFCS control loop and the process repeats.

Because IFCS cannot rely on the propulsion system to deliver the requested control, it uses a PID feedback controller to minimize the error between the desired state and the measured state. Such controllers are used by the Primary Control System to calculate the optimal force and torque to carry out the pilot’s control commands, as well as the Reaction Control System to maintain attitude stability.

PID controllers can be tuned to provide a range of response characteristics. Using velocity control as an example, an overdamped controller will accelerate quickly toward the reference velocity, overshoot, then oscillate as it settles into the final velocity. An underdamped controller will accelerate more slowly, settling into the reference velocity without any overshoot. A critically damped controller will accelerate at the optimal rate to settle in minimum time without any overshoot. The Primary Control System controllers that provide linear and angular velocity control are dynamically tuned. Based on pilot input magnitude, they can range from a subtle to aggressive acceleration response. In addition, individual pilots may prefer a more or less stiff acceleration response.

The actual response time of IFCS controllers is dependent not only on the tuning parameters, but also on the response time of its propulsion system components.