A controller is the primary intelligence within a CNC router system. It is generally comprised of a micro-processor, integrated circuitry and highly sophisticated firmware and software. In practical application the control accepts toolpath program input (from a CAM package) in the form of a G-Code file, and based on that input maps the exact path, speed and acceleration applied in executing motion.

The control in significant measure determines how fast a system can cut and still maintain accuracy, while simultaneously dictating how smoothly the system transitions when negotiating the acceleration / de-acceleration motions associated with the frequent starts/stops and direction reversals that are the consistent diet of CNC router motion.

Achieving accurate, high speed, vibration free motion is dependent on a control's: circuit quality, speed and a range of mathematically complex algorithms matched precisely to a system's unique real world mechanics and their associated levels of stiffness, damping, load capacity and oscillating frequencies.

Algorithms are mathematical tools employed within a control that manage electrical current levels. When applied correctly (matched to system mechanical characteristics) they effectively mitigate operational instability, such as, vibration and velocity disturbances.

It's worth noting, that if a system's mechanical components, structure and jointed conditions feature high levels of deflection and poor damping characteristics, even the best of controllers will be unable to effectively compensate for such mechanical weaknesses. And, overall system functionality in terms of cut quality, accuracy and repeatability will be largely compromised.

Stepper / Servo Control Comparison
Often, based on a few isolated details, an impassioned and meaningless debate is conducted over the relative merits of stepper versus servo control.
Fact is, all stepper means is an electric motor who's shaft position and speed are driven by discrete pulses received from a control. And, all servo means is an electric motor whose shaft position and speed are managed by a control and a feedback loop. So, knowing a CNC router features servo or stepper control doesn't really tell you much.

A control is the most complex component on a CNC router, and be it stepper or servo, a control's performance is dependent on a complicated, multifaceted, interaction of system hardware,firmware and software elements. And, the quality and integration of those elements can vary dramatically from system to system.

Judging a control (stepper or servo) can only be reasonably accomplished by understanding how effectively that particular control performs within the context of a system as a whole. Controls, be they servo or stepper, are but one part in a complex mix of technologies. How well a control performs will in large measure be determined by the successful synergy of those various technologies.