The drive system designed for the Lobo CNC mill is a unique servo control system, which uses standard stepper motors with encoders and drives them as brushless servo motors. Below the break you'll find a bit more discussion about why this system offers unique advantages.
|Servos vs. Steppers
There are two types of motors and motor drivers used in CNC systems:
The other drawback of steppers is that when you turn the motor drives off to move the axes by hand, you completely lose track of the motor position. This means that making all the fine adjustments for locating your part relative to the tool tip must be done with jog controls, rather than by turning the hand cranks. Practically speaking, manual machining operations are difficult to impossible.
Servo motors are the almost exclusive choice for high-end CNC machines. They are faster, smoother, but also more complex. The main advantage, though, is that the position feedback on each motor allows the controller to constantly monitor the servo position error. If any motor starts to deviate from the path, either all motors can be slowed down to maintain accuracy, or they can all be shut down to avoid cuttng an errant swath across your part.
Servo motors also allow you to constantly display the position of the motors even when the drivers are turned off. This allows you to freely reposiiton the machine without losing your homing information. You can also perform manual machining opeations using the motor feedback as a DRO (digital readout).
The Lobo CNC control system takes advange of the low-cost and high torque of stepper motors, but adds the feedback and reliability of servo control. Rather than driving the motors step-wise (step-step-step...), the Lobo CNC system drives the stepper motors continuously like brushless servo motors. The encoder feedback then is used with a PID control system position the motor accurately. (Exactly how this is done is a fairly complex discussion.)
Why all the bother? First and foremost, the servo control provides much more reliable operation. You never have motors simply stalling out (like with steppers) . The motors can recover smoothly from small position errors rather than stalling. You will always have some amount of servo position error, but the software can continuously monitor the error and prevent catastrophies.
Secondly, because the servo is tolerant of small errors, you can operate right at the published torque-speed curve for each motor. With a stepper, exceeding the torque-speed curve, even just slightly, will cause the motor to stall. This means that in practice, the exact same motor run as a servo can be run much faster than if driven in normal stepper mode.
Lastly, and most conveniently, you can turn off the drivers and do manual machining operations, using the motor feedback as a DRO. Unless you are doing production work where every last little operation has been programmed, you will find this to be a huge advantage.