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Motor Control
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.

Motor specifications:
Size:   NEMA 17, single stack (SOYO SY42STH33-0956B)
Running torque:   22 oz-in@<100rpm, 20 oz-in@450rpm, 15 oz-in@770rpm, 11oz-in@1000rpm (24v driver)
Encoder:   400 line A/B quadrature [1600 counts per rev.] (US Digital E8P-400-197-S-D-D-B)
Cable:   28" with 1x8 0.100" header (compatible with connectors on driver board)
Pin
1
2
3
4
5
6
7
8
Signal
Winding A+

Winding A-

Winding B+

Winding B-

Encoder A

GND

Encoder B

+5v
S3x3 Driver board specifications (see board data sheet for more complete details):
S3x3 Driver Board
Data Sheet
Number of axes:   3 motor control channels + multifunction I/O
Controller chip:   3 PIC-SERVO SC chips modified for use with stepper motors
1 PIC-I/O chip
Driver type:   Bipolar chopping, 0.75 amp / phase
Digital I/O:   1 encoder input channel per axis
2 limit switch inputs plus 1 homing input per axis
3 8-bit analog input
1 TTL PWM output, 1 high current PWM output (1 amp)
1 counter input
2 auxiliary I/O (can be used as inputs or ouptuts)
3 reset lines
Commnuications:   USB (type B connector)
Input power:   24v, 2.5 amp (2.5mm x 5.5mm barrel connector)
Software compatibility: LoboCNC control program (USB)
Step & Direction software (like Mach3) - note: this mode uses the limit switch inputs for
    Step & Direction signals, which are then not available for normal limit switch connections.

Servos vs. Steppers
There are two types of motors and motor drivers used in CNC systems:
  • Steppers are known for being low-cost, high-torque, and wiith simple drive electronics. However, they can be tricky to drive fast (>500 RPM).  With no feedback, they can be prone to stalling, which can be disasterous for machining.
  • Servos are typically DC motors with position feedback.  They require a PID servo algorithm to read the position and constantly adjust the  motor voltage or current.  They are more efficient, but run at higher speeds and lower torques.
Stepper motors are the overwhelmingly popular choice for low-cost CNC systems.  But because a single stalled stepper motor can easily ruin hours of work, motors must be oversized significantly to ensure reliable operation.  It is not uncommon to see people couple a 250 oz-in stepper motor to a 1/4-20 drive screw.  This combination can be produce almost 500lb of force, easily enough to damage a small machine.

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.

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