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      J E F F R E Y   K E R R,  L L C





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Our PIC-SERVO SC controllers can be used for coordinated motion in one of two ways:

  1. They can be used in Step and Direction mode with CNC or other types of control systems designed for use with stepper motors and drivers.

  2. They can be used in "path control" mode, designed to support coordinated motions through serial port commands.

With the first method, you can simply drop in a PIC-SERVO SC controller and servo motor in place of your existing stepper motor and driver and see an improvement in performance and reliability (see our application note SD_CNC.ZIP for more details.)  However, doing this ignores many of the additional advantages of using servo control technology.  The next section decribes how the PIC-SERVO SC's path mode features are used to create coordinated motions.


Coordinated Motion
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Most motion control applications have motors that move independently.  One motor starts and then stops, then the next motor goes.  Some require motors to start at the same time, but their motion is otherwise independent.  More demanding applications, however, require motors to move in a highly coordinated fashion.  CNC machine control is one such example where two motors might have to move in unison to create a highly precise circular path.

To create a coordinated motion, or path, two things must happen: 1. Individual coordinates or positions for each motor must be calculated for closely spaced points along the path, and 2. The motion controller must, in real-time, move each motor to the next path point coordinate at the same time.  The PIC-SERVO SC motion controllers solve this problem in a unique fashion.   Firstly, the PIC-SERVO SC controllers rely on the power of your host computer (PC, etc.) to do the complex calculations to create the path point coordinates.  Once calculated, these coordinates are then downloaded into path point buffers in each of the PIC-SERVO SC controllers.  With a single command, the host can start all controller motions simultaneously, and the crystal controlled clocks on each controller ensure that the path points are all reached simultaneously.

Path points are spaced at uniform timing intervals of either 30, 60 or 120 Hz spacing.   When moving from one path point to the next, the PIC-SERVO SC does not simply jerk the motor to the next point, it moves between points at a constant velocity.  The net effect is that multiple motors will create motion from one path point to the next along a straight line.  This drastically reduces the number of calculated path points for achieving smooth, accurate motion.  The efficient serial communications protocol allows up to 16 motors to be coordinated from a single serial port.

In addition, the buffers in the PIC-SERVO SC can be loaded dynamically while your motors are moving.  This lets you to create arbitrarily long motions, with your host only intermittently required to refill the buffers.  Please see the PIC-SERVO SC data sheet and our software page for more information on using path mode.

The demo video PSDEMO.WMV demonstrates the flexibility of the PIC-SERVO SC's coordination capabilities, where a non-linear 5-bar mechanism is used as a 2-D plotter.


CNC Application
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The PIC-SERVO SC controllers are ideal for creating low-cost, high performance CNC machine control systems.   To assist in developing CNC controller applications, we have developed a sample program for you to use or modify as needed.  The PSCNC program is a complete 3-axis milling machine controller with a basic G-Code interpreter.  It is designed for use with three PIC-SERVO SC controllers. 

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This application will allow designers of machine control software to evaluate the operation of the PIC-SERVO SC, as well as providing source code which can be modified or included in specific applications.   PSCNC can be used with a variety of different machines by modifying parameters in its initialization file.   Also provided is an application note for retrofitting a small desktop milling machine with DC motors and PIC-SERVO SC controllers.  Please note that this sample program is not supported as a product, nor is it designed to be completely failsafe.

download PSCNC.ZIP                download DESKTOP.PDF



Steppers vs. Servos

Most low and medium cost CNC control systems are designed to be used with stepper motors and drivers. However, the load and speed variations of CNC control are not ideal for stepper technology.

PIC-SERVO SC controllers however, are no more expensive than comparable stepper controllers, and can give your CNC machine improved performance and reliability.