Motion Control Technology

   Lynton Manufacturing Service

Because this is a closed loop system the Encoders are not an optional extra, they  are an essential requirement, in fact the
Accuracy  of the system depends on the feedback from the encoders. Positions are held to +/- 1 encoder pulse  but note:-
                                
 IF THE ENCODERS  DO  NOT  REFLECT THE TRUE AXIS  POSITION  THEN  ERRORS WILL GO  UNDETECTED !!!

 Careful consideration should be given to the placement of encoders within the mechanical  drive chain to ensure that the feedback
 is an  accurate reading of  any  slide  movement and not backlash.

 Linear encoders fitted directly to the axis, measure actual slide movement  regardless of any leadscrew  backlash  and is  the
preferred feedback method used  for most Digital Readout equipment.
 Rotary encoders are usually used in conjunction with Precision Preloaded Ballscrews and are common on  many types of  CNC
machines.
 
Both of these configurations maintain good accuracy and the choice will usually depend on  price or type of application.
 Linear encoder scales are more expensive than Rotary  encoders but they  are a lot less expensive than  precision  ballscrews.

As a general  guide I would suggest the following configurations:-
 
    A:           If you intend to replace the machine leadscrew  with a precision  ballscrew  then fit a rotary  encoder  directly
                   to one end of the ballscrew.      
    B:           If you intend to retain the standard leadscrew then you should fit Linear Encoder  scales.

This choice of  hardware configuration is an important factor when  comparing the relative costs of  Servo and Stepper systems.
Stepper systems rely  on  the motor   for positional  accuracy,  and this carries a  hidden  expense. Converting motor steps into
accurate, backlash free, slide movement requires the direct  connection of relatively cheap motors to the expensive preloaded
 precision  ballscrews.

 There is a wide choice of  encoders  which are suitable for this type of project. Most major manufactures can supply these
 as standard stock items but they   can be a bit expensive for individuals who are converting small personal machines.
 I am including a couple of links to companies which offer  economical options for both Rotary and Linear encoders.

  I have used encoders supplied by US Digital  and I use the LS7266R1 encoder chip on the system interface board.
  www.usdigital.com  

  www.digitalreadoutsystem.com  This company is importing the ‘Janix’ linear encoder from  S.Korea.  I have read
  some very  favourable reports and for a 5micron  resolution they  competitively priced.
  
  www.machine-DRO.co.uk     The GS300 Series Standard Glass Scales

 

Whichever hardware configuration i s used  to provide the positional  feedback data  the  encoder  signals must conform to the following standards.

  

       1:-    Incremental  outputs
       2:-   ‘A’ & ‘B’ Channel  Quadrature Square Waves
       3:-    TTL compatible.   ( 0 to 5 volt)

Single or differential  signals
Single ended encoder outputs are not usually recommended for applications with cable lengths in excess of  6 feet,
 or in environments where electrical noise might cause interference problems. Using differential outputs to carry the signals  will help
eliminate  electrical interference and increase the distance over which signals can be transmitted.
 Line drivers and receivers  are used to increase the signal strength and convert  single ended outputs into differential signals.

  Line drivers required
 Diver      ST26C31        (farnel 1075789)


 Receiver AM26C32CN

The accuracy of  the machine will depend upon your hardware installation  

 ENCODERS

 ENCODER SIGNALS

 Ch ‘A’                                              5 volt

                                                           0 volt
                                                           5 volt

 Ch ‘B’                                                 0 volt