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Pulse Coder alarms Troubleshooting APC/SPC (absolute pulse coder or serial pulse coder

What can causes a pulse coder alarm?
Faulty pulse coder, cable, drive, improper setting of parameters, or load in the axis.


Warning If an absolute pulse coder is used, you will have to reset home position if the cable is disconnected. Tool change or pallet change position could also be affected if the zero return position is set improperly. Also if not set properly the axis could be moved beyond its allowable stroke causing severe damage to way covers, ballscrew and much more.

Basic encoder troubleshooting:

  1. Coolant contamination in plug check both sides of each plug connector.

  2. Wires all connected to pins

  3. Drive plug connected

  4. Encoder cable Shield thumb screws snug

  5. Pins seated in plug

  6. Extra circuit board, junction block or plug between motor and drive.

  7. Check inside encoder cap

  8. While powered up tap on the connector with the back of a screw driver to check for movement or vibration problem.

  9. Move encoder cables around to see if it alarms. PLEASE SEE IMPORTANT NOTE TO RIGHT.

  10. Inspect any wires that move in cable tracks for damage. Check at the entry and exit area of the tracking too.

  11. Megohm the motor to see if there is a slight short to ground. (See procedure Servo Motor Testing)

  12. Parameters changed recently.

Detailed explanation on encoder problem troubleshooting:

Check motor cable connection for coolant contamination. Pull apart the cable end. Many times coolant can get into the cable conduit and run into the plug. Use an airline to blow through the pins. Spray some contact cleaner inside and blow through.

Also check to make sure pins are seated properly and wires are all connected in both connectors.

Coolant contamination can cause corrosion and cause wires to break off. Pull gently on each wire to make sure the shrink tubing in not the only thing holding the wires on.

Sometimes there is an extra circuit board or an extra plug that the encoder may go through before it goes to the drive. Check these connections and board to to see if there is coolant or oil covered.

Moving encoder cables could cause damage to the cables, do as last resort. Cables usually take a long time to get if you damage them. Coolant can make them brittle and not flexible.

Parameters usually are not the problem unless someone accidentally changed something.

Advanced encoder troubleshooting technics

  1. Swap sides of the drive to see if it transfers with the cable and encoder. To accomplish this just swap the encoder feedback and the motor leads.

  2. Swap the drive completely if there is a another drive in the same or different machine with the exact same part number.

Note: In some cases it may be much easier and cheaper to order a new drive or encoder and pay the restocking fee instead of swapping components. Depending on the type of machine and axis this may be recommended from a service professional. Remember if a mistake is made you could have two down machines instead on one. You may also take out another drive (though this is usually rare).

Detailed explanation:

Usually swapping sides of a drive is performed if the alarm is constant or will come up just by moving the axis. DO NOT try running program.

Warning watch to make sure you are well within the limits of the machine. The machine does not know the axis is swapped. There also may be uncontrolled motion on power up. There may also be parameters stored in drive and not in the control. The drives that hold the parameters themselves are usually the drives for tool magazine, tool changer, turret or pallet changer.

Questions to ask the operator or to observe when the encoder alarm occurs. Which may help locate the problem.

  1. Does it come usually in rapid movement?
  2. Machining with a tool that puts a lot of vibration into machine?
  3. Does it alarm in the same point in program?
  4. Does it alarm around the same machine position?
  5. Does it alarm when nothing is moving?
  6. Is there any equipment running in area when it alarms?
  7. Is it abnormally hot in the building?
  8. During a certain shift only?
  9. Has anyone been in the parameters recently changing anything?