Troubleshooting notes

  If you are making your own cables for the optocoupler and servo boards I suggest that before you power up the system, you use a continuity checker to check for proper connections to the IC’s. The control board is divided in two sections by an optocoupler, on one side is the amp and the other is the servo error generator and the PID filter. Both sides of the circuit need there own power supply that are electrically isolated from one another. When you are testing for continuity you will need to make sure that you are connected to the right ground for the side of the circuit you are trying to measure. If I refer to a chip on the amp board it means you should be using the middle ground pin of the three pin header as a ground reference. If I refer to a chip on the servo or PID board, you will have to make sure that that the ground lead of your test equipment is moved to the ground pin on the 6 pin header. The ground pin is the one on the inside or middle of the board.

Continuity checks on the servo side with the ground lead connected to the 6 pin header

1. The 74169 and the 74283 on the servo board should have ground on pins 8 and 5v on pins 16.

2. The 7414 on the servo board should be connected to ground on pin 7 and 5v on pin 14

3. All opamps and the LM311 on the PID should have -12v on pin 4 and +12v on pin 8.

4. Pin 2 of the 7084 on the servo board should be connected to 5v


Continuity checks on the amp side with the ground lead connected to the 3 pin header

5. Both IR2110 on the amp board should have 5v on pin 9 and 12v on pin 8

6. The 6N137 on the amp board should have 5v on pin 8

7. Both LM311 on the amp board should have 5v on pin 8

3. The 7486 on the amp board should have ground on pin 7 and 5v on   pin 14


When you power up the system the motor will do one of four things:

1. Rotate rapidly until the encoder counters = the computer counters, this is on the order of maybe 30 degrees depending on the resolution of the encoders. This initial jerk is normal but you should be aware of it so you can keep accidents from happening. The start up jerk is not a problem unless you turn the system off with the tool bit touching the work or the table up against the stops. What happens is that there is 50% chance that the initial jerk will be in a direction that will ether break the tool bit or run the table even further into the stops. If the table runs into the stops and fuses are not installed the amp will fail.

2. Oscillate back and forth, this will happen if the power leads to the motor are hooked up in reverse. There is a 50% chance that the motor will be hooked up in reverse when connected the first time.

3. Take off running at full speed, if this happens there is probably something wrong with the H bridge.

4. Just sit there and do nothing, no resistance when the motor shaft is turned by hand.

Servo board Troubleshooting Section

  If the motor takes off or just sets there its time to use your test equipment. The first thing you need to do is disconnect the power line that hooks up to terminal block. Then you need to know is whether the pulses from CHA and CHB on the encoder are making it to the quadrature decoder chip on the servo board. On pins 4 and 5 on the 7084 you should see pulses when you turn the motor by hand. The 7084 converts the CHA and CHB signals into step and direction signals that go to pin 1 and 2 of the counter chips, IC3 and IC7. If the pulses are making it to the counter chips check pins 11-14 on IC3 and IC7 to see if the 8 bits of counter output are good. The next place to check is the 8 inputs to the DAC0800 these are pins 5-12. When you turn the motor shaft the output of the DAC on pin 4 should vary between +8v and -8v. If the voltage on pin 4 does not change when you turn the motor shaft the DAC must be bad or there is a problem with the reference voltage from the zener. The reference voltage can be checked at the junction of R8 and ZN1. From the DAC the signal is sent to input of buffer IC9. Pin 3. The output of the buffer is pin 1 and the signal should look the same as the input. From the buffer the signal passes through the edge card connector to the PID board.

PID Board Troubleshooting Section

  To test the PID board check pin 2 of the LM311 it should have a 16 kHz triangle wave on it. Next check pin 3 while slowly turning the motor shaft, this is the modified signal coming from the DAC and it should change as you turn the shaft. The amplitude of this signal on pin 3 can be varied with the torque limit adjustment VR2 this limits how far the duty cycle can swing.

  The output of the PID board comes from pin 7 of  the LM311 and should be a 16 kHz square wave that varies between max. and min. duty cycle as the shaft is turned. From here the signal passes out of a edge card connector to the amp board. All measurements so far have been made with the ground lead of the test equipment hooked to ground on the servo side of the circuit. Now when you move on to test the amp side you will have to move your ground lead to the ground on the amp side.

Amp Board Troubleshooting Section

  The input to the amp is pin 2 of the 6N137, the output is at pin 6 and should look like the input. The signal is then sent to pins 2 and 4 on the 7486 where one of them is inverted. The two outputs from the 7486 are on pins 3 and 6 and they should be the opposite of each other. These two signals feed pins 10 and 12 on the two IR2110 MOSFET drivers. The output of the driver chips are pins 1 and 7 and should be a 16 kHz 12v square wave, with one of them being inverted. If the output from MOSFET drivers is good then you can apply power to output stage through the 4 position terminal block. After the power is applied to the MOSFETs you will notice that the top side of the driver, pin7, is now a square wave that rides on top of what ever voltage is used for the motor.

  If the motor holds its position but does not turn when commanded to, there are three things that could be causing the problem:

1. The wrong port address has been set in the PR program that comes with Maxnc.

2. The optocoupler board is bad.

3. The servo board has bad IC’s. The IC’s that could be bad are IC1, IC5 and IC8

  The first thing to check is to see if there are pulses making their way to the servo board input buffer (IC8 7414). The step and direction signals go into pins 1 and 3 of  IC8 and out pins 3 and 4. From here the step and direction signals go to pins 1 and 2 of IC1 and  IC5, the computer counters.

  To check the counters that count pulses from the computer give the command G00X1000 and check the counter step input pin 2 of IC1 IC5, you should see pulses, if not then the pulses are not making their way out of the buffer. Pin 1 on IC1 and IC5 is the direction input and should be high or low depending on the direction you command it. The output of computer counters is on pins 11-14  0f  IC1 and IC5. You should see a square wave on each of these pins if you don’t then there is something wrong with the counter chips. If the counter chips are OK move on to the inputs of the DAC which are pins 5-12 of IC4 you should see square wave signals going to each of these pins. If the inputs to the DAC look OK check the output of the DAC on pin4 of IC4, you should see a ramp or sawtooth waveform, if not then the DAC is bad or there is a problem with the DAC reference voltage.

                               Controller adjustments and notes

1. If you are building your own system using my control cards make sure that you use two electrically isolated power supplies to power the amp side and the servo side of the circuit.

2. It is very important to use fuses to protect the amps from meltdown in case you run the machine up against the stops. A 5 amp fast blow fuse should be used in the line that feeds the amps and a 3 amp slow  blow fuse inline with the motor. If a heatsink of proper size is installed on D5-D8 the size of the fuses can be increased. If a common heat sink is installed on these diodes make sure that you use isolation kits on the diodes to electrically isolate them from each other.

3. The maximum voltage that should be used to power the motors is 36v, 55v or 75v depending on which model you have.

4. When running cables try to keep them as short as possible and if shielded cables are used make sure to only ground one end so that you do not get ground loops.

   The two boards that plug into the base board are the servo error generator and the PID filter, the smaller of the two is the PID filter and it plugs into the edge card connector at the edge of the base board. The two boards should have there parts side facing away from each other and the two bottom sides of the boards towards each other. There are holes on the bottom edge of both plug in boards and the base board. These holes are for plastic tie downs to hold the boards in place.

   On the PID board there are two adjustments that can be made, these are the gain and torque limit. VR1 controls the gain and should be adjusted so the system is as stiff as it can be without oscillating. The encoder resolution also has an effect on the gain as the gain increases proportionally with the encoder resolution. If you change the jumper setting on the servo board remember to adjust the gain because it will now be too high or to low. It is normal for an oscillation to happen now and then at the end of a move but they should die down after a second or so.

   VR2 is the torque limit pot and is used to limit the pulse width of the power output. One of the benefits of this feature is that it allows you to run motors at a higher voltage than they are rated for. By limiting the pulse width you can limit the average current in the motor and this is more important than how much voltage you are using. If you were running a 12 volt motor at 24 volts you might want to limit the pulse width to 75%, but this is not mandatory as I usually run my motors at twice there rated voltage with no apparent damage to them. Using a higher than rated voltage has the effect of making the system stiffer. As long as your motor is not getting too hot you are probably OK as far as supply voltage goes. The maximum pulse width can be set at is 99%. If you allow the pulse width to go to 100% there are no pulse to charge up the upper MOSFET driver the result of this is that no power is applied to the motor until rotates far enough to put in back in the lock range. Full clockwise adjustment of VR2 will give you the minimum duty cycle and counter clockwise will adjust it to the maximum.

   The jumper on the servo error generator board controls how many pulse per encoder line will be sent to the counters. With the jumper installed 1 pulse per encoder line will be sent to the counters and with it removed the counters will receive 4 pulse per encoder line.