The chain drive system consists of three major components. The chain driver control card, the
QMC switching servo amplifier and the brushless drive motor. The chain driver control card is
the heart of the closed loop control system and contains a microcontroller, input, output and
status LEDs. This card receives commands from the machine controller and generates the drive
signal for the switching servo amplifier. The chain driver control card's inputs include limit
switches, encoder positioning and home pulses. When the system is operating normally, LT4
will be the only status LED, which is illuminated. When a failure condition exists, the
microcontroller may illuminate status LEDs that will aid in troubleshooting.
The QMC amplifier also has four fault lamps. The are OV (over voltage), UV (under voltage),
CF (Commutation fault/over temp) and SF. If any of these indicators come on or fail to
extinguish after power up, the drive and/or brushless servomotor are most likely defective. The
over voltage and under voltage indicators generally indicate if the high voltage dc supply is
within the correct operating range. However, a faulty drive can cause one of these indicators to
illuminate.
When a chain fault occurs (message from diagnostics or lamp), the cause is typically caused by
the chain having a time-out limit. In other words, the chain did not advance to the proper
position in the time allotted. This type of failure is common and typically is caused by one of the
following problems:
Power dip (brown out)
Mechanical bind in chain.
Loose drive belts, chains, sprockets or pulleys.
Switching servo amplifier failure.
Switching servo motor failure.
Chain driver card failure.
Encoder failure.
1. The best troubleshooting method for this system is to first eliminate the obvious. Palm down
and power down the machine. Unplug the drive motor connections and attempt to rotate the
motor by hand. If the motor, drive or chain is binding, make necessary repairs.
2. Verify the AC power is stable, use a line conditioner or UPS to stabilize the power or use a
line monitor.
3. Try swapping the chain driver card. Generally this can be done first because it is easiest.
4. Look for loose drive belts, chains, sprockets, pulleys or other linkages.
5. Eliminate the suspect sequencer drive by replacing it with a known good drive. If the trouble
does not end after swapping the drive, it is best to re-install the original drive. Try replacing
the motor with a known good motor. Again, reinstall the original motor if the problem
remains.
6. Replace any encoder with new. Generally encoder will result in positioning errors.
The heart of the chain driver system is the chain driver card. This card accepts commands from
the machine controller, generates velocity commands for the QMC amplifier, and has quadrature
detection for the rotary encoder and limit switch inputs.
The QMC Switching Servo Amplifier is a brushless servo motor drive. The drive provides three
motor drive signals (plus ground), the motor has a tachometer feedback as well as three phase
pulse feedback to the QMC drive. The drive's input is a velocity command from the chain driver
card. A control relay connects and disconnects the motor from the drive as part of the E-stop
circuit.
Typical control sequence
The chain driver receives a command from the machine controller to index. The chain driver
issues a velocity command (DC voltage) to the input of the QMC drive (Drive). The drive
outputs drive signals to the motor, the motor turns. The turning motor causes the belt, coupling
and encoder to turn. The encoder outputs pulses (differential A and B) to the chain driver card.
The chain driver card receives the encoder pulses and counts the relative change in position of
the chain by using a quadrature detection of the rising and falling edges of both the A and B
encoder channels. The chain continues to run until the chain index position is near based on the
counted encoder pulses. The chain driver ramps down the velocity signal to slow down and stop
the chain from indexing. If the complete cycle occurs within the correct time frame, the process
repeats. If the process does not occur, or if the chain driver does not receive the encoder pulses,
the chain driver will assert a chain fault. A chain fault indication is then output to the on-board
LEDs on the chain driver card as well as a register bit is set so the machine controller executive
program can read the status and recognize the fault. Limit switches detect and cause the chain
driver card to the process and again causing a status indication on the card LEDs and
register.