General Information 1-7
MN1269
Functional Description
The “LD” Servo Control consists of (1) a power supply, (2) a velocity control amplifier and
(3) fault logic circuitry. The functional concepts are detailed below.
Power Supply
The power supply of a PWM bridge normally acts as a low output impedance voltage
source. However, if an analog signal representing motor current is applied to the front
end of the amplifier together with a current command, the impedance of the PWM bridge
then becomes high. These high impedance bridges exhibit a unique characteristic
referred to as current sourcing. That is, motor current and therefore motor torque
becomes proportional to commanded input current.
Current Sense Coupler
The current sense coupler within the power amplifier serves as the current loop feedback
mechanism. The unit isolates the high voltage motor signal from the low level control
circuitry.
Switching Configuration
The power amplifier contains a transistor bridge consisting of 2 limbs each containing 2
main switches. These switches enable a limb output to be connected alternately from the
minus bus to the plus bus. Both limb output signals or motor lines are normally zero with
respect to the minus bus. However, if current is commanded as described above, the
average voltage on only one of the limbs will increase while the other limb remains
connected to the minus bus. This switching configuration is referred to as the lower
circulation method.
Velocity Control Amplifier (VCA)
The high gain front–end velocity control amplifier accepts a speed command (Signal) and
a tachometer voltage (Tach) through the input connector. The VCA clamped error signal
(C adjustment) is applied to the power amplifier as a current command.
Excess Current Limit
The long–term current clamp senses motor current and responds to overload conditions
beyond rated output. If the comparator is tripped, it disables the drive, lights the Excess
Current (EC) indicator and pulls the interlock line “low” (if P4 is in latch position).
Surge Current Sensor
The surge current sensor pulls the interlock line low internally if the peak output current is
exceeded. Once stopped, the drive must be cleared by connecting the reset line to
common momentarily (reset switch) or by interrupting logic power for 2 seconds.
Overheat Sensor
A thermal switch with significant hysteresis attached to the heatsink sets the EC (Excess
Current) latch if the heatsink temperature exceeds a preset level. The latch operates the
EC fault indicator. The fault condition is cleared after the heatsink temperature returns to
normal and a reset is given.
Left Limit, Right Limit
Limit switches are typically mounted on machinery to restrict the load from moving into
catastrophic over–travel conditions. Limit switch polarity of the voltage on this line is the
same as that on the signal pin of the input connector. The motor is wired such that
movement produces a positive voltage on NIML (Non–Inverting Motor Line) with respect
to IML (Inverting Motor Line). Therefore, a positive input signal will cause the axis to
move in a rightward direction. The limit switch leads are then wired confidently to the
appropriate pins of the input connector.
Decoupled Current Sense
The Decoupled Current Sense (DCS) pin is isolated from the high voltage motor lines.
Therefore, motor current is evaluated safely by connecting an oscilloscope or voltmeter to
the DCS pin of the input connector and common (TP1).
Tachometer Filter
The tachometer filter rejects tachometer commutator noise by reducing the tach path
bandwidth so that the high gain velocity control amplifier can be responsive yet reject
tach noise.
Servo Systems Co. • 115 Main Road • P.O. Box 97 • Montville, NJ, 07045-0097
(973) 335-1007 • Toll Free: (800) 922-1103 • Fax: (973) 335-1661
www.servosystems.com
