CONTREX CXB2040 - Manual (Page 19)

CXB1525 and CXB2040 MANUAL
8900 Zachary Lane N., Maple Grove, MN 55369 U.S.A. 16
2.6 THE ADVANTAGES AND DISADVANTAGES OF A TRAPEZOIDAL
AMPLIFIER SYSTEM:
A trapezoidal motor has three stator windings and together with the rotor magnets are designed so
that the magnetic flux coupling between them produce a constant torque. The torque of the motor is
proportional to the three stator phase currents which are 120
o
out-of-phase to the other two. Shaft
position sensors are required to provide the commutation signals to commutate the motor. The two most
common sensor types are Hall-effect sensors and an optical encoder with commutation tracks.
A common class of applications for trapezoidal amplifiers is for motor speed control. Classically,
this is implemented by adding a brushless DC tachometer to the motor shaft and driving the motor
through a velocity controlled servo loop. A high performance velocity loop can be implemented in this
manner.
Another way of implementating the motor speed control is by using a simulated digital tachometer
synthesized by the motor commutation signals. The commutation signals are used to trigger an one shot
signal at every transition of the commutation signals. After smoothing, a voltage proportional to velocity
(RPM) is obtained.
Two additional system features were implementated in the synthesized tachometer design:
1 At 100% of full RPM, the PSEUDO-TACH voltage is limited by the power supply voltage. If an
RPM is commanded above 100% RPM, the servo will run away. To prevent this from occuring, the
absolute value of the PSEUDO-TACH signal is compared to a 95% of full RPM reference. If the
PSEUDO-TACH signal exceeds this value, an over speed latch is set and the servo is disabled.
2 The PSEUDO-TACH one shot pulse is buffered and brought to the control interface. The controller
can use this signal to determine the exact velocity (RPM) of the motor.
2.7 CURRENT MODE vs. VELOCITY MODE:
The fundamental difference between current mode and velocity mode is that in current mode, an
external command signal controls the torque of the motor, rather than the velocity. In velocity mode, an
external command signal controls the velocity (RPM) of the motor, rather than the torque. In a current
mode amplifier, the command signal is proportional to the motor current, thus it is also proportional to the
torque of the motor. In a velocity mode amplifier, the current loop amplifier stage is preceded by a high
gain error amplifier which compares the command signal and the tachometer feedback signal.
Current mode amplifiers are usually used in Position Control Systems where no tachometer feedback
is required. While velocity mode amplifiers are usually used in Classic Cascaded Contol Systems where
there are position, velocity and current loops in the system. Velocity loops tend to have a higher
bandwidth and operate better near zero speed.