13 December 2001
Direct Torque Control Technology = 'Smart' Control
MC: How does direct torque control (DTC) relate to sensorless vector control (SVC)? We're referring to SVC derived from full field-oriented control.
KG: The evolution of SVC is from what you call "full field-oriented control" (FOC). In other words, people conceived the encoder-based full field-oriented control first and then tried to modify it for sensorless torque control algorithms to eliminate the encoder. So most of the SVC algorithms use the FOC control architecture as the starting point and then try to estimate the speed using motor current and voltage information [instead of the encoder]. On the other hand, DTC was developed as a "native" sensorless torque control (not a vector control) method. From the beginning, this technology was pioneered as a sensorless control architecture. I think this is one of the major factors for its success.
MC: Is the separation of flux- and torque-producing currents part of DTC's algorithms?
KG: That's partly answered by my previous comment. In SVC, because of FOC, the flux- and torque-producing currents [two separate current regulators] are produced in the machine to control torque and flux in the machine. In DTC, flux and torque of the machine are estimated and two hysteretic loops are closed without the intermediate torque- and flux-producing current loops. The output of these hysteretic control loops is the most optimum inverter state to achieve the desired flux and torque quickly and accurately. This hysteretic loop runs every 25 microseconds.
MC: There's no modulator element in DTC. What takes the place (or function) of the modulator?
KG: As mentioned previously, the hysteretic control loops, based on the torque and flux errors, select the inverter output state. That state is applied for the next 25 microseconds. Since, in DTC, there are no explicit current regulators, there is no explicit inverter voltage command and therefore no need for the traditional pulse width modulator.
MC: Accurate adaptive motor model is used in DTC. What inputs are required from the user?
KG: A user supplies only the nameplate data of the motor. The drive identifies the necessary machine parameters using the "identification" process. The simple identification procedure does not require spinning the motor and is generally adequate. The sophisticated identification procedure is also available, but it involves spinning the motor at no load and is rarely needed.
MC: Is DTC used exclusively for all AC drives at ABB?
KG: The premium ABB drives [600, 1000, and 6000] use DTC technique. The general-purpose and HVAC drives [100, 140, 160, and 400 series] do not use DTC technique.
MC: Does the block diagram of DTC illustrate this native sensorless control concept?
MC: What does DTC mean from an operating point of view?
KG: It means mills like the Georgia Pacific Gypsum Paper Mill in San Leandro, Calif., now operate their entire paper machine with encoderless drives. This is a first. A second G-P mill already has retrofitted in this technology, and more mills will follow suit. It removes all the difficulties and downtime that can be associated with trying to keep those glass disk encoders operable in extremely harsh environments, such as those used historically in paper mills and cement plants.
Note to readers: To better understand what's "smart" about ABB's AC 600 controllers, Motion Control interviewed Kalyan Gohkale, manager, AC Drives R&D, ABB Inc., Automation Technology Products division, Drives & Power Electronics.