Generally FOC consists of the following components,
- PID controller for motor ﬂux and toque
- Current and/or voltage decoupling network
- Complex coordinate transformation
- Two axis to three axis transformation
- voltage or current modulator
- Flux and torque estimator
- PID speed controller
In a direct torque control the ﬁrst ﬁve components are replaced by two hysteresis comparators and a selection table.The direct torque control is diﬀerent from vector control in such a way, that it does not control the ﬂux through the current control, but it directly controls the ﬂux itself. The direct torque control is also diﬀerent from vector control in the sense that the reference frame here is stator ﬂux instead of rotor ﬂux used in vector control.The input to the direct toque controller are the torque error, error in magnitude of the stator ﬂux space vector, and the angle of the stator ﬂux space vector, from which the states of the power switches are determined. Based on this information, a certain voltage vector or combination of voltage vectors is directly applied to the inverter with a certain average timing. This gives the induction motor drives a very fast response.
the DTC scheme has the following features.
1.There are no current loops; hence the current is not regulated directly.
2. Coordinate transformation is not required.
3. Stator ﬂux vector and torque estimation is required.
for more information you can refer to the book "High Performance AC Drives Modelling Analysis and Control by Mukhtar Ahmad"