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ADW71205WSTZRL Datasheet(Arkusz danych) 10 Page  Analog Devices 

10 page AD2S1205 Rev. A  Page 9 of 20 THEORY OF OPERATION The AD2S1205’s operation is based on a Type II tracking closed loop principle. The digitally implemented tracking loop continually tracks the position and velocity of the resolver without the need for external convert and wait states. As the resolver moves through a position equivalent to the least significant bit weighting, the tracking loop output is updated by 1 LSB. The converter tracks the shaft angle (θ) by producing an output angle (ϕ) that is fed back and compared with the input angle (θ); the difference between the two angles is the error, which is driven towards 0 when the converter is correctly tracking the input angle. To measure the error, S3 − S1 is multiplied by Cosϕ and S2 − S4 is multiplied by Sinϕ to give S4 S2 for ) ( S1 S3 for ) ( 0 0 − × − × Sinφ Cosθ ωt Sin E Cosφ Sinθ ωt Sin E (2) The difference is taken, giving ) ( ) ( 0 Sinφ Cosθ Cos Sinθ ωt Sin E − φ × (3) This signal is demodulated using the internally generated synthetic reference, yielding ) ( 0 φ − φ Sin Cosθ Cos Sinθ E (4) Equation 4 is equivalent to E0Sin(θ − ϕ), which is approximately equal to E0(θ − ϕ) for small values of θ − ϕ, where θ − ϕ is the angular error. The value E0(θ − ϕ) is the difference between the angular error of the rotor and the digital angle output of the converter. A phasesensitive demodulator, some integrators, and a compen sation filter form a closedloop system that seeks to null the error signal. If this is accomplished, ϕ equals the resolver angle, θ, within the rated accuracy of the converter. A Type II tracking loop is used so that constant velocity inputs can be tracked without inherent error. For more information about the operation of the converter, see the Circuit Dynamics section. FAULT DETECTION CIRCUIT The AD2S1205 fault detection circuit can sense loss of resolver signals, outofrange input signals, input signal mismatch, or loss of position tracking; however, the position indicated by the AD2S1205 may differ significantly from the actual shaft position of the resolver. MONITOR SIGNAL The AD2S1205 generates a monitor signal by comparing the angle in the position register to the incoming Sin and Cos signals from the resolver. The monitor signal is created in a similar fashion to the error signal (described in the Theory of Operation section). The incoming Sinθ and Cosθ signals are multiplied by the Sin and Cos of the output angle, respectively, and then these values are added together: ) ( ) ( Cosφ Cosθ A2 Sinφ Sinθ A1 Monitor × × + × × = (5) where: A1 is the amplitude of the incoming Sin signal (A1 × Sinθ). A2 is the amplitude of the incoming Cos signal (A2 × Cosθ). θ is the resolver angle. ϕ is the angle stored in the position register. Note that Equation 5 is shown after demodulation with the carrier signal Sin(ωt) removed. Also note that for a matched input signal (that is, a no fault condition), A1 is equal to A2. When A1 is equal to A2 and the converter is tracking (therefore, θ is equal to ϕ), the monitor signal output has a constant magnitude of A1 (Monitor = A1 × (Sin2θ + Cos2θ) = A1), which is independent of the shaft angle. When A1 does not equal A2, the monitor signal magnitude alternates between A1 and A2 at twice the rate of the shaft rotation. The monitor signal is used to detect degradation or loss of input signals. LOSS OF SIGNAL DETECTION Loss of signal (LOS) is detected when either resolver input (Sin or Cos) falls below the specified LOS Sin/Cos threshold. The AD2S1205 detects this by comparing the monitor signal to a fixed minimum value. Without the use of external circuitry, the AD2S1205 can detect the loss of up to three of the four connections from the resolver. The addition of two external 68 kΩ resistors, as outlined in Figure 5, ensures that the loss of all 4 connections, that is, complete removal of the resolver, may also be detected. LOS is indicated by both DOS and LOT latching as logic low outputs. The DOS and LOT pins are reset to the no fault state by a rising edge of SAMPLE. The LOS condition has priority over both the DOS and LOT conditions, as shown in . LOS is indicated within 57° of the angular output error (worst case). Table 4 
