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AD2S1200YST Scheda tecnica(PDF) 9 Page - Analog Devices |
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AD2S1200YST Scheda tecnica(HTML) 9 Page - Analog Devices |
9 / 24 page AD2S1200 Rev. 0 | Page 9 of 24 PRINCIPLE OF OPERATION The AD2S1200 operates on a Type II tracking closed-loop principle. The output continually tracks the position 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 output is updated by one LSB. The converter tracks the shaft angle θ by producing an output angle ϕ that is fed back and compared to the input angle θ, and the resulting error between the two 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 4 2 3 1 0 0 S to S Sin Cos t Sin E S to S Cos Sin t Sin E φ θ ω φ θ ω × × The difference is taken, giving ) ( 0 φ θ − φ θ × ω Sin Cos Cos Sin t Sin E Equation 2. This signal is demodulated using the internally generated synthetic reference, yielding ) ( 0 φ θ φ θ Sin Cos Cos Sin E − Equation 3. Equation 3 is equivalent to E0 Sin (θ − ϕ), which is approximately equal to E0 (θ − ϕ) for small values of θ − ϕ, where θ − ϕ = angular error. The value E0 (θ − ϕ) is the difference between the angular error of the rotor and the converter’s digital angle output. A phase-sensitive demodulator, integrators, and a compensation filter form a closed-loop system that seeks to null the error signal. When 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 AD2S1200 fault detection circuit will detect loss of resolver signals, out of range input signals, input signal mismatch, or loss of position tracking. In these cases, the position indicated by the AD2S1200 may differ significantly from the actual shaft position of the resolver. Monitor Signal The AD2S1200 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 Principle of Operation section. The incoming signals Sinθ and Cosθ are multiplied by the Sin and Cos of the output angle, respectively, and then added together as shown below: φ × θ × + φ θ × = Cos Cos A Sin x Sin A Monitor 2 1 Equation 4. 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, and ϕ is the angle stored in the position register. Note that Equation 4 is shown after demodula- tion, with the carrier signal Sinωt removed. Also note that for matched input signal (i.e., no-fault condition), A1 = A2. When A1 = A2 and the converter is tracking (θ = ϕ), the monitor signal output has a constant magnitude of A1 (Monitor = A1 × (Sin2 θ + Cos2 θ) = A1), independent of shaft angle. When A1 ≠ A2, the monitor signal magnitude varies between A1 and A2 at twice the rate of shaft rotation. The monitor signal is used as described in the following sections 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 by comparing the monitor signal to a fixed minimum value. 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 Table 4. LOS is indicated within 45° of angular output error worst case. |
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