Pulse encoders

The phasor /, and /, are separated and then controlled separately as discussed later. For more precise speed control a pulse encoder feedback device can also be employed. The characteristics now improve to Figure 6.10. The torque can now be maintained constant at any speed, even at zero speed. 

Pulse encoder-To feed back actual speed ol the motor and the angular position of the rotor with respect to the stator at a particular instant. 

The method by which a particular coherence transfer pathway is selected using field gradients is illustrated opposite. The first gradient pulse encodes a spatially dependent phase, 2 where... 

Figure Bl.14.1. Spin warp spin-echo imaging pulse sequence. A spin echo is refocused by a non-selective 180° pulse. A slice is selected perpendicular to the z-direction. To frequency-encode the v-coordinate the echo SE is acquired in the presence of the readout gradient. Phase-encoding of the > -dimension is achieved by incrementmg the gradient pulse G...

Figure Bl.14.2. Gradient-recalled echo pulse sequence. The echo is generated by deliberately dephasing and refocusing transverse magnetization with the readout gradient. A slice is selected in the z-direction and v- and y-dimension are frequency and phase encoded, respectively.

In order to encode displacement as opposed to average position, the gradient is applied in such a maimer as to ensure that ). Generally, this means applying gradient pulses in bipolar pairs or applying uni-... 

Figure Bl.14.9. Imaging pulse sequence including flow and/or diflfiision encoding. Gradient pulses before and after the inversion pulse are supplemented in any of the spatial dimensions of the standard spin-echo imaging sequence. Motion weighting is achieved by switching a strong gradient pulse pair G, (see solid black line). The steady-state distribution of flow (coherent motion) as well as diffusion (spatially...

Pulse Inputs Where the sensor within the measurement device is digital in nature, analog-to-digital conversion can be avoided. For rotational devices, the rotational element can be outfitted with a shaft encoder that generates a known number of pulses per revolution. The digital system can process such inputs in either of the following ways ... 

Position Feedback Encoders generate 512 pulses per revolution of the motor. One pulse correlates to the me2tsurement of 0.023 degree in angular displacement of the mandrel. 

The variable ERRORmj n represents the error in the position of the mandrel over an increment in TIME, in seconds. ERRORman is calculated by subtracting the actual pulses accumulated, PULSEman, from the desired number of pulses that would be generated under perfect control. The desired number of pulses for perfect control is determined by the set point speed, RPSman, revolutions per second and the mechanical gear reduction. The constant 15630 is the product of encoder counts per revolution and the thirty to one gear reduction of the mandrel. 

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