|
2. Background Information | 4. Detailed Circuit Description | |
Lateral effect transistors (figure 8) are devices having a large photosensitive area (~20x20mm), which when illuminated by a strong light beam such as a laser, will give positional information (See later, section 4). In this application, only a horizontal position is required; taking the difference between the outputs from the two side connections of the transistor, (subtracting them) will gjve a voltage that is linearly dependent on the horizontal position of the beam on the detector surface. However, this alone would not be sufficient as the condition specified in figure 6b would not be met; ie when both diffracted beams shift by the same amount, the output would not be zero. Therefore two lateral effect transistors are necessary; the difference between their position voltages will not alter if the
diffracted beams merely shift, but will change as required if the beams move closer together or further apart.
Figure 5 shows the experimental arrangement, and figure 7 shows a circuit block diagram that will achieve the required positive feedback. The output of this circuit can then be used to control a servo-motor, that can change the length of the muscle. In this way, any deviation in the sarcomere length during muscle stimulation will immediately be detected and corrected by the motor. Note also, with reference to the block diagram, figure 7, that some sort of Automatic Gain Control (AGC) circuit is needed, so that the changes in intensity between the diffracted beams (section 2.2 and figure 4) do not distort the output. This will be discussed in more detail in the next section. Not shown on the block diagram is the power supply: this must convert the 240 V AC mains voltage into positive and negative 15
volt supplies for the op-amps.
|
2. Background Information | 4. Detailed Circuit Description | |