Pressure sensor has been implemented as a green feature in building to conserve

Question

Pressure sensor has been implemented as a green feature in building to conserve energy or example, escalators make pressure sensors which activate only when there is human traffic. The escalators are stationary when not in use. At the same time, pressure sensors located underneath the escalator mat, will detect human traffic towards the escalators, and initiate their movement accordingly Escalator activated when pressure sensors activated by buman traffic Escalator mat with Figure 8 using pressure sensors to conserve energy in escalator Your task is to design a pressure system prototype for human traffc detection. The output of pressure sensor will be converted to voltage and sent to the escalator computer control system so as to activate the lift for 20 seconds; Encautors Pressure Piate Eiguree Pressure detection system forthe sensor capet

Explanation / Answer

The best choice for the pressure transducer in this case will be a differential capacitive transducer attached with diaphragm or bellows. We will prefer diaphragms for their more linearity. Now generally a transducer is low power application, so it won't give you 30 volts. In general transducers are designed to give output voltage within the range of 0-5 volts as per ISA norms. That's why you require the amplifier. we are choosing differential capacitance sensors here because they are highly accurate, stable and rugged and have a wide operating range. Since the operating range is large, it provides good linearity in yhe output. Accordingly the amplifier must have a gain of 6 to give output within the range of 0-30 volts.

Now for the case where for less than 10 bar pressure a green LED will turn on anf for more than 10 bar pressure a red LED will turn on, we will require a comparator connected to the output of the amplifier separately (parallely). Since 0-30 bar pressure input will correspond to an output voltage of 0-30V after amplification so reference voltage for comparison corresponding to 10 bar pressure will be 10V. The reference is connected to inverting terminal of OPAMP and output from signal amplifier is connected to non-inverting terminal of OPAMP. Parallely, the output of signal amplifier is connected to a low-pass filter to suppress high frequency components. Back to the comparator, the output of the comparator will be connected to two LEDs which will be in parallel configuration and red one will be forward biased while the green one will be reverse biased as connected in a full bridge rectifier mode. Now the LEDs will be ON between the input range of 0-25V since the bias for OPAMP is -15V to +15V and input above 25V will give output greater than 15V where the opamp will already be in saturation with a voltage of +15V. Above 25 V input the output will be in saturation and still the red LED can be ON. So for input range of 0-10V the output from the comparator will be in the range of -10V to 0V, so at that time the green LED connected in reverse bias will turn ON while the red one in forward biased configuration will be off. When input is exactly 10V, none of the LEDs will glow as it will result in 0V output from the comparator. When the input voltage is between 10-30V, the output of the comparator will range from 0-15V (since in saturation it cannot exceed above 15V) and the red LED will be ON while the green one is OFF.

Now, you are required to send a signal of 24V to computer for activating the escalator. What i understand from the question is that whatever the output of the signal amplifier be, the computer must be sent a 24V signal. In that case, it is preferable to use a programmable gain amplifier (PGA) in series with the filter output which will adjust the range of 0-30V signal to 24V fixed by adjusting the gains automatically using external analog or digital control signals. Alternatively, you can also use a zener diode of 24V connected parallely to the output of the filter in reverse bias but this has the disadvantage compared to PGA in that for high breakdown voltage of zener diode its temperature coefficient will also rise drastically.

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