The SCR In Figure 1 Requires A 3 V Trigger Using Mults

The SCR In Figure 1 Below Requires A 3 V Trigger Using Multsim Desig

The SCR in Figure 1 below requires a 3 V trigger. Using Multisim, design a system by which the gears are shifted when a CdS photocell resistance drops below 4k. Figure 1 Using Multisim, design a system by which a control signal of 4 to 20 mA is converted into a force of 200 to 1000N. Use a pneumatic actuator and specify the required diaphragm area if the pressure output is to be in the range of 20 to 100kPa. An I/P converter is available that converts 0 to 5 V into 20 to 100 kPa.

A block diagraph of the system is shown below. (Hint: Use a differential amplifier. You are only designing the circuit to interface into the I/P below)

Paper For Above instruction

The design challenge presented involves creating two interconnected systems utilizing Multisim: one for triggering an SCR based on photocell resistance and another for converting an electrical control signal into a pneumatic force. Both systems are fundamental in automation and control engineering, and their integration requires careful consideration of electronic, pneumatic, and interface components.

Designing the SCR Trigger System Based on Photocell Resistance

The first part of the task is to design a circuit that activates an SCR when the resistance of a CdS photoconductive cell drops below 4kΩ. Photocells or LDRs (Light Dependent Resistors) are variable resistors that decrease in resistance with increasing light intensity. When light hits the photocell, its resistance drops, influencing the voltage divider circuit. Using Multisim, a typical approach involves connecting the photocell in series with a fixed resistor to form a voltage divider. The voltage across the photocell can then be monitored.

An operational amplifier (op-amp) comparator circuit can compare this voltage with a reference voltage corresponding to the 4kΩ resistance threshold. When the photocell’s resistance drops below 4kΩ, the voltage exceeds the reference, switching the comparator output. This output then triggers the SCR, which can be driven via an appropriate gate circuit. The SCR's triggering voltage requirement of 3 V is achieved by selecting the correct biasing and gate resistor. This circuit ensures that the SCR turns on precisely when the photocell resistance indicates sufficient light presence.

This setup involves careful calibration of the voltage reference and ensuring the comparator's output voltage exceeds the SCR's gate trigger threshold. The design also considers the response time and hysteresis to prevent false triggering due to noise or fluctuations in ambient light.

Converting 4-20 mA Control Signal to Pneumatic Force

The second part involves interfacing a control signal of 4 to 20 mA with a pneumatic actuator to generate a force between 200 and 1000 N. The system requires translating an electrical current signal into a pneumatic pressure that actuates a diaphragm. An important consideration here is the use of a differential amplifier to process the current signal into an appropriate voltage input for the I/P converter.

The I/P (current-to-pressure) converter accepts a 0-5 V signal and transforms it into a pressure range of 20 to 100 kPa. To interface the 4-20 mA signal with the I/P converter, a resistor converts current to voltage (using Ohm's law). For example, a resistor R can be chosen such that at 20 mA, the voltage reaches 5 V stimulus level, thus R = 5 V / 0.02 A = 250 Ω.

A differential amplifier subtracts the baseline (corresponding to the 4 mA at the lower end) and amplifies the difference to generate a proportional voltage output (0-5 V) aligned with the 20-100 kPa range. The calibration ensures that 4 mA corresponds to 20 kPa and 20 mA corresponds to 100 kPa.

The pneumatic actuator's force output F is related to pressure P and diaphragm area A by F = P × A. To generate a force between 200 N and 1000 N within the pressure range, the diaphragm area must be designed accordingly. For example, for a maximum pressure of 100 kPa and desired maximum force of 1000 N, the diaphragm area A = F / P = 1000 N / 100,000 Pa = 0.01 m² (or 100 cm²). This area ensures the force output aligns with the control signal range.

Thus, the overall system includes the current-to-voltage resistor network, the differential amplifier for signal conditioning, and the I/P converter feeding into the pneumatic actuator. Proper calibration and shielding are essential to maintain accuracy and responsiveness.

Conclusion

This integrated control system combines electronic circuitry with pneumatic actuators through signal conditioning and interface components. The precise triggering of the SCR via photocell sensing enhances automation in gear shifting applications, while the current-to-force conversion streamlines actuator control in industrial settings. Implementing such a system involves meticulous design, calibration, and testing within simulation tools like Multisim, ensuring reliable, efficient operation in real-world environments.

References

• Sedra, A. S., & Smith, K. C. (2015). Microelectronic Circuits (7th ed.). Oxford University Press.
• Millman, J., & Grabel, A. (1987). Microelectronics (2nd ed.). McGraw-Hill Education.
• Boylestad, R. (2009). Electronic Devices and Circuit Theory (11th ed.). Pearson.
• Ogata, K. (2010). Modern Control Engineering (5th ed.). Prentice Hall.
• Kuo, B. C. (2003). Automatic Control Systems (7th ed.). Wiley.
• Shinskey, F. G. (1996). Process Control Systems: Application, Design, and Tuning. McGraw-Hill.
• Rao, S. S. (2019). Mechanical Vibrations (6th ed.). Pearson.
• Hurst, P. (2016). Pneumatic Control Systems. Elsevier.
• Harper, J. K., & Maher, P. H. (2018). Control of Pneumatic Actuator Systems. Journal of Automation Engineering, 12(3), 152-162.
• Multisim Circuit Design Guide (National Instruments). Retrieved from https://www.ni.com/en-us/innovations/what-is-multisim.html