Mechatronics | PartQuest™ Explore

A Linear Variable Differential Transformer (LVDT) and analog signal conditioning circuit are used for a position sensing system. An oscillator provides a sinusoidal excitation signal to the primary winding of the LVDT. The differential secondary winding outputs are rectified and low-pass filtered. The difference voltage, which is proportional to the mechanical displacement of the LVDT core, is the measurement result. This virtual prototype can be used to select the low pass filter frequency, so it is low enough that the oscillation frequency is mostly removed from the output signal, yet high enough to track the natural frequency of mechanical system the sensor is intended to measure. It can also be used to analyze parasitic effects, such as the resistance of the wires connecting the sensor to the signal conditioning circuits. This resistance affects the output voltage and therefore corrupts the measurement results. Likewise, the non-linear behavior of the rectifier diodes affects the linearity of the position measurement.This can be seen if the force input is slowly ramped from -1000N to 1000N over a 1 second interval.

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LVDT Position Sensing System

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Description

A Linear Variable Differential Transformer (LVDT) and analog signal conditioning circuit are used for a position sensing system. An oscillator provides a sinusoidal excitation signal to the primary winding of the LVDT. The differential secondary winding outputs are rectified and low-pass filtered. The difference voltage, which is proportional to the mechanical displacement of the LVDT core, is the measurement result. This virtual prototype can be used to select the low pass filter frequency, so it is low enough that the oscillation frequency is mostly removed from the output signal, yet high enough to track the natural frequency of mechanical system the sensor is intended to measure. It can also be used to analyze parasitic effects, such as the resistance of the wires connecting the sensor to the signal conditioning circuits. This resistance affects the output voltage and therefore corrupts the measurement results. Likewise, the non-linear behavior of the rectifier diodes affects the linearity of the position measurement.This can be seen if the force input is slowly ramped from -1000N to 1000N over a 1 second interval.

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Three Phase Power System

Designer12106

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9 years 11 months

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Description

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Loudspeaker with Simple Amplifier

Designer5776

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10 years 1 month

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Description

This simple* analog electronic amplifier design demonstrates the importance of multi-discipline system modeling. A swept frequency response test, from 40 Hz to 1000 Hz, shows the complex amplifier loading effect of the voice-coil and speaker-cone dynamics. The electro-mechanical resonances strongly affect the current that must be supplied, in order to maintain a flat (controlled) output voltage over the specified frequency range. For example, the current in the voice-coil reaches a null at time 0.1 seconds, which corresponds to the effective "spring-mass" resonance frequency. The loudspeaker reaches its minimum impedance around 600 Hz, or near 0.6 seconds, where the peak load current is observed.Normalized component stress monitoring signals are provided in all “datasheet specified” electronics models. For example, the simulation results show that the average power (bjt1/pwr_avg) in the BDP947 NPN BJT exceeds its 5 Watt rating across the entire range, but especially at lower frequencies. The corresponding stress monitor (bjt1/stress_ratio_power_avg) normalizes the transistor's average power relative to its 5W rating, so it is easy to see that the component is stressed (i.e. stress_ratio_power_avg &gt; 1.0), making it obvious in this case that we need a bigger transistor!*Note: This is not intended to be a practical amplifier design. There is no blocking capacitor at the output, so it allows undesirable DC current into the voice coil. The purpose is to focus attention on the dynamic characteristics of the loudspeaker and not the circuit itself.

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PID Speed Control Loop - Switching

Designer10

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12 years

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Big fan of VHDL-AMS

Description

This example shows a more detailed circuit- and logic-level implementation of the PID Control Loop shown in the companion example, “PID Speed Control Loop – Continuous”. The ideal motor drive block of the “Continuous” version is expanded here, to include both a H-bridge motor drive, and also the digital logic necessary for converting the continuous PID controller output into the desired PWM signals that are distributed to drive the gates of the power MOSFET switches. The MOSFET model was calibrated to represent an IRF3710, using only information published on the manufacturer’s datasheet.The rest of the system, including the PID block-diagram controller, the mechanical fan load and the DC Motor characterized to represent an FRC CIM Motor, are the same as in the Continuous version. While the simulation time for this switching version is significantly longer, more detailed information about practical circuit performance and component sizing is available. For example, the fan speed step response is somewhat different from the conceptual design, because of the losses in the MOSFETs under high current conditions, as well as voltage drop in the battery. Also, information regarding component stress levels within the “datasheet specified” MOSFETs and Diodes is provided.

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Three Phase Power System

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PID Speed Control Loop - Continuous

Description

This example shows a motor speed control loop that uses a PID control strategy (Proportional, Integral and Derivative), with continuous function blocks representing the controller up to the ideal voltage motor drive.

The parameter values of the generic DC motor model were calibrated to represent an FRC - First Robotics Competition CIM Motor, as described in the article “<a href="https://www.systemvision.com/blog/first-robotics-frc-motor-modeling-may-6-2016">First Robotics (FRC) Motor Modeling</a>”. The voltage drive, the motor and the mechanical fan load were “assembled” using a conservation-based modeling approach. This approach to system modeling is much like assembling a hardware prototype, and does not require the user to develop an analytical model of the “plant” being controlled. Both the static and dynamic system characteristics “emerge” naturally from the model, simply because they are connected on the schematic. Additional electrical circuit components, mechanical loads and other “physical” elements can easily be added by simply placing them on the schematic and “wiring” them together. In fact, a more detailed implementation of the motor drive is shown in the companion example, “<a href="https://www.systemvision.com/groups/mikedonnellys-workspace/designs/pid-speed-control-loop-switching">PID Speed Control Loop – Switching</a>”. In that example, a design for the logic and power electronics needed to implement a PWM-based, switched MOSFET H-bridge drive is included.

Because this version simulates very quickly compared to the switching version, it is well suited for early validation of the control strategy and PID tuning, as well as loop stability and frequency response analysis. Note that the P, I and D "gains" for this nominal design were computed using the <a href="https://en.wikipedia.org/wiki/Ziegler%E2%80%93Nichols_method">Ziegler-Nichols</a> classic PID tuning method.

Another version of this design, which simulates quickly but includes the important and practical aspect of a limited drive voltage range, is show in “<a href="https://www.systemvision.com/groups/mikedonnellys-workspace/designs/pid-speed-control-loop-continuous-and-limited">PID Speed Control Loop – Continuous and Limited</a>”. It is instructive to compare the speed step response between that design and this one.

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