Electromechanical Simulation Software and the Role of Circuit Analysis

July 13, 2020 Cadence PCB Solutions

Key Takeaways

  • Electromechanical systems need power and control circuitry to run properly and make efficient use of input power.

  • To maximize power conversion efficiency and power delivery to motor controllers, your circuits need to be carefully designed and simulated.

  • Circuit simulations should be performed before performing mechanical simulations to ensure your system will work properly and that the right components will appear in your PCB layout.

Electromechanical simulation software for motor control

Electromechanical design is much easier with circuit simulation and electromechanical simulation software.

Electromechanical systems take multiple pieces of software to design successfully, including electromechanical simulation software.  Your electromechanical simulation software and your mechanical modeling tools ensure your system will behave as you intended mechanically, but they can’t address the specific functions of your circuits before you create mechanical and electromechanical simulations.

Your electrical design and analysis tools are important for designing your motor control circuits and power delivery systems, which then ensure your mechanical system can operate as you intend. Real electromechanical simulation software will use a 3D model of your system for analysis, but this requires creating your schematics and PCB layout first. If you can finish your circuit design and PCB layout before you start your mechanical design and electromechanical simulations, you’ll be able to create a system that meets your electrical and mechanical requirements.

Why Perform Circuit Simulations Before Using Electromechanical Simulation Software?

The electromechanical system design process requires close collaboration between an electrical designer and a mechanical designer. The mechanical designer’s job can be difficult because they have to consider things like enclosure design, range of motion for an enclosure or package, and the power delivered to motors in the system. These aspects of mechanical design, particularly the power delivered to a motor and range of motion, can only be properly considered after power and motor control circuits are designed.

The two critical circuits that must be simulated are your power delivery/management and motor control systems. These circuits need to deliver the required power to mechanical components with minimal loss (i.e., highest possible efficiency). Neither of these systems can be 100% efficient, but you can get as close as possible with the right circuit design and analysis tools

Here’s what you should examine in each set of circuits to ensure your mechanical components receive the required power with minimal losses.

Power Regulation and Delivery

Your power delivery system typically needs to accommodate the input power from an unregulated DC source or AC mains. In either case, you’ll need to design a power regulation stage to provide power to downstream motors on your board. Because motors come in the AC and DC varieties, it’s best to match the motor type to the input power type. The power regulation and delivery strategy need to be matched to the power source being used.

For more complex systems that will run on AC power, but need a DC motor or stepper motor, you’ll likely need a power factor correction (PFC) circuit with a switching regulator to ensure stable current and high-efficiency power delivery to your electromechanical system. For conversion to pulsating DC from single-phase power, a standard rectifier circuit with an output capacitor can be used. For three-phase conversion to DC power, use the circuit shown below.

A circuit schematic for converting a three-phase AC source to DC

Three-phase rectifier circuit for AC-DC conversion.

In your simulation software, you can model three-phase AC input power as a set of three AC sources, each with the phase offset of 120°. Simply sweep the source amplitudes with your diode models to examine the magnitude of the resulting ripple and calculate the power factor. A very stringent European standard on three-phase AC power conversion systems is IEC 61000-3, which limits total harmonic distortion (THD) of input current to not exceed 48% at 16 to 75 A RMS input current per phase. Lower power devices, up to 16 A RMS input current per phase, are limited to no more than 33.8% of THD.

The power factor and THD of your system are related, and excessively high THD will cause your electromechanical system to waste power when THD is high. However, a bus capacitor is required in DC power conversion to provide low pass filtering and more stable input DC power.

In your simulation, you’ll need to experiment with different capacitors to see which values (including their ESL and ESR values) will provide the highest power factor conversion to your electromechanical components. This can be done with parametric sweeps while sweeping capacitor values and calculating the total output power efficiency.

Motor Control

The term “motor control” encompasses speed and torque control on a running motor. This is provided by adjusting one of the parameters in the driving signal. For practical AC driving circuits, a typical motor control method for AC motors is to use a potentiometer with a diac and triac to adjust the power delivered to the motor. 

An example with single-phase power is shown below. In such a simple strategy, you can simulate the potentiometer as a fixed resistor and simply vary its value in a parametric sweep while simulating the power delivered to the motor (M1 below).

Yield curve histogram in a yield optimization simulation

A speed control circuit for a single-phase AC motor. A similar circuit can be built with a three-phase motor.

The most common method for AC motor control is to adjust the frequency of the driving signal. When constructing a variable frequency drive circuit, you’ll need to simulate the output power as a function of source voltage/current, either for single-phase or three-phase input power. In a SPICE simulation, you can test your circuit design using frequency sweeps.

For DC control, the exact method depends on the type of motor being used. The table below summarizes the different methods used to control DC motor speed, and thus, torque.

DC Motor Type

Driving/Control Method

Brushless

Change the voltage applied to the armature. This might use a power transistor, which acts as a linear voltage regulator.

Brushed

Same as a brushless motor.

Servo (Brushed motor with feedback control)

Typically uses a PWM signal for driving with a positive feedback signal for precise speed selection.

Stepper

Source a PWM signal with a duty cycle matched to the spacing between points on the motor.

Note that brushed and brushless motors can also use a PWM signal where the average voltage received by the motor is equal to the duty cycle multiplied by the line voltage. In this way, the motor can be controlled electronically with a programmable PWM generator, such as in an MCU.

Software Solutions for Design, Simulation, and Analysis 

Power delivery circuits and motor control circuits are easy to design and layout when you have access to the best PCB design and analysis software. The front-end design features from Cadence integrate with the powerful PSpice Simulator for circuit design and simulation, followed by PCB layout. Cadence also has a suite of SI/PI Analysis Point Tools for post-layout verification and simulation. Once you’ve created your circuits and your PCB, you can import it into electromechanical simulation software to examine your system’s mechanical behavior.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

About the Author

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