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Motors and Mechatronic Systems: Analog Circuits and Beyond!

Say the word “analog” to most EEs, and thoughts turn to op-amps, power devices, I/O, or signal conditioning circuits. But, the system beyond the circuit is also full of “analog,” if we include everything described by continuous variables and behavior, e.g., the “mechanical” aspects. The word “mechatronics” has been coined for technology that combines electronic and mechanical elements, including the motors and sensors that perform at the interfaces.

In this blog post I’d like to show a few example mechatronic systems that can be modeled and simulated in SystemVision Cloud, a free online schematic capture and simulation platform. The examples demonstrate not only analog circuit designs, but also key elements of the external system including controls, motors/actuators, and dynamic mechanical loads that are essential to understanding the performance of the entire system. The examples also demonstrate the early stage “concept exploration” vs. later stage “implementation verification” phases of the design process.

Stepper Motor for Open-Loop Angle Control (Click to open a live version)1

Stepper Motor for Open-Loop Angle Control (Click to open a live version )1

This first design example shows a stepper motor's ability to control a load angle, not by using a rotation angle sensor for feedback, but rather by simply counting steps. In this case, eight steps are taken in the forward direction, followed by two steps in the reverse direction, repeating this cycle every one second.

The load angle (brown waveform) is seen to increase and decrease as commanded by the direction control signal (light blue waveform) and a 100ms periodic clock input (not shown). But on the fourth step of the second cycle, the windup spring's torque exceeds the capability of the stepper motor, causing it to “snap back” suddenly to a negative angle, below the initial starting point! This “dramatic” result illustrates the need to include all the relevant “system context” when designing a motor and its control electronics.

Note that the switching order of the phase currents of the motor (red and dark blue waveforms) reflect the sequencing of the drive switches in the forward and reverse direction. At this phase of the design process we are trying to explore and verify proper switch sequencing, adequate motor stall torque, step ringing/settling times, and perhaps maximum step rate limits. These design aspects do not require detailed power MOSFET switches and complex gate drive circuits for proper assessment, so ideal digitally controlled switches are being used as they generally allow faster simulation.

This second design example extends the range of interacting technologies, including more detailed analog and digital electronics as well as fluidic aspects. The system includes a DC-motor/pump/pressure-regulator, an electro-fluidic fuel injector, and a mixed-signal drive circuit that regulates the injector current.

The design shows the ability to reduce the injector drive current after injection-start, for the purpose of improved efficiency. This leverages a characteristic that is common to electro-magnets, that a lower current is needed to “hold” the magnet closed than is needed for initial “pull-in”. The simulation results shown in the upper left include the commanded injector current set-point (dark blue waveform) and the actual current (orange waveform) which is being regulated by PWM switching of the Power MOSFET. The current set-point is initially set to 3.0A, but is reduced after 10msec to just 1.3A to hold.

The results shown in the upper right include the regulated pressure at the injector inlet (green waveform) and the injector plunger position (magenta waveform) during the pull-in and release cycle. Both the pressure regulating pump and the fuel injector component models (shaded areas) are created by “assembling” more primitive models of key physical effects. For the pump, this includes the DC motor, an ideal fixed-displacement pump, and a bypass valve. The injector model includes the electro-magnet, spring/mass/damper, and travel-limiting hard-stop, as well as the injection valve and a fixed orifice representing the spray nozzle. The nozzle flow rate (light blue waveform) is integrated to give the fuel volume delivered to the engine (red waveform) a key performance metric for an injection event.

Ideally this fueling volume depends only on the duration of the commanded injection time. But in

practice, it also depends on the dynamic response of the drive circuit and the injector, the regulated pressure from the pump, and other factors. The ability to observe and analyze these multi-discipline interactions is critical for this type of mechatronic system development.

In this last example we look at design concept exploration that can occur at the beginning of a system development process. The schematic represents a Permanent Magnet Synchronous Motor (PMSM) driving a simple mechanical load, which again includes a windup spring to test the torque capability of the machine. Details of the drive electronics have not yet been defined, so continuous Clarke/Park Transformation blocks apply ideal sinusoidal phase voltages and read the phase currents of the motor. The light-blue waveform is the quadrature current (Iq, the “torque current”) that is commanded for the test. The orange waveform shows the corresponding load shaft angle response, which in steady-state is proportional to the motor torque because of the spring (Hooke’s Law). Also, the motor internal torque and Phase A current are shown (green and dark blue waveforms, respectively).

Because the control models are continuous, the simulations run very quickly. This version is useful for system-level assessment of the overall design performance. But, you can also explore a later-stage version of this design, which includes ideal switches and verifies the space-vector modulation algorithm that drives them here: PMSM Motor and PWM Drive1 . Another version with a complete drive circuit implementation, in which the stress levels of the Power MOSFET switches can be analyzed, is also available here: PMSM Motor and PWM NMOS Drive1 .

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In Summary , Analog electronics plays a key role in mechatronic systems. But, an effective design process must include a holistic view of the multiple interacting technologies. It must also support the progression of development from early/abstract/conceptual to final/concrete/detailed-implementation, with appropriate analysis and trade-off assessment at each stage.

1 If you click on a link to open the “live” version of any design, you can observe signals on any net or within any component and view component parameter values. You can also modify and re-simulate a copy of the design, to see the effect of the changes you make.

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