Today's industrial and appliance machine builders must straddle a deep divide between remaining competitive in the marketplace and addressing regulatory requirements for safer, more efficient and yet feature-rich products. From a time-to-market standpoint, the traditional approach of buying off-the-shelf prepackaged motor drives, then using a system integrator to put together the system, is no longer a viable option. And, with the growing complexity of the motor drive, the option of bringing the entire development in-house is also no longer a viable option.
A myriad of specialized skills that blends control algorithms and coding techniques with analog circuit and power management design is now necessary, escalating the effort beyond the comfort zone of most system engineers. A new approach is needed that reduces overall manufacturing cost, development schedules, and associated technical risks in bringing to life these complex yet low-cost motion control systems. To answer the call, an integrated design platform and a companion mixed signal chipset have been developed specifically for motor control applications.
This integrated design platform, called iMOTION™, is comprised of four major elements:
A digital control technology, labeled Accelerator™, which uses configurable control engine with parallel processing and customizable peripherals to execute high-performance field-oriented control and sensorless algorithms.
An analog control section that uses monolithic high-voltage IC (HVIC) technology to integrate the gate drive, current sensing, voltage sensing, power supply control, and other analog functions that interface between the digital controller and the power stage.
A power silicon technology combining thin wafer, non-punch-through (NPT) IGBT with ultrafast, soft recovery diode to achieve lower conduction and switching losses without excessive noise generation.
Integrated power module technology, PlugNDrive™, using insulated metal substrate (IMS) with over-molded package cost-effectively integrates the 3-phase inverter power stage with gate driver in a single compact package.
Elements of this platform are codeveloped to take advantage of the linkage offered by the simplified interface and interlocking features among the digital, analog, and power stages. For example, the HVIC current sensing signal output is formatted to directly connect to the customized peripheral port of the Accelerator. The reduced latency and higher signal integrity feeding into the current control loop contributes to higher bandwidth demonstrated to greater than 5 kHz. Another example is the integration of temperature and current sensing elements with the power stage that are designed to directly interface with the HVIC chip. The result is better overload protection and added level of diagnostic signals that can be processed by the HVIC and digital controller to avoid nuisance trip.
Now, let's discuss the technology, application, development system, and performance examples of an encoder-based platform for servo drive and a sensorless platform for spindle and appliance drives.
The digital control technology is based on the concept of a configurable control engine and customizable peripherals. You can achieve design flexibility several ways:
- An adjustable FOC algorithm with host CPU access to all important drive parameters and monitor points. The ability to make some of the more common changes to the algorithm structure without changing the code should be included.
- Allow customization of the fundamental algorithm structure by building it from a library of motion IP core modules.
- Express the algorithm design in a clear block-diagram format for understandability — preferably one that shows control action.
The preconfigured adjustable algorithm is described by a system block diagram, shown in Fig. 1, which shows how the control acts on the various data paths, giving the user better understanding of system behavior. This enhanced system perspective is important in creating robust controls with fewer soft failures.
Traditional programmable DSPs or microcontrollers have a processing engine at their core that's designed to perform a diverse range of functions combined with a set of I/O peripherals. These resources are permanently fixed in hardware. Because one size does not fit all in motor drive control, this results in suboptimal processing for specialized tasks that motor controls require involving high-speed timing and low latency, and intensive computation.
Another drawback of the fixed core and peripherals is this architecture can't support changes, as newly created peripheral functions are required to be processed by the dedicated hardware. An example in servo motor control is in changing from a six line incremental encoder to a 14-bit parallel absolute encoder or a resolver, which requires an analog-to-digital interface function. Manufacturers using software-based controllers typically go through the costly process of re-hosting on a new DSP or microcontroller to gain the specialized processing capability they need.
Using the flexible approach that Accelerator provides, users can focus on their application rather than on rigorous software design methods requiring expertise with complex tools. Detailed module testing of the design by the user isn't required because this was done while the FOC design was being built. Tests were carried out on a module level using digital simulation tools followed by system level testing as a whole using real motors and load conditions. The result is that implementation of the design is much easier because the algorithm is flexible, and its structure and control behavior are better visualized.
You can implement Accelerator technology in field programmable gate arrays (FPGA) or dedicated silicon chips. The object libraries are kept on a host computer for creating and downloading into the target execution hardware — a full function motor drive. The processor can be configured for specific functions without changing the physical component hardware. Until recently, reconfigurable computing hardware was used only for coprocessors for the main CPU to optimize performance and off-load the main CPU. Accelerator technology goes a step further and pulls speed and current control loop functionality and peripherals out of the main CPU, placing them into dedicated silicon.
The integrated design platform is also supplied with ServoDesigner™, a configuration software package providing control diagnostic testing that includes complete access to all internal registers, embedded buffering of test parameters, and a graphical per-cycle test point monitor. Using this software package, designers can alter the control structure of the system without any programming effort. Structural and parametric changes are possible by selecting optional hardware logic switches through the RS232C communications port of the host PC. Once configuration is complete, parameters are stored in the EEROM to facilitate rapid power-up-and-run operation, as shown in Fig. 2.
Servo Drive Design Platform
Combining the iMOTION family of codeveloped mixed-signal chipsets, including the IGBT module, monolithic current sensing high-voltage IC with configurable control engine implemented by hardware logics in the FPGA, a complete servo drive development system is constructed. As seen in Fig. 3, this development system has a simple, low-cost, and flexible structure, while performing complete servo amplifier functions. Key features include the following:
- High bandwidth torque loop response up to 5 kHz (see Fig. 4).
- Adjustable speed loop bandwidth, typically at 400 Hz.
- Loss minimization space vector PWM with maximum frequency up to 70 kHz.
- Current loop execution time less than 6µs.
- Flexible drive configuration (PMAC or induction motor).
- Quadrature encoder interface.
- RS232C/RS422 and fast 6-MHz SPI interface.
- Built-in 8-kByte trace buffer memory for diagnostics and monitor function.
- Parallel interface for microcontroller expansion or debug port.
- Complete IP library (IRMCO201) available as licensable code.
With this design platform (IRMCS201), users can configure the drive parameters for specific applications. To complete a configuration, there's no need to implement logic by text line-based coding, as necessary in traditional programming. Instead, the user just chooses appropriate functions by selecting switches and writing values to the associated parameters. With the ServoDesigner™ PC tool, typical configuration can be completed in hours instead of months.
Sensorless Control Design Platform
Likewise, a complete development system for a sensorless control of permanent magnet ac motors with sinusoidal back EMF is based on a configurable control engine for the FOC and a patented sensorless algorithm to achieve high starting torque and smooth speed ramping without the use of sensors or voltage feedback sensing. Fig. 5 shows these components, together with Xilinx Spartan2E-400, simplify hardware construction and perform complete motor control functions. The system also uses the iMOTION family of codeveloped mixed signal chipset, including the IGBT module, and monolithic current sensing high-voltage IC.
Important features for this sensorless control design platform include the following:
- Direct interface to IR2175 for current sensing with 10-bit resolution and 8µs latency.
- No voltage feedback sensing required.
- High starting torque and smooth speed ramp-up with adjustable current limit (Figs. 6a and 6b).
- High-speed operation up to 100,000 rpm (2-pole motor), speed operating range from 10% to 100%, and speed accuracy at 0.01%.
- Versatile loss minimization Space Vector PWM with maximum frequency up to 70 kHz.
- Complete sensorless control computation time less than 10µs.
- RS232C/RS422 and fast 6MHz SPI interface.
- Parallel interface for microcontroller expansion or debug port.
- Complete IP library (IRMCO203) available as licensable code.
Some applications require sinusoidal sensorless control with PM motor instead of traditional trapezoidal control. Heating pumps and high-speed spindles are two examples that require less torque ripple, less acoustics noise, and less temperature rise of the motor.
A sensorless control design platform (IRMCS203) is well-suited as a design verification tool for these applications. This development system comes with automated parameter generation, based on the motor name plate data. With this tool, which is part of ServoDesigner™ PC configuration toolbox, the user can configure the sensorless control system within an hour.
In Part 2 of this article, we'll discuss the mixed signal chipsets that are designed to support the two iMOTION™ platforms. We'll also present actual hardware implementation integrating the chipset into system board.
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