Tweet

Posted on 05 August 2019

Decentralized Drive Technology Solution with MPU

Free Bodo's Power Magazines!

 

 

 

In factory automation, decentralized drive solutions have many benefits over centralized drive solutions. For the latter, the frequency inverter and control unit is mounted inside a control cabinet and it requires multiple cabinets as well as star cabling topology for each drive.

By Thomas Mauer, Texas Instruments

Decentralized drives have the frequency inverter and control unit either directly attached or positioned close to the drive – this simplifies cabling as line topology can be used and reduces the overall number of installed control cabinets. In particular, production sites with many drives benefit from a decentralized solution as it can be found in manufacturing streets with belt conveyer or luggage transportation belt at airports or high rack warehouses.

Many manufacturers of decentralized drive solutions follow a modular system approach with the following building blocks: The central control or supervisor unit, the motor control, the frequency inverter, power stage or gate driver, motor current sensing and position encoder, motor protection unit and the industrial Ethernet or fieldbus interface. Often those blocks are spread over multiple subsystems with individual microcontrollers, microprocessors, digital signal processors (DSPs) and FPGAs/ASICs.

Decentralized drive system block diagram

This modular system approach has the benefit that each subsystem can be developed or replaced by itself. But it also has trade-offs like component sourcing from multiple vendors, non-compatible application programming interfaces (APIs) between the subsystems and it requires a large PCB footprint. All this makes the production of decentralized drive solutions more expensive.

The integrated system approach

Manufacturers of decentralized drive solutions can save production cost with the Sitara AM437x microprocessor family from Texas Instruments (TI) which integrates many of those building blocks into a single device. The powerful ARM Cortex-A9 processor handles many of the application tasks including motor control, fieldbus stack and central control unit. The industrial Ethernet, motor position encoder and real-time critical tasks are resourced by the industrial communication subsystem (ICSS). The remaining decentralized drive building blocks are sourced by integrated peripherals, for example the enhanced pulse-width modulator (ePWM), the enhanced quadrature encoder peripheral (eQEP) and the on-chip ADCs. Overall the Sitara AM437x solution allows architecting the decentralized drive solution across multiple subsystems while integrated into a single microprocessor device.

How decentralized drive building blocks are integrated into the Sitara AM437x

Many motor control loop algorithms exist and the IP used is the differentiating mark of the manufacturer’s products. One of the algorithms, field oriented control (FOC), is used to drive a 3-phase brushless DC (BLDC) motor. For this algorithm the motor control loop is processed, as an example, every 100μs (10kHz). In the control loop the next PWM’s duty cycle is calculated. With the usage of a real-time operating system (RTOS), like SysBios from TI, the ARM Cortex-A9 can execute the motor control loop. The floating-point support within the ARM Cortex-A9, as well as the processor speed between 300MHz and 1GHz, helps to reduce the execution time of the motor control loop – hence there is more execution time available for other tasks. Those tasks may include the fieldbus stack for industrial Ethernet, the control/supervisor unit, statistic database and web/maintenance services.

Once the motor control loop has finished its calculations the ePWM peripheral is updated with new duty cycles. The ePWM output acts as frequency inverter, which is fed into the power stage to create the 3-phase power signal for spinning the motor.

The ePWM block also generates the trigger signal to start the measurement of phase currents and motor position feedback. Those measurement results are used by the next cycle of the motor control loop. The phase currents are translated into voltage levels and sampled by the on-chip ADC. An external high precision position encoder with an EnDAT 2.2 measures the motor position or BiSS interface. The ICSS interfaces with the EnDAT/BiSS encoder and acquires the motor position for the motor control loop. Alternatively the enhanced quadrature encoder (eQEP) with lower position resolution or any other feedback system via the parallel or serial interfaces can be used.

The fieldbus interface, nowadays based on one of the many industrial Ethernet solutions, is used to exchange drive control commands and process data with the programmable logic controller (PLC). The Sitara AM437x has a flexible industrial Ethernet interface based on ICSS. The ARM Cortex-A9 application loads an ICSS firmware at system start time with the manufactures preferred real-time Ethernet (RTE) protocol. TI and its partners offer the most common RTE protocol solutions including EtherCAT, Profinet RT/IRT, Sercos 3, Ethernet/IP and Power-Link. The Sitara AM437x also supports legacy serial based fieldbus solutions like Profbus DPv1 slave and CAN.

Additional integrated peripherals complement the decentralized drive building blocks. For example the enhanced direct memory access (EDMA) for transferring memory blocks of data without processor involvement, USB for interfacing to a service terminal on a laptop, or GBit Ethernet MAC to connect to an Ethernet infrastructure.

The industrial development kit

The AM437x industrial development kit (IDK) combines the entire set of building blocks for a decentralized drive solution. The development kit supports a 3-phase motor with the DRV8313 power stage and has protection against fault conditions like overheating and current overload. The fault signal of DRV8313 is fed into the PWMs TRIPZONE input which immediately disables ePWM signal generation in order to protect the power stage.

The IDK supports a M12 connector to directly interface to an EnDAT 2.2 position encoder from Heidenhain. Here the SN65HVD78D transceiver converts the high-speed EnDAT 2.2 levels into signal levels accessible by ICSS.

IDK system block diagram

Two instances of TLK105L, an Ethernet PHY for industrial and harsh environments, are used for the industrial Ethernet interface. The PHY has additional diagnostic capabilities for measuring signal quality or the distance at which a cable break occurred.

The IDK has expansion connectors to support evaluation of applications beyond the baseline IDK use cases: This could be multiple motors, higher motor voltages, high precision ADCs with Sigma-Delta interface and additional fieldbus interfaces.

The power supply subsystem on IDK is build up by a discrete solution based on TPS5402D. Alternatively the AM437x supports the integrated power management unit (PMU) with TPS65218 which is used by the AM437x Starter Kit boards.

Conclusion

The Sitara AM437x saves production cost for manufacturers of decentralized drive solutions by integrating major system building blocks into the microprocessor. This saves component cost and PCB space. For maximum flexibility the AM437x supports external interfaces for connecting optional system building block and resources.

TI offers the AM437x IDK bundled with the industrial software development kit (industrial SDK) to customers of decentralized drive solutions. The kit allows evaluating and developing application with a motor control framework, position encoder EnDAT 2.2 interface, current feedback and industrial Ethernet protocols.

 

VN:F [1.9.17_1161]
Rating: 0.0/6 (0 votes cast)

This post was written by:

- who has written 791 posts on PowerGuru - Power Electronics Information Portal.


Contact the author

Leave a Response

You must be logged in to post a comment.