DSP Approach

Implementation of advanced motor drive systems requires the following features from a typical motor controller

• Capability of generating multiple high frequency, high-resolution PWM waveforms.
• Fast processing to implement advanced algorithms to minimize torque ripple, on line parameter adaptation, precise speed control etc.
• Implementing multiple features using the same controller (motor control, power factor correction, communication, etc.)
• Making the complete implementation as simple as possible (reduced component count, simple board layout and manufacturing etc.)
• Implementing a flexible solution so that future modi-fication can be realized by changing software instead of redesigning a separate hardware platform.

A new class of DSP controllers has addressed these issues effectively. These controllers provide the computational capability of a DSP core and integrate useful peripherals on chip to reduce the total chip count. The TI's DSP2000 family controller is becoming aviable option for even the most cost sensitive applications like appliances, HVAC systems etc.

In addition to traditional mathematical functions like digital filter, FFT implementations, this new class of DSPs integrates all the important power electronics peripherals to simplify the overall system implementation. This integrationlowers over all part count of the system and reduces the board size. The DSP controller TMS320F240 from Texas Instruments is for motor control applications.

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Fig. 7 shows the overall block diagram of TMS320F240. TMS320F40 has a 20MIPs 16 bit fixed point DSP core. It also integrates the following power electronics peripherals - 12 PWM channels (out of which 9 are independent), three 16 bit multi-mode general purpose timers, 16 channel 10 bit ADC with simultaneous conversion capability, four capture pins, encoder interface capability, SCI, SPI, Watch Dog etc. Six PWM channels (PWM1 through PWM6) control the three phase voltage source inverter. These six PWM channels are grouped in three pairs (PWM 1&2, PWM 3&4, PWM 5&6). Three compare registers, called Full Compare, are associated with each PWM channel pair. The compare register values are updated to obtain the proper PWM output. The on-chip software programmable, dead band module provides sufficient dead time to avoid shoot through fault. There are three more PWM channels left to implement other functions like power factor correction.To put TMS320F240 chip into application for motor control, the gate driver must be used to amplify the logic signals to 15V and powerful enough to turn on and turn off the transistors. Because the transistors have different collectors and gates at different voltage levels as shown in Fig. 2, the gate drivers (amplifiers) cannot use a common ground. Therefore, the dc power supply for those gate drivers should be isolated.

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A DSP card for motor drive is used, as shown in Fig. 8. The card has the following on-card facilities:

• Socket for a 64k x 16 EPROM, accessible as I/O space
• 128k x 16 RAM, accessible as 64k program and 32k external data space
• Serial ROM with non-volatile storage for 256 x 16 bits of data
• Switch-mode power supply to generate all on-card supplies
• On-card DIP switches and status LEDs
• Power-fail circuitry.


The card also supports the following peripheral interfaces:

• RS-232/RS-422/RS-485 serial interface
• High speed clocked serial peripheral interface
• 2 off isolated digital inputs
• 2 off MOSFET switch isolated outputs
• 1 off relay output, c/o contact
• 2 off AC current inputs
• 2 off differential AC voltage inputs,
• 1 off differential DC voltage input
• 1 off flexible temperature sensor input
• 2 off potentiometer analog inputs
• 2 off general purpose analog inputs (± 10V default)
• 8 off complementary isolated gate driver PWM outputs, with common fault interrupt
• +24V isolated field supply
• JTAG port for software development.
• Quadrature Position Encoder input with Index more PWM channels left to implement other functions like power factor correction.