Basic Aspects of Control System Design of Microprocessor Based Variable Speed Drives:

Based on the examples of drives discussed in the foregoing, general outlines may be drawn for designing the Control System Design of variable drives using a microprocessor. It is evident that it is rather involved and time consuming to design and develop a microprocessor based variable speed drive. The Control System Design design starts once the detailed specifications of drive are available.

The main stages in the design are

• system analysis
• computer simulation
• hardware design
• software design
• system integration, debugging and performance.

System analysis: In this stage the complete structure of the system is formu­lated. All the components of the system are specified and their functions are identified, e.g., the power circuit configurations analysis of their commutation to decide the optimum values of commutation circuit components, etc. The performance specifications also decide the type of converter to be employed. If regeneration is not required half controlled converters with advantages of im­proved power factor, reduced ripple, etc. can be employed. If a PWM inverter is employed the line side converter can be a diode rectifier if no regeneration is required. The analysis also gives the details of the voltage waveforms, feed­back signals, their generation and estimation. It is also useful in writing the algorithms for control purposes.

Computer simulation: If the system is complicated with several control loops and variables, a computer simulation of the system may be useful, to arrive at the optimised system and at the variables at every stage of Control System Design.

Hardware design: In this stage a decision is made, before selecting a microprocessor, about the boundary between the local hardware and microprocessor software. The functions to be carried out by the dedicated hardware circuits defined. If the microprocessor is sufficiently fast, it may be assigned to per­form the functions necessary for the Control System Design. On the other hand, if it is slow the hardware oriented circuits may be used to cover the jobs of signal gener­ation for the inverter, high speed protection, firing angle generation for the converter, etc. The microprocessor can be saved to perform the other jobs.

While selecting a microprocessor the following points need consideration:

1. Suitability of the performance of the microprocessor for the intended This is identified by the bit size, operating time, memory capacity, memory access, etc. The resolution required determines the bit size of the microprocessor. The sampling time of the processor is a measure of the resolution. The bit resolution to sampling time is identified as the figure of merit.
2. Suitability of the functions of the processor for intended purpose: The functional capacity of a microprocessor is identified by the kind and number of instructions, microprocessor control, I/O interfaces, peripheral LSI, num­ber of microprocessor devices, etc. Direct multiplication/division capability, functional integration, etc. show the effectiveness of the microprocessor in the computation of feedback signals in the Control System Design.
3. The reliability of the system under operating conditions and temperature.
4. Cost of the equipment.
5. Software and hardware support.
6. Possibility for the expansion of the system to meet future needs.

Depending upon the program and data size ROM and RAM memory size may be designed conservatively. EPROM may be preferred to ROM in the initial stages. In many cases a single chip microprocessor with built in memory capacity may be sufficient, if aided by external hardware, to perform a few of the Control System Design functions in order to increase the speed. In case of multitasking a careful partitioning of the tasks may be necessary. Suitable methods may be used to increase the speed of operation.

Software design: Many of the functions of the microprocessor are software supported. The implementation of compensators, generation of PWM signals of a PWM inverter, etc. are carried out by suitable software. This makes them adaptable to various operating conditions. This is an important stage in the design of the system. The design of powerful software actually improves the adaptability and reliability of the system. The principal factors that decide the effective design are the language, bit size, sampling time and the identifica­tion of the function of the microprocessor. The resolution required in the operation decides the sampling time and bit size of the processor.

Once the above stages are completed, all the components are systematically integrated and the system may be tested for its performance.