3.2 Microprocessor control svstem.

Microprocessors are used increasingly in scientific instruments to hardle control functions which would previously have been carried out by a dedicated electronic analogue control circuit A microprocessor control system outperforms its less complex predecessor in almost all respects. It is programmable, so that modifications or specific applicatlon requirements may easily be applied without total redesign. The system may be programmed to make detailed calculations and statistical analysis, and adapted to make corrections for non-linearity in the measurement device; hence greater accuracy is achieved. Data logging can be accomplished with ease, so that operation is totally automatic. Calibration of the instrument also becomes simple, and results can be displayed in a manner superior to that obtainable from an analogue unit. Such advantages justify the additional complexity and cost of a microprocessor based system. For these reasons a microprocessor control unit was designed to control the viscometer mechanism. Of course, a standard commercial computer such as a PC could have been used. However, the expense and inconvenience of dedicating a powerful computer was felt to be excessive, and a self-contained unit was considered more attractive.

A block diagram of the circuit used is shown in figure 5. Appendix A may be consulted for a full schematic.
 

Processor Unit:

An 8-bit Z80B microprocessor was chosen to form the heart of the system. This processor was certainly the most powerful of its generation and despite today's proliferation of faster  16 and 32-bit processors, the Z80 still has much to commend it. Literature on the device abounds; and in particular, designing a system around the Z80 is considerably simpler than for a more powerful processor, leading to a lower component count and cost. Running at a speed of 3 MIHz, the processing power is ample in this application.

The processor unit also includes the necessary logic, 32K of Random Access Memory (RAM) and 8K of Electrically Erasable Programmable Read Only Memory FEPROM). The former is used for variables and data logging, while the latter holds the program itself. EEPROM provides data retention while the system is off, but is electrically erasable as opposed to earlier Ultra-Violet light erasable types. This facilitates reprogramming in situ.

Interface circuit:

This section provides connection between the processor unit and the host computer that was used to develop software; downloading of programs is accomplished without interfering with normal viscometer operation.

Display:

A 6 digit 7-segment LED display and its associteted decoder/driver chip provides a digital relout for the system. The display is programmable by the microprocessor and appears as six output ports. A latch has also been incorporated, such that blanking of any or all of the digits is possible: this greatly enhances the clarity of the display by allowing leaning zeros to be removed for example.

Keyboard:

A 20-key numerical membrane keypad, together with decoding logic, allows the user to control the viscometer, enter calibration viscosities, set time intervals, etc.

5OHz counter:

An 8-bit counter is clocked at the 50Hz mains frequency. By reading this input port, the microprocessor can operate a real-time clock and measare time intervals.

Timers:

Two 24-bit counters and input ports time the photo diode illumination and beam broken periods. The counters are clocked at 6 MHz; hence rotation rates down to 21 rpm may be measured, while at the motor's maximum speed of 300 rpm, the periods are accurate to about 1 part in a million (although in practice the precision of the 6 MHz crystal itself limits the accuracy to 30 parts per million).

D-A Converter:

A digital to analogue converter and latch give the microprocessor accurate control over the motor speed. A power operational amplifier buffers the output to a voltage and current sufficient to drive the DC motor. The output voltage is from 0 to +9 volts in 65536 steps, with a differential linearity error of only 0.001%.

Mains power supply unit:

A 12-0-12 volt toroidal transformer, rectifiers and voltage regulators form a standard power supply with +/- 12 V outputs at lA for the D-A conveeter and buffer. A high efliciency switched mode regulator provides +5V for the digital circuits, which consume a power of approximately 5 W. Use of this unusual type of regulator results in considerably lower overall power consumption and heat dissipation.
 
 

3.1 Viscometer Mechanism

Contents

3.3 Operating Software