Electronics Engineering Magazine

Thursday, February 23, 2012

Custom Built Test Instruments

I designed and built many custom "engineered" Test Instruments. They were not prefect, as only one or two were fabricated. That means it will be costly to produce such instruments. It is just like tooling, you have to make some numbers to recover costs, or price it very high to compensate for the wastages and iterations. But things are better off now for building custom instruments. Yet a design firm has to make modules and keep ready, then put together a solution/system, program and ship.

Custom Built Test Instruments

When making custom equipment, you have to have an extended service agreement like 5 years. Also request a minimum order of at least 5. Make your R&D and Inventory Modular. That way it is profitable and some money available for reinvestment on instruments and Equipment for calibration and quality.

Transistor Beta and Vce Tester

This was a Beta Hfe and Vce Measuring Instrument used in Incoming Inspection of Power Transistors made by me 20 years back.

Transistor Beta and Vce Tester

It is a Test and Measurement Instrument to Evaluate the Beta of a Semiconductor Transistor at different Ic and Vce. This was an early custom test instrument.

Tuesday, February 21, 2012

Analog Interface for PC Ports

Here are examples of an analog Input and Output interface for Printer Port. There are two visual basic demo programs with source to test out idea.

Analog Interface for PC Ports
USB and wireless interface to PC may be more in vogue, but a higher speed is possible in printer-port. For analog and digital fast data acquisition, PC cards to full Access to PC bus are the best choice.

Analog Interface for PC Ports

The Sensors and Actuators interfaced with a computer form a basic automation system or even a robot. So learning to interface Sensors (input devices like a thermocouple or pH sensor) and Actuators (output devices like a motor or heater) with a PC is the best way to start. It is by studying examples and experimenting cautiously can one learn the science, book reading alone will never help.

See the Hardware Book connector details, cables, adapters, etc. The RS232-Serial Port, The Parallel- Centronics port, The USB-firefire port are good to start with. Then you can interface by LAN or Wireless- LAN or TCP/IP-Internet to make Robots or Automation extend to multiple places and computers. The possibilities are endless, imagination is the limit for students. But entrepreneurs need to be practical and only make equipment that have a utility and sensible market scope.

The circuits and code have some errors, The documentation is not complete.
DVM = digital volt meter MVS = milli volt source

Friday, February 10, 2012

Temperature Control - Part 2

A Two Point Process Controller or Temperature Controller would look like this. When a High-Low Alarm Protection for uC based Controller was needed the same was modified and used.

Sometimes uC based instruments just forget and that can be costly, the watchdog timer has solved this problem. In Industrial Process Control we cannot take any chances, so a two tier or even three tier protection is required, especially so if the Job or Raw material being processed is expensive. So Real Time Analog Trip + uC Controller is safe. Then you can still have Electro Mechanical Devices for Extra protection against Fire or Meltdown.

Two Setpoint Temperature Controller

Temperature is the most common process parameter, Pressure seems to be next. Flow, Level, RPM, Distance and many more follow.

When a physical parameter let us say distance is measured, we first need a sensor that converts it into electrical values, digital or analog. The sensor used for length is a Linear encoder and it creates the electrical information for the DRO to Display.

Now many sensors like a strain gauge produce very low and weak signals and some are non-linear. So the signals undergo Amplifying, Temperature compensation and Linearizing together called Signal Conditioning. This stage outputs some standard values like 4-20mA, 0-10 V, 1-5V, or even directly digital or wireless in the new technologies. Even when a digital interfacing standard is used, amplification and some signal conditioning is required at sensor end. Compensation and Linearizing can be better done with a uC using Math or Look up Tables.

When you have many different parameters then it is better to use process controllers with a 4-20mA input standard. Then the inventory of controllers can be reused for any process.

It is always better to use sensor-end transmitters to get 4-20mA to your Controller, Recorder or SCADA System.Weak signals are corrupted by noise and dont travel long wire distance. Junction EMF at joints and terminal blocks add to errors. These are overcome by the 4-20mA signal that has the juice and punch to transmit the data over the shop floor to the control panel rooms.

Read more here Temperature Measurement and Control