130-28176 Parallax Inc, 130-28176 Datasheet - Page 228

• Manufacturer Part Number: 130-28176
• Description: KIT PARTS PROCESS CONTROL
• Manufacturer: Parallax Inc

Specifications of 130-28176

• Accessory Type: Education Kit
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: Board of Education Full Kit
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant

Page 218 · Process Control
CONCLUSION
The ability to measure temperature, or other analog parameters, is dependent on the
resolution of the system. Using the ADC0831, an 8-bit analog-to-digital converter, a
span of voltages from 0 to 5 V can only be resolved to 255 steps. To increase the
resolution, the incoming signal may be amplified and offset to narrow in on the range of
voltages of interest. The ADC0831 has built in features to set the span and offset using
its Vin(-), Vin(+) and Vref pins. By setting the voltages on these pins, the ADC can be
configured to convert a defined range to byte values of 0 to 255. Using filtered PWM,
the BASIC Stamp can be programmed to automatically set these pins and adjust the range
of conversion for the ADC0831. Combining this device with the LM34 temperature
sensor, which has an output of 0.01V/°F, systems temperatures can be monitored to a
high degree of resolution.
Another means to increase the resolution would be to use an ADC with higher resolution,
such as 12-bit ADC. This type of ADC resolves a range of temperatures to 4095 steps
12
(2
− 1). Over the range of 0 to 500 °F (0 to 5 V), this would provide a resolution of
0.122 °F. Another popular style of ADC converts the analog voltage to 10-bits for a
range of 0 to 1023.
PWM can also be used to control the power supplied to the control element, a resistive
heater in these experiments. Using PWM, the transistor is cycled on quickly, effectively
controlling the power supplied to the heater. Recall from Chapter 5 that the filtered
PWM may also be used to control the voltage to the element, but the transistor will be
controlled in the linear region, causing it dissipate large amounts of power, causing it to
heat.
By adjusting the power to the heater, a value of PWM can be found that will control the
temperature in the band at or near the setpoint. A drawback to this manual control is that
as conditions change, the setting may no longer be appropriate for control at the setpoint.
In the next chapter, we will study control scenarios to automatically control a system at
the setpoint.