Industrial Control (Students guide, 1999, v1.1 )

Experiment #7: Real-time Control and Data Logging

Some examples include heating controls for buildings to set the temperature lower after working hours to conserve power; annealing a metal by heating at different temperatures for specific time periods to temper or strengthen the metal; and logging data over long periods of time for later retrieval and analysis.

This experiment will explore taking action based on specific time of day, action taken based on time intervals, and logging and retrieving data. For this we will need to add real-time features to the circuit built in Experiment #5. Connect the DS-1302 Real Time Clock (RTC) as illustrated in Figure 7.1c and the pushbutton in Figure 7.1d. Figure 7.2 is a board layout sample for placing all the components.

The DS1302 RTC uses an external 32.767kHz crystal oscillator for a time base. Be sure to connect the crystal as close as possible to the IC to maximize time reliability (distance will create more error due to the capacitance effects of the breadboard). The RTC is similar to the BASIC Stamp in that data is stored in registers (RAM memory) that may be written and read. These registers hold the time and date as seconds, minutes, hours, month, etc. The DS1302 also has RAM available for general data storage by the user. For our purposes we will use only the time of day features of the chip. Please see the DS1302 data sheets and Parallax application notes concerning additional features.

Just as with the ADC0831 A/D converter, data is serially shifted into the BASIC Stamp 2 from the IC. This will allow us to access the current time as maintained by the DS1302. In order to set the current time, we can place the IC in a 'write-mode' and serially shift data from the BASIC Stamp 2 into the IC.

Data for the time (and date) is maintained in the DS1302 as Binary Coded Decimals (BCD). This is a subset of the Hexadecimal number base. In Hexadecimal (base 16) a byte is broken up into a high and low nibble, and each is read as a digit.

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Figure 7.1: Complete Circuit with Real Time Clock

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Figure 7.2: Sample Component Layout for Experiment #7

Take for example the binary number: 01000110. In binary, each place is a higher power of 2, and the decimal equivalent would be: 64+4+2 = 70. With an 8-bit binary number we have a possible decimal range of 0-255.

In Hexadecimal, the byte is broken down into nibbles and converted individually: 0100 0110

4 6

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This hexadecimal number is typically written as 86H (Intel format), $86 (Motorola format) or 8616 (Scientific format). Since a single unique number represents each nibble, we have a range in binary from 0000 to 1111, or decimal 0-15. In Hexadecimal the values of 10 to 15 are represented using the letters A to F. The hexadecimal full range of a byte of data is $00 to $FF. The binary number 10101110 is be represented in Hexadecimal as $AE.

In Binary Coded Decimal, each nibble is again used to represent a single digit, but since it is a coded DECIMAL number, the valid ranges can only be from 0-9 for each nibble. Our first example of 01000110 is 46BCD (a valid BCD number) and $46 (a valid hexadecimal number). Our second example of 10101110 would be $AE (valid hexadecimal), but an INVALID BCD value since A and E are not valid decimal numbers.

Our programs will use the hexadecimal notation in setting, or checking the times of the RTC. Additionally, the RTC is set to use a 24-hour clock, so a time of 1:00 PM will be 13:00.

Exercise #1: Real Time Control

In this first exercise we will simulate a night-setback thermostat of a building. During normal working hours, temperature will be kept at a higher temperature than during the night when energy conservation is needed. To simplify our code, we will use On/Off control instead of the more appropriate control of differential-gap.

Since we will be using our incubator, and to ensure the temperature is above your room temperature, we will use values 100F for working hours and 90 F for nighttime hours. The times for adjusting the temperature up for the day is at 6:00 AM, or 06:00 hours. The time to turn the thermostat down will be 6:00 PM or 18:00 hours.

Program 7.1 is the code for experiment.

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'Program 7.1 - Real Time On/Off-Control.

'This program will adjust temperture to 100F between the hours 'of 06:00 - 18:00 and 90F between 18:00 and 06:00.

'Define RTC Constants

'Register values in the RTC

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HIGH 8

RETURN

Off:

LOW 8

RETURN

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Display: 'Display real time and time for next change and send data for plotting DEBUG "!USRS Current Time:", HEX2 hours,":",HEX2 Minutes,":",HEX2 seconds DEBUG " Next Change at:",HEX2 Chours,":",HEX2 CMinutes,CR

DEBUG DEC Temp,CR

DEBUG IBIN OUT8,CR RETURN

SetTime:

'****** Initialize the real time clock to start time RTemp = $10 : RTCCmd = CtrlReg : GOSUB WriteRTC

'Clear Write Protect bit in control register

'Set write-protect bit in control register

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Figure 7.3: Night Setback Flowchart

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Note that the program will reset the time to 05:53 every time the BASIC Stamp is reset. This can occur from program loading, manual Stamp reset, power supply cycling, and sometimes the COM port or computer cycling. The start time is appropriate for what we are discussing in this section, but in later sections you may want to set the values of the start-time to actual values.

Note: If power is removed from the DS1302 Real Time Clock it will power-up with unpredictable values in the time registers with Gosub SetTime remarked out.

The times for changing temperature and their new values are also set here. GOSUB SetTime sets the real time clock to the specified time. Once the proper time is set, this line may be remarked out and downloaded again to prevent the time from being reset to 05:53 if the BASIC Stamp is reset.

The program uses two sets of variables for time, one to set/hold the current time, and another to hold the time we wish to change the thermostat. Note that the word variable of Time and CTime are further broken down into Hours and Minutes:

The variable Hours is assigned to be the high byte of the word variable Time, or those two BCD positions representing the hour. The same is true for minutes and the lower 2 positions. This is a very powerful tool when parts of a single variable need to be addressed individually.

Program 7.1 starts time 7 minutes before switching to the working-hours temperature. This should provide time for temperature to stabilize at the lower temperature. Figure 7.4 is a plot of the run.

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Figure 7.4: Time-Controlled 'Building Heating’

The StampPlot Lite user status box displays the current time and the time that the next change is set to occur. The time in the status box may appear to be changing at irregular intervals, but that is a result of timing of the BS2 in displaying the data and not the time kept by the RTC. The message area displays both the time a change occurred and the new setpoint.

The plot illustrates On/Off control at the 90 F setpoint, and the switch to the 100 F setpoint at 06:00. Using the RTC, adding more output devices, and expanding the control section of the code, we could add numerous time-based events to occur over the course of a day.

Download and run program 7.1. Monitor with StampPlot Lite through at least the 06:00 change. You will need to wait another 12 hours to see if it switches back to the low setpoint at 18:00... Have time to wait?

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