Controller Equations
RESET CONTROL FOR ELECTRONIC SYSTEMS Reset with an Electronic Controller
A dual-input electronic controller is used for resetting application.
The primary variable sensor is called “A” and is connected to the input terminals marked “ISA,” or a similar term, and the secondary variable sensor is called “B” and is connected to the input terminals marked
“ISB,” or a similar term.
The programming of electronic controllers is similar to program-ming pneumatic controllers except for the following:
1. The action of the controller for both primary and secondary vari-ables is directly set on the electronic controller.
2. Electronic controllers have individual setpoint adjusting dials so that setpoints “A” and “B” can be set directly on the controller.
3. Electronic controllers have the throttling range of the primary vari-able set directly on the controller and a setting of the throttling range of the secondary variable is made by selecting and setting the ratio.
Ratio in electronic controllers is defined as the ratio of the second-ary variable throttling range to the primsecond-ary variable throttling range.
Ratio in electronic systems is the reciprocal of authority in pneumatic systems when the spans of the pneumatic sensors are equal. The control-ler set-up includes a setting for ratio. The ratio setting may be as low as 0.5:1 to as high as 25:1. The procedure for calculating controller ratio
settings is discussed in Calculating Ratio.
The output voltage of a dual-input electronic controller can be obtained by using the following equation:
Vout = Vsp±T1± SP1
TR1 × VR ±T2± SP2
TR2 × VR (4-5)
Where:
T1, T2 = Temperatures of first and second variable respectively (°F).
SP1, SP2 = Setpoints for first and second variable respectively (°F).
TR1, TR2 = Throttling ranges for first and second variable respec-tively (°F).
Reset Action
The reset action which will occur depends on the actions of the two inputs which are used in the reset control for pneumatic systems.
Reset Schedules
A typical reset schedule for resetting supply air temperature on heating from outside air temperature would be:
Condition Coil Air Outside Air
A 75°F 65°F
B 95°F 10°F
Example of Reset Action
The determination of the type of reset action is illustrated in the following example for reset of hot water supply temperature from out-side air temperature, by controlling a 3-way valve, bypassing return water to the circulating pump based, on the following reset schedule:
Output Bypass Hot Water Outside Air Voltage Port Condition Temperature Temperature vdc Position
———— ————— ————— ——— ————
A 90°F 60°F 9 open
B 210°F 0°F 6 closed
The primary or reset variable is hot water temperature supplied to
the system and the secondary or resetting variable is outdoor air tem-perature. The reset schedule shows that a decrease in primary variable setpoint will be required upon an increase in secondary variable value, which is reverse reset. A study of the variation of voltage with outdoor temperature finds that output voltage increases as temperature in-creases, which is direct action. It is also seen that the valve bypass port is scheduled to go to closed position as hot water temperature increases to 210°F. Thus, for a valve that is normally open to bypass, the voltage must increase to close the valve.
Because an increase in hot water temperature is to cause an increase in the controller output voltage, direct action is required. The controller must be selected for direct-acting/direct-acting to give reverse reset.
Selecting the Throttling Ranges
The next parameters to be considered are the throttling ranges. A trial throttling range is assigned for the primary variable on the basis of experience factors. After the primary variable throttling range is as-signed, the throttling range of the second variable is calculated accord-ing to the followaccord-ing equations:
1. For reverse reset; using DA/DA Controller:
TR2 = T2B± T2A VdcB ± VdcA
3 vdc ± T1B± T1A TR1
(4-6)
2. For reverse reset; using RA/RA Controller:
TR2 = T2B± T2A VdcA ± VdcB
3 vdc ± T1B± T1A TR1
(4-7)
3. For direct reset; using DA/RA Controller:
TR2 = T2B± T2A
VdcA ± VdcB
3 vdc ± T1A± T1B TR1
(4-8)
4. For direct reset; using RA/DA Controller:
TR2 = T2B± T2A
VdcB ± VdcA
3 vdc ± T1A± T1B
TR1
(4-9)
Where:
TR1 = Throttling range of first variable, °F.
T1A = Temperature of first variable at condition A, °F.
T1B = Temperature of first variable at condition B, °F.
T2A = Temperature of second variable at condition A, °F.
T2B = Temperature of second variable at condition B, °F.
VdcA = Output voltage of the controller at condition A.
VdcB = Output voltage of the controller at condition B.
The voltage range of 3 vdc used in all the above equations indi-cates the voltage change between 6 to 9 volts. When using the equations for other voltage ranges, substitute the correct voltage range for 3 vdc.
Calculating Ratio
The throttling range for the second variable cannot be set up on the controller, so the ratio is set instead. Using the required throttling ranges for the primary and secondary variables, the ratio can be calculated as follows:
Ratio =TR2 TR1
(4-10)
Entering the Set-Up Parameters
For dual-input electronic controllers, the setpoint for the first vari-able is selected and set directly on the controller. The setpoint of the second variable must be calculated by use of one of the following equa-tions and set directly on the controller:
1. For reverse reset; using DA/DA Controller:
SP2= T2A+ TR2 (Vmp ± VdcA)
VR +(T1A± SP1)
TR1 (4-11)
2. For reverse reset; using RA/RA Controller:
SP2= T2A+ TR2 (VdcA ± Vmp)
VR +(T1A± SP1)
TR1 (4-12)
3. For direct reset; using DA/RA Controller:
SP2= T2A+ TR2 (Vmp ± VdcA)
VR +(T1A± SP1)
TR1 (4-13)
4. For direct-acting reset; using RA/DA Controller:
SP2= T2A+ TR2 (Vmp ± VdcA)
VR +(T1A± SP1)
TR1 (4-14)
Where:
SP1 = Setpoint temperature for primary variable, °F.
SP2 = Setpoint temperature for secondary variable, °F.
Vmp = Midpoint voltage, 7.5 vdc for 6 to 9 vdc systems.
VR = Voltage range, 3 vdc for 6 to 9 vdc systems.
For controllers which are to be calibrated at any other midpoint voltage, substitute the correct midpoint voltage.
The final step is to enter the calculated setpoint parameters onto the controller dials.
EXAMPLES FOR SET-UP OF DUAL-INPUT