Keywords: DS1821, refrigeration, evaporator, control
The evaporator of the air conditioner is a heat exchange device. If the surface temperature of the evaporator is too low, the moisture in the air is precipitated and frosted. At this time, if the refrigerant flow rate in the evaporator is not weakened, the surface of the evaporator will gradually freeze, which eventually leads to The evaporator cannot work (the wind cannot pass through and cannot exchange heat), and the air conditioner cannot be cooled. Therefore, the most basic and main control task of the air conditioner is to prevent frost on the surface of the evaporator, that is, to control the temperature of the surface of the evaporator. At present, the temperature control of the automotive air conditioner evaporator mainly includes a clutch-thermal expansion valve system (CCTXV system) and a clutch-throttle tube system (CCOT system), which are controlled by the thermostat to control the clutch, and then control the compressor to stop For defrosting purposes. The thermostat mainly has two forms: a bellows type and a bimetallic sheet type. The former is a thermal lever structure, the sensitive element is a temperature sensitive package, and the latter comes from the connection and disconnection of two metal sheet contacts with different temperature coefficients. Control clutch on and off. Their working accuracy is not high. The digital temperature sensor DS1821 has two working modes: thermostat and single bus. In the thermostat working mode, it has a temperature switch function, which can be used to control the surface temperature of the car air conditioner evaporator.
1 The function and packaging of DS1821 DS1821 is a digital temperature sensor produced by American Dallas company. The connection with the microcontroller is simple. It uses a single bus (1-Wire Bus), that is, it is connected to the microcontroller with a data line. The single-bus communication protocol communicates with the microcontroller. The operating temperature range is -55 Â° C to 125 Â° C, and the accuracy at 0 Â° C to 80 Â° C is Â± 1 Â° C, which fully meets the requirements of automotive air conditioning control. There are two types of packages: 8-pin SOIC package and 3-pin PR35 package, as shown in Figure 1.
QD: data input / output line;
Vdd: connect to power supply;
DS1821 can be used as both an 8-bit temperature sensor and a thermostat. When used as a thermostat, DS1821 has the function of a temperature switch. The alarm trigger points TH and TL can be set by the user. The level (POL) of the DQ pin set when triggered is stored in the DS1821's internal EEPROM, that is, stored in the TH register, TL register, and working register.
2 The working principle of DS1821 The working principle of DS1821 is shown in Figure 2.
The DS1821 measures temperature by recording the number of clock cycles. There are two oscillators in DS1821: low temperature coefficient oscillator and high temperature coefficient oscillator. The number of clock cycles recorded is the number of clock cycles within the gate time determined by the low temperature coefficient oscillator within the high temperature coefficient oscillator. The counter of the low temperature coefficient oscillator is preset to the base corresponding to -55 Â° C. If the counter counts to zero within the gate time, the temperature register preset to -55 Â° C is increased by 1 Â° C, and the counter is operated by the slope accumulator The determined value is preset again,
This slope accumulator circuit can also compensate the parabolic characteristic of the oscillator corresponding to temperature. The counter restarts counting. If the counter counts to zero within the gate time determined by the high temperature coefficient oscillator, the value of the temperature register increases by 1 Â° C and the counter is reset. Repeat the above steps until the time when the counter counts to zero exceeds the gate time determined by the high temperature coefficient oscillator. After this process is completed, the value in the temperature register is the current temperature.
3 Structure of DS1821 The structure of DS1821 is shown in Figure 3.
Both the TH register and the TL register are non-volatile, electrically erasable memory EEPROM, and the user can read and write the TH and TL registers in the single bus working mode. The TH and TL registers are 8-bit registers, whose format is as follows:
The S bit is the sign bit, 0 is positive and 1 is negative.
The format of the temperature register is the same as that of the TH and TL registers, with 1 sign bit and 7 valid data. Its value is the corresponding temperature value (Celsius temperature). â„ƒ.
The working register is also an 8-bit register with the following format:
T / R: working mode control bit, when T / R = 1, it is thermostat mode, T / R = 0, it is single bus mode. In the single-bus working mode, data transmission should follow the single-bus protocol. For the single-bus protocol and operation, see Literature 1;
POL: the state of DQ when the temperature exceeds the alarm point, DQ is 1 when POL = 1, and DQ is 0 when POL = 0;
THF: In the thermostat operating mode, when the temperature exceeds the value set in TH, THF is set to 1 and remains 1 until the user resets it to 0;
TLF: Like THF, in the thermostat operating mode, when the temperature is lower than the value set in TL, TLF is set to 1 until the user clears it to 0;
THF and TLF provide information about whether the temperature exceeds the alarm point within a certain period of time.
1SHOT: In single bus working mode, this bit decides whether to use continuous temperature detection mode or single step detection mode, when 1SHOT = 0, it is continuous detection mode, when 1SHOT = 1, it is single step detection mode, in thermostat working mode Next, it is always continuous detection mode;
DONE: In the single bus mode, when 1SHOT = 1, the user can query the DONE bit to determine whether the temperature detection is completed, when DONE = 0 is being detected, and DONE = 1 is completed;
NVB: Status query when writing EEPROM, NVB = 0, writing EEPROM, NVB = 1, writing is complete.
In the working register, THF, TLF, T / R, POL, 1SHOT these 5 bits are stored in the non-volatile memory EEPROM.
4 DS1821 thermostat working mode When T / R = 1, the DS1821 is in the thermostat working mode after power-on and continuously detects the temperature. DQ is used as the signal output pin of DS1821. To use the thermostat function of DS1821, the TH and TL registers must be set in single-bus mode. The working principle of DS1821 as a thermostat is shown in Figure 4.
Set POL = 1, when DS1821 detects that the temperature exceeds the value set in the TH register (alarm point), the state of DQ changes from 0 to 1 (DQ changes from 1 to 0 when POL = 0) and maintains the 1 state, when DS1821 detects When the temperature is lower than the value in TL, DQ changes from 1 to 0, as the cycle repeats. Users can check whether the temperature exceeds the alarm point by querying the status of THF and TLF.
5 The application of DS1821 in the temperature control of automotive air conditioner evaporator is based on the analysis of the working mode of DS1821 thermostat. DS1821 has the function of temperature switch. When the temperature exceeds the alarm point, the DQ pin can be used to drive the execution component to complete a certain action. When controlling the surface temperature of the evaporator of the air conditioner, when the surface of the evaporator is frosted (the temperature is lower than 0 â„ƒ), the DQ outputs a signal to control the electromagnetic clutch, disconnect the connection between the engine and the compressor, and start defrosting. The circuit schematic is shown in Figure 5.
Set TH = 4 Â° C, TL = 0 Â° C, POL = 1, T / R = 1, the temperature when the air conditioner starts to work must exceed TH, the DQ output of DS1821 is 1, after the reverse phase is 0, open the light isolation, electromagnetic clutch Connect the compressor and the engine to start cooling, and the temperature of the evaporator surface also starts to drop. When the surface temperature of the evaporator drops to more than 0 â„ƒ, the DQ output is 0, and the reverse phase is 1, the light barrier is turned off, and the electromagnetic clutch is disengaged When connected to the compressor, the compressor stops running and starts to defrost. The temperature on the surface of the evaporator gradually increases. When the temperature is higher than the set value of TH, DQ becomes 1, the light barrier is turned on, and the electromagnetic clutch is turned on again. The compressor and the compressor start to refrigerate, and the cycle is repeated, thereby controlling the surface temperature of the evaporator and preventing frost formation.
DS1821 can also be used as an ambient temperature switch in automotive air conditioning control. When the outside temperature is higher than a certain set value (TH), the electromagnetic clutch is closed, the compressor starts to work, and when the outside temperature is lower than a certain setting At value (TL), the electromagnetic clutch is disconnected and the compressor stops working.
DS1821 can be used as a temperature sensor for temperature measurement in single-bus working mode. Because it is digital, it does not require other devices such as A / D conversion when measuring temperature, and its data can be directly used by the microcontroller, as long as both Data transmission between follow the single bus protocol. When connecting with MCS-51 series microcontrollers, MCS-51 series does not have a single bus interface, so you need to use software to simulate the operation of a single bus. In addition, unlike the DS1820 and other single-bus devices, the DS1821 cannot be addressed. Therefore, only one DS1821 can be hung on a bus. When multi-point measurement is required, the microcontroller resources are occupied, which limits Its use as a temperature sensor.
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