Digital Control of a Mobile Air
ME 313 project
Professor N. R. Miller
When automotive air conditioners are started up on a hot day, they are known to create
whistling and hissing noises. These noises have resulted in customer complaints. It has
been shown at the University of Illinois that the acoustic noise results from high speed,
single phase vapor refrigerant flowing through the evaporator. It was then proposed to
digitally control the A/C system to resolve this and other problems. It has also been
proven at the University of Illinois that closing the expansion device can reduce the
torque imposed on the compressor at startup. These two remedies along with a PID
control algorithm where combined to formulate a control routine for a mobile air
conditioning system. The algorithm is implemented by Dan Block’s DSP, and controls a
mobile A/C system that uses an electric expansion valve (EEV) as its expansion device.
A schematic flow diagram showing the basic components of the vapor
compression refrigeration system is shown in figure 1. Some typical temperatures for air
conditioning applications are indicated. Refrigerant fluid circulates through the piping
and equipment in the direction shown.
Process 1 - 2.
At point (1), the refrigerant is in the liquid state at a relatively high
pressure and high temperature. It flows to (2) through a restriction, called the flow
control device or expansion device. The refrigerant loses pressure going though the
restriction. The pressure at (2) is so low that a small portion of the refrigerant flashes
(vaporizes) into a gas. But in order to vaporize it must gain heat (which it takes from that
portion of the refrigerant that did not vaporize), thus cooling the mixture and resulting in
a low temperature at (2).
Process 2 - 3.
This is when the refrigerant passes through the evaporator. During
this time, the refrigerant vaporizes, taking heat from the environment to do so. This is
how an air conditioner cools air.
The vapor compression refrigeration system.
The expansion device can play a dual role. The most obvious purpose of an
expansion device is to expand some of the liquid into gas by imposing a pressure drop.
The expansion device can also be responsible for regulating the refrigerant flow
according to the load.
One of the two most common traditional methods of refrigerant control in
automobiles is the orifice tube. The orifice tube is only able to exhibit a fixed restriction,
and is not able to actively control the flow of refrigerant. The orifice tube has been
traditionally used because of its low cost and high reliability due to the fact of no moving
The second of the two most common traditional methods of refrigerant control in
automobiles is the thermal expansion valve. The thermal expansion valve is able to
actively change the flow of refrigerant as needed by the system. A bulb filled with a fluid
is strapped to the outlet of the evaporator, and thus, senses the temperature at this point.
This bulb is connected to the valve by a tube in a manner so that the pressure of the fluid
in the bulb tends to open the valve more, against a closing spring pressure. If the load in
the system increases, then the refrigerant in the evaporator picks up more heat, and the
temperature and pressure of the fluid in the bulb increases. This action is able to open the
valve more to handle the increased load. Their disadvantage is they are less reliable than
orifice tubes due to their moving parts and their relatively fragile capillary tubes.
Electronic expansion valves have not been traditionally used for automotive
applications due to the expense on such a system. An electronic expansion valve (EEV)
system requires a computer to monitor the temperatures at the inlet and the outlet of the
evaporator. From this feedback, the computer actively controls the valve setting. The
versatility and efficiency of an EEV make it ideal for an automotive air conditioner, but,
the cost is simply too great.
DSP / AC Interfacing
Dan Block’s DSP unit was chosen to initially control the mobile AC unit. The
DSP is equipped with four digital to analog and to digital converters. Each has twelve
bits of resolution and operates at a voltage range of -10 volts to +10 volts. There are also
32 digital I/O lines, where 16 are input and 16 are output. These lines operate at +5 volts
for logic high, and 0 volts for logic low.
At the AC unit a thermocouple will be placed that the inlet of the evaporator and
one will be placed at the outlet of the evaporator. An Omega linearized, isolated
thermocouple input will be used to change -100
oC to 400oC temperature to a 0 volt to 5
volt signal. But, the greatest range that the thermocouples would experience is from
oC to 30oC. This corresponds to a 0.9 volt to a 1.3 volt range. That is only 2% of the
resolution of the DSP. It would be ideal to have -10
oC correspond to -10 volts and 30oC
correspond to a +10 volts.
With constraints that -10
oC correspond to -10 volts and 30oC correspond to a +10
volts, the relationship between the input of the desired circuit to the output of the desired
circuit is found to be:
out = 50*Vin- 55
This would correspond to the following circuit
Op-Amp circuit used to increase the resolution of the thermocouples.
This circuit uses two inverting op-amps. The first op-amp has a gain of
output would feed into a relay circuit where logic high (+5 volts) would engage the clutch
and a low signal (0 volts) would disengage the clutch. The final interfacing between the
A/C unit and the DSP is shown in figure 5.
Interfacing diagram between the DSP and A/C unit.
0V or +12V
0V or +5V
The algorithm to control a mobile A/C system that is presented in this paper will
address three issues:
reduce compressor wear at startup
reduce noise caused by high speed vapor through the evaporator
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