application note 1 of 4 www.xicor.com rev 1.1 11/12/02 AN155 advanced mixed-signal-approach for modern hvac systems introduction: the increasing complexity of modern hvac-control-systems (heating, ventilating, air- conditioning), requires more and more the capability of precise and quick system-calibration during production. the following article will demonstrate a cost effective advanced mixed-signal approach . the general hvac concept the amount of analog inputs that need to be sensed by a modern hvac feedback control system usually will include heater , ac , inside air , outside air and multiple seat temperature inputs . however, other criteria such as sunlight-intensity as well as the inside humidity will be additional parameters based on which the hvac will be controlling the compartment s climate as well. this sensor matrix leads to the following generic block diagram: figure 1. hvac block diagram modern hvac-control-systems need to sense several analog parameters in order to achieve a high level of passenger comfort ref. temp. vector sensor matrix heater temp. ac temperature inside temp. outside temp. seat 1 temp. seat 1 temp. driver sunlight intensity passenger sunlight intensity inside humidity signal conditioning matrix c based process unit adc vector v pwm ac fan speed heat flow valve heater fan speed duct control 1 driver temp. setp. pass. temp. setp. e c t o r duct control 2
2 of 4 AN155 application note www.xicor.com rev 1.1 11/12/02 as pointed out in the above block diagram, the average compartment temperature is not any longer the only physical quantity to be monitored, but also humidity as well as the sunlight intensity . in case of this last one, what usually is being measured will be the local light intensity generated by the sun and therefore the amount of ir-energy ( e.g. heat ) locally provided. this becomes especially an issue, when the individual comfort of driver and passengers deviate from each other depending whom is seated in the shadow or in the sun. based on this difference of heat, the duct controls for driver and passenger need to be individually controlled. on the other hand humidity becomes especially a key parameter in terms of safety when the passenger compartment air is almost saturated and it might condense at the inner surface of the windscreen. the hvac s target will therefore be to cool down the compartment air and to make sure to condense the mois- ture inside of the ac before being supplied to the compartment. based on the above comments, ?ure 1 can be trans- formed into the following feedback control system block diagram: figure 2. hvac feedback control system while moisture and temperature are the physical quantities that need to be dynamically controlled, the sunlight intensity becom es a distortion parameter within the temperature closed loop that will be additionally monitored . the system level challenge with the above comments it becomes evident, that the whole sensor matrix needs to be calibrated properly in order to ensure a high level of comfort for any individual passenger. this will either mean to perform an analog signal conditioning of all matrix values (e.g. high preci- sion) before providing them to the mcu or leave the sensor s outputs as they are and perform a digital signal conditioning (e.g. high sw overhead). however, due to this high level of complexity as well as due to the thermal behavior of the sensor matrix itself, at least some of the sensor vector outputs need to experience analog signal conditioning. temperature hvac compartment 4 5 3 humidity 2 1 moisture control temp. control setpoint setpoint + + ambient moisture human body transpiration sunlight intensity + + + + + compartment humidity user temperature
3 of 4 AN155 application note www.xicor.com rev 1.1 11/12/02 this indicates the following are needed as a minimum to make sure that: the complete temperature sensor vector shows the same absolute values for a given reference temperature (e.g. rel. temp. offset = 0!) the photo detector vector provides the same values for a determined light intensity (e.g. relative sunlight intensity offset = 0!) and that the humidity sensor shows a speci? wanted value for a given moisture level (e.g. absol. humidity offset = 0!) it is evident that the challenge will therefore be to perform the calibration of the complete sensor matrix within a reasonable time , no matter whether this is done in the analog or in the digital domain. the following concept will be based on a mixed-signal-approach that is meant to deal with the following targets: implementation of a high precision analog signal conditioning solution that will keep the error vector generated by the sensor matrix (noise, offset, non- linearity, etc.) within an acceptable tolerance. introduction of programmability in order to keep the required time for a whole calibration cycle (e.g. costs) as low as possible. figure 3. mixed signal approach for a generic hvac system analog signals of temperature, light and moisture detectors are conditioned in the analog domain, while their calibration will be mcu supported. i to v conv. photo detector r pot-1 r to v conver. temp. sensor v ref - + c 1 c to v conv. c s = c o + ? c moisture detector - + xdcp logic pot-4 reg. pot-3 reg. pot-2 reg. pot-1 reg. mcu adc vector external calibration equipment v = v = i v = -v ref ?r pot3 ?f ?c c f r r v ref r +r pot1 r pot-2 r pot-2 r pot-3 v ref i
4 of 4 AN155 application note www.xicor.com rev 1.1 11/12/02 the ?st xdcp (xicor digitally controlled potentiom- eter) pot-1 acts as a resistor-to-voltage converter within a voltage divider circuit. its output might be passed through an additional ampli?r circuit, which will require another variable resistor in order to tune the gain. the second xdcp pot-2 acts as a current-to-voltage converter in combination with an operational ampli?r circuit. using a dc-voltage-source between the photo- diode and ground might expand the bandwidth of this circuit. the last circuit with the moisture sensitivity sensor is based on the so-called switched-capacitor-technique . in that case f ?represents the frequency, which the switch will be driven with. this might either be gener- ated by a switch-mode logic or directly coming from the mcu. c c represents an average value between c s and c 1 . as the moisture to be sensed starts to change, the capacitor s dielectric constant and therefore its capaci- tance will change as well. based on the very well known effect ? s = q/v? pot-3 will act ?similar as pot-2 ? within the ampli?r circuit as a capacitance to voltage converter. the bene?s although the above circuit had been built up with stan- dard blocks that need further accommodation within a real hvac system, it evidently points out the necessity of relative-offset compensation when talking about a speci? sensor-vector. each sensor bears ?depending on the way it is being used ?an absolute and a relative offset. the above used photo detector might, for example, exhibit an absolute offset if used (not in the above circuit) with an additional voltage source. however, if the number of photo detectors within a matrix becomes bigger then one, each light sensor will also exhibit an additional relative offset as compared to the other once. in such a case the main concern will be at least to reduce the relative offset within the analog domain, before the individual vector values are being applied to the adc-vector. since these principles are applicable to all other sensor vectors (in this case the temp. and moisture sensor vector), it is very clear that a pure analog signal condi- tioning approach will require an extensive amount of time for the complete system s calibration. on the other hand, a pure digital approach will most probably require the use of: additional digital peripherals like an extensive memory array (in order to store all sensor character- istic data) a bigger and faster mcu (in order to administrate all activities in the fastest possible way) a dsp (in order to calculate all required values) this amount of overhead might not be justi?d in terms of the return on investment within these types of applica- tions. using this advanced mixed-signal approach will help to keep the direct (high-end components, sw) and indirect costs (system calibration, assembly) at an acceptable level. with the above-presented advanced mixed-signal solution, all sensor vectors can be tuned to a predetermined value at a high precision analog level through a serial interface in an automated way . the xicor approach the key to successful cost reduction will be the use of xicor s xdcps. for the above circuit two quad xdcps and one single xdpc will be enough to cover all variable analog parameters. the above mentioned application could be built up with two x9259-xdcps (= 2 x 4 x 256 steps/pot) and one x9279-xdcp (= 1 x 256 steps/pot). both are fully i 2 c-addressable (400 khz), and include an additional on-chip e 2 prom register set. since xicor s xdcps will automatically recall during power-up the stored wiper positions in these registers, there will be no further need for digital communication between any xdcp and a mcu, as soon as the calibra- tion had been successfully completed. cost reduction will therefore be achieved by eliminating any manual adjustment performed up-to now and by keeping the costs for additional peripheral devices within a reason- able range, with the highest possible level of analog accuracy.
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