Respiratory Sensor System (RSS) for In Situ Breath ... · PDF fileRespiratory Sensor System...

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Respiratory Sensor System (RSS) for In Situ Breath Measurement:

O2, CO2 and Flow,based on a MSP430F149

Dipl.-Ing. Timo KirschkeDipl.-Ing Jörg Heisig

supported by the European Space Agency ESA

Institute for Aerospace EngineeringProfessorship for Space Systems and Utilization


Contents

• RSS 1.5 – explanation and principles• Hardware• Software• Control Algorithms• Sensor Modelling and Errors• Trivia• Results• System Evolution• Discussion, Questions



RSS 1.5 – What's that?

Abstract

The TU Dresden, Institute for Aerospace Engineering, is developing a Respiratory Sensor System in a project, sponsored by the European Space Agency. Essential parts of the system are: solid electrolyte gas sensors, an electronics box and the visualisation software, written in LabView and running on a PC. The initial target of the system is to observe the fitness of astronauts during their activities at the International Space Station ISS. Main terrestrial applications for this system are the fitness market and/or other medical applications, but also many gas sensing applications (e.g. vacuum coating techniques, combustion control...).

The electronics of this RSS were built around the TI MSP430F149. The electronic box contains analogue signal conditioning circuitry, voltage and current excitation for the sensors, a flow turbine pulse shaping circuit, two classD (PWM) power output stages for sensor heaters, bidirectional RS232 communication, some power supply parts and a reset circuit.

The main challenge was the realisation of some DSP-like functionalities on a low power µController. The heater control algorithm uses a cycle time of 1ms. Two PID-controls for the sensor heaters, all signal precalculations and the bidirectional communication with the PC had to be realised inside this time frame.

This presentation will introduce some interesting parts of the electronics, especially the power output section, and the challenges of the firmware development to meet the timing requirements, following the slogan: „Squeezing the last bit“.

The Respiratory Sensor System will be demonstrated in function.



RSS 1.5 Advantages

• Low response time allowing a breath-to-breath, or even a ”one-breath” analysis

• Capability of measuring O2, CO2, and flow rate simultaneously

• Measurement in mainstream (the inspired/expired flow path of a human subject)

• Miniaturised design, „low-weight“• Low power loss of the electronics• Possibility of mobile/portable use of systems incorporating

the new sensor



Principle of the O2 Sensors

● Oxygen ions can be conducted through a hot (>600°C) solid state electrolyte● Ions can be “pumped” from the cathode to the anode by an applied DC-voltage● The oxygen flux to the cathode is limited by a diffusion barrier and the sensor

temperature is kept constant

A

diffusionbarrier cathode

current anode

O2-

O2

O2

solidelectrolyte

constantvoltage

Anode

Electrolyte

Cathode

Substrate

Heater

Planar Design

Size:20 x 3,5 x 0,5 mm



Flow Measurement● Sensor dissipates heat to

surroundings through:- Radiation- Free convection- Forced convection- Heat conduction to sensor plug

● Need to stabilise the temperature with a controller:- Heating by electrical power

● By measuring this power the original fluid flow rate can be computed (similar to “hot wire/hot film anemometer”)

VFluid

Radiation Free convection Forced convection Heating

Heat conduction


Insert Company Logo Here• Sensor signal

conditioning (INA, TIA, PGA: 5 gain ranges, 8ch. ADC)

• DAC 6/8ch. 16Bit• 2ch. PWM heater

power amplifier • Temperature

measurement, (programmable RTD current excitation)

• Flow turbine (SW quadrature dec.)

• JTAG + RS232

Hardware Structure

O2-sensor

CO2-sensor

PC

V24 protocolserial data

port

TI MSP430F149

serial on boardprogramming port

display port

LC – displayStatus-LEDs

digital heating control

additional port

SPI

digital input port

keypad

PowerAmp

DAC

DAC

ADC

PGA

DAC

PowerAmp

DAC

PGA

Reference

ADC-Filter

Temperature measurement



Linear Power Stage

previous design:Linear Power Amplifier

main advantages:simple,fast,stable

disadvantage:high powerloss



Triangle PWM Power Stage2nd approach:

natural sampling PWM, clocked by MSP430

advantage:low power loss

disadvantage:complex design, inherent output impedance makes output voltagemeasurementnecessary(no internal

control loop)



Hysteretic Modulator OPS

1st quadrant modification of an Audio classD amplifier principleself oscillating, needs no clock, internal voltage control loop

3rd approach: Hysteretic ModulatorInstitute for Aerospace EngineeringProfessorship for Space Systems and Utilization


Hyst. Modulator Simulation

● 0 ≤ M ≤ 1 possible, load independent over a wide range● constant voltage ripple over varying modulation index● good linearity (in Audio applications: fidelity)● not sensitive against power supply variations



Power Stage Schematic

Small, simple, stable, efficient (>91% >5W), fast, accurateminor disadvantage: noisy at extreme modulation indexes



Power Stage Efficiency

Total power consumption of the complete electronics box is approx. 2.5W (working condition)

Additional power loss of the output stage remains below 1W, even at full power (25W)

0,1 1,0 10,0 100,00

10

20

30

40

50

60

70

80

90

100Modulator Efficiency vs. output Power

Eta [%] @ 100Ω V 1_0Eta [%] @ 20Ω V 1_0Eta [%] @ 100Ω V 1_5Eta [%] @ 20Ω V 1_5

Output Power [W]

Effi

cien

cy E

ta [%

]

Main Working RangeVersion 1.0

Version 1.5



Software Summary

● Development of a dedicated sensor model for control design

● Analytical and PC-based numerical parametrisation possible

● Development and verification of different control algorithms● Working principle of the sensors and their applicability was

demonstrated● Decision for a control algorithm depends on the sensor

structure and its parameters● LabView-Software + DDE-interface available



Data Flux in the System

Analogue sensor

signals

Micro-controller

(heat control anddigitalisation)

Sensor(measurement of gas

mixture and flow)

MC-Driver(for data

acquisition andcalibration)

PC

Raw data inRS 232 buffer

Time, Flow,O2 and CO2 datain DDE buffer

UserProgram

(for data processing, storageand visualisation)

Digital sensor signals via serial port

Serial port DDE interface

Current sampling rates:

1000 Hz in MC250 Hz in PC

Analogue sensor

signals

Micro-controller

(heat control anddigitalisation)

Sensor(measurement of gas

mixture and flow)

MC-Driver(for data

acquisition andcalibration)

PC

Raw data inRS 232 buffer

Time, Flow,O2 and CO2 datain DDE buffer

UserProgram

(for data processing, storageand visualisation)

Digital sensor signals via serial port

Serial port DDE interface

Current sampling rates:

1000 Hz in MC250 Hz in PC



Function Calls (Modules)

main

read ADC

Initialisation Oscillator ADC Timer A Timer B UART0 UART1 SPI0 WakeUpDAC Temp. Sensor

receive interrupt

convert ASCII->Hex

convert Hex->ASCII

load parameters set control switch

init sensorapplication

control switch PRBS PID_Heater PID_Sensor 3point_heater 3point_sensor Mod 3point_heater Mod 3point_sensor PARC PVR OR2/2 (OR4/4) cascaded_heater cascaded_sensor

upload values fill send buffer

wake up DACs

control cycle

one shot

transmit interrupt



Control Algorithms● Control algorithm requirements:

- fast (quick response on disturbances, short cycle time),- robust (non-sensitive against parameter errors)- adjusted to the control path - high control quality;- continuous (smooth) controlling variable- low (better: no) remaining offset- low consumption of resources

calculation timememory space

● Suitable algorithms: 3-point-controlPID control

● Literature example: spra083.pdf on TI's website“PID and Deadbeat Controllers With the TMS320 Family”Finally implemented formulas No. 45 + 46



PID Control Formulas

PID- Parameter Loading

K1 = Kp + 2•Kd/T + Ki•T/2K2 = Ki•T – 4•Kd/TK3 = Ki•T/2 + 2•Kd/T – Kp

Calculation of the output voltage (discrete algorithm)

u(n) = u(n-2) + K1*e(n) + K2*e(n-1) + K3*e(n-2)



PID Control ProgrammingInstitute for Aerospace EngineeringProfessorship for Space Systems and Utilization

PID- Parameter Loading (P is shown only, I and D are similar)

If (Parameter[0]=='P' && Parameter[1]=='F' && Parameter[2]=='O') // O2 sensor PID control proportional factor{ P_Factor_O2 = 10*Value; K1_O2 = P_Factor_O2 + 2 * D_Factor_O2 + (I_Factor_O2 >> 1); K2_O2 = I_Factor_O2 - 4 * D_Factor_O2; K3_O2 = 2 * D_Factor_O2 - P_Factor_O2 + (I_Factor_O2 >> 1); return;}

Calculation of the output voltage(small modifications are dependent on the control path)

y[k0] = y[k1] + ((K1_O2 * (RTsoll - RTist[k0])) >> 10)+ ((K2_O2 * (RTsoll - RTist[k1])) >> 10)+ ((K3_O2 * (RTsoll - RTist[k2])) >> 10);

ControlOut = ((y[k0] + DAC_Offset) * DAC_Factor) >> 10;


3-Point Control PrincipleInstitute for Aerospace EngineeringProfessorship for Space Systems and Utilization

UT>UTsoll+Hysteresis?

UHmid

send to DAC,output acquired data

yes

no

yes

UT<UTsoll-Hysteresis?

UHhighUHlow

yes

no


3-Point Control Programming

// calculate output voltage

if (RTist[k0] > (RTsoll + Hysteresis)){ y[k0] = y_Low; // set out voltage = lower limit P5OUT &= ~BIT7; // switch LED 4 off}else if (RTist[k0] < (RTsoll - Hysteresis)){ y[k0] = y_High; // set out voltage = upper limit P5OUT |= BIT7; // switch LED 4 on}else y[k0] = y_Mid; // set out voltage = mid point

if (RTist[k0] > RT_UpLimit) // overheat?{ y[k0] = 1000; // reset control output value P5OUT |= BIT5; // switch LED 2 on}

ControlOut = ((y[k0] + DAC_Offset) * DAC_Factor) >> 10;



Sensor Modelling (1)Institute for Aerospace EngineeringProfessorship for Space Systems and Utilization

Hea

ter

Tem

pera

ture

sen

sor


Sensor Modelling (2)

u voltagei currentP powerR resistanceQ heat

flowϑ temperatureG transfer

functionH heater sideS sensor sidedr drain offdist disturbance

GRϑH

GPH

GϑH GϑS

GiH

uH

RH

RH

RHuH RH

ϑS

GRHuH

iH

PH

ϑH

● ●

QdrH+QdistH

● ●

QdrS+QdistS

●

QLeit



Quantisation EffectsInstitute for Aerospace EngineeringProfessorship for Space Systems and Utilization

Discretisation• Time intervals (1ms)• Amplitude (12 Bit = 4095 values)Measuring range is:• Heater voltage 24 V ∆u = 6mV• Heater current 2.5 A ∆i = 610µA

• Measurement error is higher than the aimed temperature accuracypossible solution:significant enhancement of the measurement resolution

• Fast and accurate control algorithms get a two-point-behavior due to - the limited resolution and - the time delay between heater and temperature sensor

possible solution:concentration of control path parameterse.g. heater = temperature measurement resistor


Measurement Quantisation and Resulting Reading Error

Peak to peak reading errors

Heater voltage±0.05% of range

Heater current±0.05% of range

Heater resistance±0.3% of reading(!)



Heater Control Results

● If optimised for speed, a PID control shows similar behaviour as a 3-point control:

● Control variable deviation (green)

● Control variable noise (blue)

● Response time


PID control | 3-point control

0.1s

< 0.

1%


Division Routines● 32Bit/32Bit via Libraries takes 68µs @ 8MHz● 2 divisions per control cycle necessary

approx. 16% of computational time● Lookup Table division possible?● Resolution of the measured values, acting as the divisor, is

12 Bits● Rearrange the division into a multiplication with the divisor

reciprocals, put them into a L.U.T. (vector)● Expand the reciprocals by 12Bit to keep the resolution,

even for the smallest values● Split the L.U.T. into 3 blocks to stay inside the 16Bit-range

(scaling by 16/20/24 Bits)● 12Bit/12Bit via L.U.T. takes 12µs @ 8MHz



Serial Data Transmission

● Control cycle time: 1ms● Max. Baudrate: 115.2kBd, limited by the PC● Data to be transmitted: 8 values at 4 ASCII-symbols each● Overhead: 6 ASCII-symbols sum: 38 Bytes● Parameters 8N1: 9 Bits per data Byte● Theoretical data rate: 336 value sets per second● Data transmission: every 4ms● Function calls inside data conversion and buffer filling

routines avoided● Keep overhead small



Transmit Results via RS232void FillSendBuf(unsigned int UARTx){ static unsigned int i = 0; static int result; { SendCRC = 0; SendBuf1[37] = 0x02; // STX

result = 0x0F & ((int)(SumRHist >> 12)); // convert numbers hex->ASCII if (result <= 9) SendBuf1[36] = result + '0'; else SendBuf1[36] = result + ('A' - 10);

result = 0x0F & ((int)(SumRHist >> 8)); if (result <= 9) SendBuf1[35] = result + '0'; else SendBuf1[35] = result + ('A' – 10);... for (i = 36; i >= 3; i--) // build CRC exclusive STX/ETX { SendCRC = SendCRC ^ (SendBuf1[i]); // bytewise EXOR } SendBuf1[2] = Digit1(SendCRC); // ASCII CRC SendBuf1[1] = Digit0(SendCRC); // ASCII CRC

SendBuf1[0] = 0x03; // ETX SendBufCnt1 = 37; // counter for bytes to be sent TXBUF1 = SendBuf1[SendBufCnt1]; // load 1st value into buffer } // initiates UART send routine}



Used PeripheralsInstitute for Aerospace EngineeringProfessorship for Space Systems and Utilization


3 rd Hardware Generation● All elements integrated

onto 1 PCB (3 PCBs)● Full SMT except big

caps and connectors● External power supply● O2 and CO2 sensor

channels (O2 only)● Multiple measurement

ranges per sensor channel (1 range only)

● PWM output stages(linear OPS)

● ¼ size, ⅓ weight


3rd (1st) hardware generation


The Complete RSS PRO 1.5● Mask, containing

O2 and CO2 sensors

● Sensor cables, interface connection

● Electronics box ● External power

supply ● PC with LabView-

software



Used Tools

Schematic: Orcad - not the optimum, many chrashes, sometimes corrupting the design

PCB-Layout: Orcad (sub-contractor)Software: first IAR, later Crossworks – no way back ☺



The Final

Thanks to:

● The ESA/ESTEC for the financial support of our project● Texas Instruments for their nice controllers and support● The web community (e.g. MSP430 Yahoo-forum)● Paul Curtis (Rowley) for his generous help● You, for your attention and patience

Enjoy your projects!



Slides - Discussion1. Title2. Contents3. RSS 1.5 – What's that?4. RSS 1.5 Advantages5. Principle of the O2 Sensors6. Flow Measurement7. Hardware Structure8. Linear Power Stage9. Triangle PWM Power Stage10. Hysteretic Modulator OPS11. Hyst. Modulator Simulation12. Power Stage Schematic13. Power Stage Efficiency14. Software Summary15. Data Flux in the System16. Function Calls (Modules)17. Control Algorithms18. PID Control Formulas19. PID Control Programming


20. 3-Point Control Principle21. 3-Point Control Programming22. Sensor Modelling (1)23. Sensor Modelling (2)24. Quantisation Effects25. Measurement Quantisation and

Resulting Reading Error26. Heater Control Results27. Division Routines28. Serial Data Transmission29. Transmit Results via RS23230. Used Peripherals31. 3rd Hardware Generation32. The Complete RSS PRO 1.533. Used Tools34. The Final35. Slides - Discussion

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