Autopilot voor zeilboten (automatisch stuursysteem)

Code

• Schets stuurautomaat (voor Uno)
• WatchDog-schets (voor Nano)

Schets stuurautomaat (voor Uno)Arduino

/* Deze schets fungeert als stuurautomaat voor kleine zeilboten, door Marco Zonca, 2019 Arduino UNO als CPU, Arduino Nano als waakhond, GPS BT-220 nmea, stappenmotor + controller, rf433Mhz RC, 6 knoppen, zoemer, i2c display, 2 leds, i2c 24c04 eeprom, Mux 4051 voor sensoren, lipo 2s 7.4v 2600mA, 7805 spanningsregelaar, thermistor;*/#include #include #include # include String inputString ="";String nm_time ="00:00:00";String nm_validity ="V";String nm_latitude ="ddmm.mmmm'N";String nm_longitude ="dddmm.mmmm'E ";String nm_knots ="0.0kn";float nmf_knots =0.0;String nm_truecourse ="360";float nmf_truecourse =360;String nm_date ="dd/mm/jjjj";String nm_routetofollow ="000";float nmf_routetofollow =0; unsigned long previousStearingMillis =0; unsigned long currentStearingMillis =0; unsigned long prevCheckSensorsMillis =0; unsigned long currCheckSensorsMillis =0;int CheckSensorsInterval =10000;bool stringComplete =false;bool isfirstfix =true;bool ispause =true;bool isStearing =false;bool isSetup =false;int s=0;int y=0;int z=0;int d=0;int rfRemoteControlValue =0;int HWButtonValue =0;int SetupParameter =0;float calcmove =0;float cm =0;float Stearing =0;float prevStearing =0;float t =0;int EEdisk =0x50;int EEid1 =0x29;int EEid2 =0x00;unsigned int EEaddress =0;unsigned int EEbytes =12;byte EEdata[12];byte EEbytedata;int EEerr =0;float SensorVBatt=0;float SensorVRes=0;float SensorTemp=0;float SensormAmp=0;// volgende parameters zijn de standaardwaarden, maar zijn lezen/schrijven in eeprom// eeprom wordt geïnitialiseerd als op adressen 0 en 1 de inhoud verschillend is adrestype opmerkingen// 0-255 bytes bij 0x50 EEdisk, 256-512 bytes bij 0x51 (niet gebruikt) ---------- -------------------------------------------------- ---// 0 1B byte 01001001 (0x29 als stuurautomaatproject id1)// 1 1B byte 00000000 (0x00 " " id2)int StearingInterval =2000; // millis tussen proberen en terug 2 2B int StearingInterval 1000-5000 stappen 100int StearingMinToMove =2; // compass_degrees 4 2B int StearingMinToMove 0-20 stappen 1int StearingMaxMove =40; // compass_degrees 6 2B int StearingMaxMove 10-45 stappen 1float StearingCoeffMove =1.5; // gebruikt als (compass_degrees * coeff) 8 4B float StearingCoeffMove 0.1-4 stappen 0.1// 12 free//byte bStearingInterval[sizeof(int)];byte bStearingMinToMove[sizeof(int)];byte bStearingMaxMove[sizeof(int)];byte bStearingCoeffMove[sizeof(float)];int prev_StearingInterval=0;int prev_StearingMinToMove=0;int prev_StearingMaxMove=0;float prev_StearingCoeffMove=0;const int ledpause 7Pin =2;const int =2;const int =// 00=Vin 01=Vbatt 10=Tempconst int MuxSelBit1Pin =6; // const int motorenABenablePin =13;const int MuxIOPin =14;const int ButtonsPin =15;const int rfRemoteControlPin =16;const int speakerPin =17;const int RCleftbutton =201;const int RCrightbutton =202;const int RCleft10button =203; const int RCright10knop =204;const int HWlinksknop =101;const int HWrightknop =102;const int HWpauzeknop =103;const int HWsetupknop =104;const int HWleft10knop =105;const int HWright10knop =106;const intRevolution motorSteps; // 200 voor model 23LM, 54 stappen =1/4 van omwenteling LiquidCrystal_I2C lcd (0x27, 2, 1, 0, 4, 5, 6, 7, 3, POSITIEF); Stappenmotor (motorStepsPerRevolution, 9, 10, 11, 12 ); ongeldige setup () { Serial.begin (4800); lcd.begin(16,2); Draad.begin(); motor.setSpeed(60); inputString.reserve(200); pinMode (motorenABenablePin, OUTPUT); pinMode (MuxSelBit0Pin, UITGANG); pinMode (MuxSelBit1Pin, UITGANG); digitalWrite (motorenABenablePin, LAAG); digitalWrite (MuxSelBit0Pin, LAAG); digitalWrite (MuxSelBit1Pin, LAAG); pinMode (ledpauzePin, UITGANG); pinMode (watchDogPin, OUTPUT); digitalWrite(ledpausePin, LAAG); digitalWrite (watchDogPin, LAAG); // read+check EEPROM (formatteren indien nieuw (of niet geïdentificeerd)) lcd.clear(); lcd.setCursor(0,0); lcd.print("Geheugencontrole..."); lcd.setCursor(0,1); voor (s =0; s =CheckSensorsInterval) { readMuxSensors(); if ((SensorVBatt <=7.0) || (SensorTemp>
=50)) { lcd.clear(); lcd.setCursor(0,0); lcd.print("Alarmsensoren! "); lcd.setCursor(1,1); lcd.print("V="); lcd.print (SensorVBatt); lcd.print(" "); lcd.print(int(SensorTemp)); lcd.schrijven (0xDF); lcd.print("C"); NewTone (speakerPin,10); vertraging (1000); geenNieuweTone(); } prevCheckSensorsMillis =currCheckSensorsMillis; } // STUURBEDIENING ---------------- currentStearingMillis =millis(); if (currentStearingMillis - previousStearingMillis>=StearingInterval) {if (isStearing ==false &&ispause ==false) { // probeer (verplaats stearing) calcmove =nmf_routetofollow - nmf_truecourse; if (calcmove <(-180)) { calcmove =calcmove + 360; } else { if (calcmove> (+180)) { calcmove =calcmove - 360; } } if (abs(calcmove)>=StearingMinToMove) { if (abs(calcmove)>=StearingMaxMove) { if (calcmove <0) { cm =(StearingMaxMove * -1); calcmove =cm; } else { cm =(StearingMaxMove * 1); calcmove =cm; } } Stearing =(calcmove * StearingCoeffMove); gomotor(int((Stuur * 216) / 360)); // 54 stappen =1/4 van omwenteling prevStearing =Sturen; isStearing =waar; } } else { // ga terug (verplaats stearing naar "nul" positie) if (isStearing ==true) { Stearing =(prevStearing * -1); gomotor(int((Stuur * 216) / 360)); // 54 stappen =1/4 van omwenteling Sturen =0; prevStearing =0; isStearing =onwaar; } } previousStearingMillis =currentStearingMillis; } // RC RF-KNOPPEN ------------------ rfRemoteControlValue =checkRfRC(); if (rfRemoteControlValue) { switch (rfRemoteControlValue) { case RCleftbutton:// Linker RC-knop goleft (); pauze; case RCrightbutton:// Rechter RC-knop goright(); pauze; behuizing RCleft10button:// Left-10 RC-knop goleft10(); pauze; behuizing RCright10button:// Rechts+10 RC-knop goright10(); pauze; } } // KNOPPEN ------------------------ HWButtonValue =checkHWButtons(); if (HWButtonValue) { switch (HWButtonValue) { case HWleftbutton:// Left (-1) HW-knop if (isSetup ==false) { goleft (); } else { setupMinus(); } pauze; case HWrightbutton:// Rechts(+1) HW-knop if (isSetup ==false) { goright(); } else { setupPlus(); } pauze; case HWpauzeknop:// Pauzeer HW-knop gopause(); pauze; case HWsetup-knop:// Setup HW-knop gosetup(); pauze; behuizing HWleft10button:// Links(-10) HW-knop goleft10(); pauze; behuizing HWright10button:// Rechts (+10) HW-knop goright10(); pauze; } } // GPS NMEA ------------------ if (stringComplete ==true) { // NMEA-zin ontvangen via seriële poort RX bool ret; ret =nmeaExtractData(); invoerString =""; stringComplete =onwaar; if (ret ==true) { RefreshDisplay(); } } // WATCHDOG FEEDING ---------------- if (digitalRead (watchDogPin) ==LAAG) {digitalWrite (watchDogPin, HIGH); } else {digitalWrite(watchDogPin, LOW); }}// lees sensoren op multiplexervoid readMuxSensors () { float Vo =0; vlotter n =0; vlotter n1 =0; float v1ad =0; float v2ad =0; vlotter corr =0; vlotter R1 =10000; float logR2 =0; vlotter R2 =0; vlotter T =0; vlotter c1 =1.09249522e-03; vlotter c2 =2.378405444e-04; vlotter c3 =2.019202697e-07; digitalWrite (MuxSelBit0Pin, LAAG); // 00=Vbatt digitalWrite (MuxSelBit1Pin, LAAG); n =analoog lezen (MuxIOPin); v1ad=n; n1=(((10.00 * n) / 1023,00)); SensorVBatt=(n1 + ((n1 * 0,0) /100)); // willekeurige correctie (niet actief =0,0%), digitalWrite (MuxSelBit0Pin, LOW); // 01=Vres digitalWrite (MuxSelBit1Pin, HOOG); n =analoog lezen (MuxIOPin); v2ad=n; n1=(((10.00 * n) / 1023,00)); SensorVRes=(n1 + ((n1 * 0,0) /100)); // willekeurige correctie (niet actief =0,0%), digitalWrite (MuxSelBit0Pin, HIGH); // 10=NTC Temp digitalWrite (MuxSelBit1Pin, LAAG); Vo =analoog lezen (MuxIOPin); R2 =R1 * (1023,0 / Vo - 1,0); logR2 =log(R2); T =(1,0 / (c1 + c2 * logR2 + c3 * logR2 * logR2 * logR2)); SensorTemp =T - 273.15; // Celsius n =(v1ad - v2ad); n1 =(n / 0,22) * 1000,00; SensormAmp =(((10.00 * n1) / 1023.00));}// gegevens extraheren uit nmea inputStringbool nmeaExtractData () { bool ret =false; //true if nmea zin =$GNRMC en geldige CHKSUM if ((inputString.substring(0,6) =="$GNRMC") &&(inputString.substring(inputString.length()-4,inputString.length()- 2) ==nmea0183_checksum(inputString))) { y=0; for (s =1; s <11; s ++) { y=inputString.indexOf(",",y); switch (s) { case 1://time z=inputString.indexOf(",",y+1); if (z>(y+1)) { nm_time=inputString.substring(y+1,y+2+1)+":"+inputString.substring(y+1+2,y+4+1)+" :"+inputString.substring(y+1+4,y+6+1); } y=z; pauze; geval 2://geldigheid z=inputString.indexOf(",",y+1); if (z>(y+1)) { nm_validity=inputString.substring(y+1,y+1+1); } y=z; pauze; geval 3://latitude z=inputString.indexOf(",",y+1); if (z>(y+1)) { nm_latitude=inputString.substring(y+1,y+2+1)+""+inputString.substring(y+1+2,y+10+1)+"' "; } y=z; pauze; geval 4://noord/zuid z=inputString.indexOf(",",y+1); if (z>(y+1)) { nm_latitude=nm_latitude + inputString.substring(y+1,y+1+1); } y=z; pauze; geval 5:// lengtegraad z=inputString.indexOf(",",y+1); if (z>(y+1)) { nm_longitude=inputString.substring(y+1,y+3+1)+""+inputString.substring(y+1+3,y+11+1)+"' "; } y=z; pauze; geval 6://east/west z=inputString.indexOf(",",y+1); if (z>(y+1)) { nm_longitude=nm_longitude + inputString.substring(y+1,y+1+1); } y=z; pauze; geval 7://snelheidsknopen z=inputString.indexOf(",",y+1); if (z>(y+1)) { nmf_knots=inputString.substring(y+1,z).toFloat(); t=roundOneDec(nmf_knots); nm_knots=String(t,1)+"kn"; } y=z; pauze; geval 8://true course z=inputString.indexOf(",",y+1); if (z>(y+1)) { nmf_truecourse=inputString.substring(y+1,z).toFloat(); d=nmf_truecourse; nm_truecourse=d; } y=z; pauze; geval 9:// datum z=inputString.indexOf(",",y+1); if (z>(y+1)) { nm_date=inputString.substring(y+1,y+2+1)+"/"+inputString.substring(y+1+2,y+4+1)+" /20"+inputString.substring(y+1+4,y+6+1); } y=z; pauze; geval 10:// uitspraken breken; standaard:// instructies breken; } } if ((isfirstfix ==true) || (ispause ==true)) { nm_routetofollow=nm_truecourse; nmf_routetofollow=nmf_truecourse; isfirstfix=false; } ret=waar; } return ret;}// verhoging (+) parameterwaarde tijdens setupvoid setupPlus () { switch (SetupParameter) { case 2:// interval StearingInterval =(StearingInterval + 100); if (StearingInterval> 5000) { StearingInterval =5000; } pauze; geval 3://min. om te bewegen StearingMinToMove =(StearingMinToMove + 1); if (StearingMinToMove> 20) { StearingMinToMove =20; } pauze; geval 4://max. move StearingMaxMove =(StearingMaxMove + 1); if (StearingMaxMove> 45) { StearingMaxMove =45; } pauze; geval 5://coëfficiënt StearingCoeffMove =(StearingCoeffMove + 0,1); if (StearingCoeffMove> 4) { StearingCoeffMove =4; } pauze; } vertraging (200); RefreshDisplay ();}// afname (-) parameterwaarde tijdens setupvoid setupMinus () { switch (SetupParameter) { geval 2:// interval StearingInterval =(StearingInterval - 100); if (StearingInterval <1000) { StearingInterval =1000; } pauze; geval 3://min. om te verplaatsen StearingMinToMove =(StearingMinToMove - 1); if (StearingMinToMove <0) { StearingMinToMove =0; } pauze; geval 4://max. move StearingMaxMove =(StearingMaxMove - 1); if (StearingMaxMove <10) { StearingMaxMove =10; } pauze; geval 5://coëfficiënt StearingCoeffMove =(StearingCoeffMove - 0,1); if (StearingCoeffMove <0,1) { StearingCoeffMove =0,1; } pauze; } vertraging (200); RefreshDisplay();}// motor control (+)=forward (-)=backwardsvoid gomotor(int stepsToMove) {digitalWrite(motorenABenablePin, HIGH); motor.stap(stappenToMove); digitalWrite(motorsABenablePin, LOW);}// refresh data on displayvoid RefreshDisplay() { if (isSetup ==false) { //---------normal lcd.clear(); lcd.setCursor(0,0); lcd.print("R"+nm_routetofollow); lcd.write(0xDF); lcd.print(" H"+nm_truecourse); lcd.write(0xDF); if (ispause ==true) { lcd.print(" STOP"); } else { if (Stearing> 0) { lcd.print(" +"); } if (Stearing ==0) { lcd.print(" "); } if (Stearing <0) { lcd.print(" "); } lcd.print(int(Stearing)); } lcd.setCursor(0,1); lcd.print(nm_time+" "+nm_knots); } if (isSetup ==true) { //-----------setup lcd.clear(); lcd.setCursor(0,0); lcd.print("setup:"); switch (SetupParameter) { case 1://display sensors readMuxSensors(); lcd.print("V="); lcd.print(SensorVBatt); lcd.setCursor(1,1); lcd.print("mA="); lcd.print(int(SensormAmp)); lcd.print(" "); lcd.print(int(SensorTemp)); lcd.write(0xDF); lcd.print("C"); break; case 2://interval lcd.print("interval"); lcd.setCursor(7,1); lcd.print(StearingInterval); lcd.print(" mSec"); break; case 3://min. to move lcd.print("minimum"); lcd.setCursor(7,1); lcd.print(StearingMinToMove); lcd.write(0xDF); break; case 4://max. move lcd.print("max"); lcd.setCursor(7,1); lcd.print(StearingMaxMove); lcd.write(0xDF); break; case 5://coefficient lcd.print("coeffic."); lcd.setCursor(7,1); lcd.print(StearingCoeffMove); lcd.print(" x "); lcd.write(0xDF); break; } }}/* SerialEvent occurs whenever a new data comes in the hardware serial RX. This routine is run between each time loop() runs, so using delay inside loop can delay response. Multiple bytes of data may be available.*/void serialEvent() { while (Serial.available()) { char inChar =(char)Serial.read(); inputString +=inChar; // if the incoming character is a newline, set a flag so the main loop can // do something about it if (inChar =='\n') { stringComplete =true; } } }//calculate checksum of nmea sentenceString nmea0183_checksum(String nmea_data) { int crc =0; String chSumString =""; int ik; // ignore the first $ sign, checksum in sentence for (i =1; i <(nmea_data.length()-5); i ++) { // remove the - 5 if no "*" + cksum + cr + lf are present crc ^=nmea_data[i]; } chSumString =String(crc,HEX); if (chSumString.length()==1) { chSumString="0"+chSumString.substring(0,1); } chSumString.toUpperCase(); return chSumString;}//check RC which button is pressedint checkRfRC() { int n =0; int res =0; n =analogRead(rfRemoteControlPin); if ((n>350) and (n<460)) { // button A res =RCleftbutton; } if ((n> 90) and (n<190)) { // button B res =RCrightbutton; } if ((n>540) and (n<640)) { // button C res =RCleft10button; } if ((n>225) and (n<325)) { // button D res =RCright10button; } return res; }//check HW which button is pressedint checkHWButtons() { int n =0; int res =0; n =analogRead(ButtonsPin); //Serial.println(n); if ((n>465) and (n<565)) { // button left res =HWleftbutton; } if ((n>290) and (n<390)) { // button right res =HWrightbutton; } if ((n>130) and (n<220)) { // button pause res =HWpausebutton; } if ((n>625) and (n<725)) { // button setup res =HWsetupbutton; } if ((n>975) and (n<1075)) { // button left-10 res =HWleft10button; } if ((n>800) and (n<900)) { // button right+10 res =HWright10button; } return res; }void gosetup() { // setup button if (isSetup ==false) { SetupParameter =1; isSetup =true; } else { if (SetupParameter <5) { SetupParameter ++; } else { if (prev_StearingInterval !=StearingInterval || prev_StearingMinToMove !=StearingMinToMove || prev_StearingMaxMove !=StearingMaxMove || prev_StearingCoeffMove !=StearingCoeffMove) { lcd.clear(); lcd.setCursor(0,0); lcd.print("updating... "); vertraging (1000); goupdateEEPROM(); if (EEerr) { lcd.print("E="); lcd.print(EEerr); vertraging (1000); } prev_StearingInterval =StearingInterval; prev_StearingMinToMove =StearingMinToMove; prev_StearingMaxMove =StearingMaxMove; prev_StearingCoeffMove =StearingCoeffMove; } isSetup =false; } } NewTone (speakerPin,2000); delay(200); noNewTone(); RefreshDisplay();}void goupdateEEPROM() { EEaddress =0; //id1 EEdata[0] =EEid1; EEbytedata =EEid1; writeEEPROM (EEdisk, EEaddress, EEbytedata); EEaddress =1; //id2 EEdata[1] =EEid2; EEbytedata =EEid2; writeEEPROM (EEdisk, EEaddress, EEbytedata); memcpy(bStearingInterval, &StearingInterval, sizeof(int)); memcpy(bStearingMinToMove, &StearingMinToMove, sizeof(int)); memcpy(bStearingMaxMove, &StearingMaxMove, sizeof(int)); memcpy(bStearingCoeffMove, &StearingCoeffMove, sizeof(float)); memcpy(EEdata+2,bStearingInterval,sizeof(int)); memcpy(EEdata+4,bStearingMinToMove,sizeof(int)); memcpy(EEdata+6,bStearingMaxMove,sizeof(int)); memcpy(EEdata+8,bStearingCoeffMove,sizeof(float)); for (s =2; s  360) { nmf_routetofollow =1; } d=nmf_routetofollow; nmf_routetofollow=d; nm_routetofollow=d; NewTone (speakerPin,800); delay(200); noNewTone(); } else { NewTone (speakerPin,1000); delay(50); noNewTone(); } RefreshDisplay();}void goright10() { // right 10x button/RC if (ispause ==false) { for (s =1; s <11; s ++) { nmf_routetofollow ++; if (nmf_routetofollow> 360) { nmf_routetofollow =1; } } d=nmf_routetofollow; nmf_routetofollow=d; nm_routetofollow=d; NewTone (speakerPin,800); delay(200); noNewTone(); } else { NewTone (speakerPin,1000); delay(50); noNewTone(); } RefreshDisplay();}void gopause() { // pause button/RC if (ispause ==true) { ispause=false; digitalWrite(ledpausePin, HIGH); NewTone (speakerPin,50); delay(200); NewTone (speakerPin,200); delay(800); noNewTone(); } else { ispause=true; digitalWrite(ledpausePin, LOW); NewTone (speakerPin,200); delay(200); NewTone (speakerPin,50); delay(800); noNewTone(); } RefreshDisplay();}// reading eeprombyte readEEPROM (int diskaddress, unsigned int memaddress) { byte rdata =0x00; Wire.beginTransmission (diskaddress); Wire.write (memaddress); if (Wire.endTransmission () ==0) { Wire.requestFrom (diskaddress,1); if (Wire.available()) { rdata =Wire.read(); } else { EEerr =1; //"READ no data available" } } else { EEerr =2; //"READ eTX error" } Wire.endTransmission (true); return rdata;}// writing eepromvoid writeEEPROM (int diskaddress, unsigned int memaddress, byte bytedata) { Wire.beginTransmission (diskaddress); Wire.write (memaddress); Wire.write (bytedata); if (Wire.endTransmission () !=0) { EEerr =3; //"WRITING eTX error" } Wire.endTransmission (true); delay(5); }// round zero decimalfloat roundZeroDec(float f) { float y, d; y =f*1; d =y - (int)y; y =(float)(int)(f*1)/1; if (d>=0.5) { y +=1; } else { if (d <-0.5) { y -=1; } } return y;}// round one decimalfloat roundOneDec(float f) { float y, d; y =f*10; d =y - (int)y; y =(float)(int)(f*10)/10; if (d>=0.5) { y +=0.1; } else { if (d <-0.5) { y -=0.1; } } return y;}// round two decimalfloat roundTwoDec(float f) { float y, d; y =f*100; d =y - (int)y; y =(float)(int)(f*100)/100; if (d>=0.5) { y +=0.01; } else { if (d <-0.5) { y -=0.01; } } return y;}

WatchDog sketch (for Nano)Arduino

/* * This sketch is a Watchdog to keep CLIENT under control, on Arduino NANO 3.0 by Marco Zonca * CLIENT must feed Whatcdog sooner then feedingInterval otherwise will be forced to restart * */const int feedingPin =14;const int ledPin =15;const int restartPin =16;const int buzzerPin =17;const long ledInterval =1000;const long feedingInterval =2500;const long timeForClientStart =16000;int ledState =LOW;int previousFeedingState =LOW;int feedingState =LOW;unsigned long previousLedMillis =0;unsigned long previousFeedingMillis =0;void setup() { digitalWrite(restartPin, HIGH); // LOW will force CLIENT to restart pinMode(ledPin, OUTPUT); pinMode(buzzerPin, OUTPUT); pinMode(restartPin, OUTPUT); pinMode(feedingPin, INPUT); delay(timeForClientStart); // let time to CLIENT to start...}void loop() { unsigned long currentMillis =millis(); // BLINK LED ------------------- if (currentMillis - previousLedMillis>=ledInterval) { previousLedMillis =currentMillis; if (ledState ==LOW) { ledState =HIGH; } else { ledState =LOW; } digitalWrite(ledPin, ledState); } // CHECK THE FEEDING ------------------- feedingState =digitalRead(feedingPin); // CLIENT must set pin HIGH -> LOW frequently to prove it's alive if (feedingState ==HIGH) { if (previousFeedingState ==LOW) { previousFeedingMillis =currentMillis; } previousFeedingState =HIGH; } else { previousFeedingState =LOW; } if (currentMillis - previousFeedingMillis> feedingInterval) { // CLIENT is sleeping ledState =HIGH; digitalWrite(ledPin, ledState); tone(buzzerPin,1500); delay(500); digitalWrite(restartPin, LOW); //restart CLIENT tone(buzzerPin,1500); delay(500); digitalWrite(restartPin, HIGH); tone(buzzerPin,1500); delay(timeForClientStart); // let CLIENT time to restart... noTone(buzzerPin); currentMillis =millis(); previousFeedingState =LOW; previousFeedingMillis =currentMillis; previousLedMillis =currentMillis; }}

Aangepaste onderdelen en behuizingen

23lm-stepper-plate-v2_PlvJaff9Hl.step 23lm-stepper-pulley-56_UhsbaWbiBt.step

Schema's