Come calcolare la frequenza di misura del sensore e scrittura su sd

Dunque ho effettuato le modifiche che mi hai detto,ma mi da un errore su
accelerometer.getAcceleration(&ax,&ay,&az);

dice

sketch_apr11a:23: error: expected constructor, destructor, or type conversion before '.' token
sketch_apr11a:24: error: expected constructor, destructor, or type conversion before '(' token
sketch_apr11a.ino: In function 'void loop()':
sketch_apr11a:66: error: incompatible types in assignment of 'const char [1]' to 'char [100]'
sketch_apr11a:71: error: incompatible types in assignment of 'char' to 'char [100]'
sketch_apr11a:72: error: no match for 'operator+=' in 'Buffer += String(ax, 2u)'
sketch_apr11a:73: error: invalid operands of types 'char [100]' and 'const char [4]' to binary 'operator+'
sketch_apr11a:73: error: in evaluation of 'operator+=(char [100], const char [4])'
sketch_apr11a:74: error: no match for 'operator+=' in 'Buffer += String(ay, 2u)'
sketch_apr11a:75: error: invalid operands of types 'char [100]' and 'const char [4]' to binary 'operator+'
sketch_apr11a:75: error: in evaluation of 'operator+=(char [100], const char [4])'
sketch_apr11a:76: error: no match for 'operator+=' in 'Buffer += String(az, 2u)'
sketch_apr11a:77: error: invalid operands of types 'char [100]' and 'const char [3]' to binary 'operator+'
sketch_apr11a:77: error: in evaluation of 'operator+=(char [100], const char [3])'
sketch_apr11a:79: error: incompatible types in assignment of 'char' to 'char [100]'
sketch_apr11a:81: error: incompatible types in assignment of 'uint8_t' to 'char [100]'
sketch_apr11a:82: error: incompatible types in assignment of 'char' to 'char [100]'
sketch_apr11a:83: error: incompatible types in assignment of 'uint8_t' to 'char [100]'
sketch_apr11a:84: error: incompatible types in assignment of 'char' to 'char [100]'
sketch_apr11a:85: error: incompatible types in assignment of 'uint16_t' to 'char [100]'
sketch_apr11a:86: error: incompatible types in assignment of 'char' to 'char [100]'
sketch_apr11a:87: error: incompatible types in assignment of 'uint8_t' to 'char [100]'
sketch_apr11a:88: error: incompatible types in assignment of 'char' to 'char [100]'
sketch_apr11a:89: error: incompatible types in assignment of 'uint8_t' to 'char [100]'
sketch_apr11a:90: error: incompatible types in assignment of 'char' to 'char [100]'
sketch_apr11a:91: error: incompatible types in assignment of 'uint8_t' to 'char [100]'
sketch_apr11a:92: error: incompatible types in assignment of 'char' to 'char [100]'

ecco il codice modificato:

#include <SD.h>
#include <Wire.h>
#include "RTClib.h"
#include <SPI.h>
#include "ADXL335.h"


ADXL335 accelerometer;

RTC_DS1307 RTC;


File Dati; // La variabile di tipo file che useremo per il log

const int chipSelect =10;

char Buffer[100];


unsigned long time;

DateTime now = RTC.now(); // leggiamo l'ora
accelerometer.getAcceleration(&ax,&ay,&az);
snprintf(buffer,100,"\t g,%d,%d,%d",ax,ay,az);

void setup ()

{
  Serial.begin(9600);
  
  Wire.begin(); // inizializziamo la libreria WIRE per usare
  RTC.begin(); //il Real Time clock basato su ds1307
  accelerometer.begin();
  
  if (! RTC.isrunning()) 
  
  {
    Serial.println("RTC NON STA FUNZIONANDO");
    
    //LA SEGUENTE ISTRUZIONE SERVE SOLO PER REGOLARE L'OROLOGIO.ATTIVARE SOLO SE NECESSARIO.
  //  RTC.adjust (DateTime(__DATE__, __TIME__)); //imposta ora e data
    }
    
  Serial.print("\nSto verificando la presenza della scheda..\n");
  pinMode(10, OUTPUT); // fa funzionare la sdlib
  
  if (!SD.begin(chipSelect))
  
  {
     Serial.println("SD card non trovata");
     return;
     
  }
 else 
   {
     Serial.println("Scheda inserita e funzionante");
     Serial.println();
    
   }
}
void loop ()

{
  Serial.print ("time:");
  time=millis();
  Buffer=""; // Nuovo giro, stringa vuota!

  float ax,ay,az;
 
  
  Buffer+=('\t');
  Buffer+=String(ax); 
  Buffer+=" g,";
  Buffer+=String(ay); 
  Buffer+=" g,";
  Buffer+=String(az);
  Buffer+=" g";
  
   Buffer+=('\t');
  
  Buffer +=(now.day()); // Componiamo la data AAAA/MM/GG
  Buffer +=('/');
  Buffer +=(now.month());
  Buffer +=('/');
  Buffer +=(now.year());
  Buffer +=('\t');
  Buffer +=(now.hour());
  Buffer +=(':');
  Buffer +=(now.minute());
  Buffer +=(':');
  Buffer +=(now.second());
  Buffer +=('\t'); // tab per separare i campi
 
  Dati = SD.open("datalog.txt", FILE_WRITE); // apriamo il file in scrittura
  Serial.println(time);
  if (Dati)
  {
   Dati.println(Buffer);  // Scriviamo in un sol colpo i dati sul file
   Dati.close();     // chiudiamo e trasferiamo su SD
   Serial.print("Dati rilevati \t");
   Serial.println(Buffer);
  }
  else
  {
    Serial.println("Errore nell'apertura di datalog.txt");
  }
  delay (0);  // cadenza dellascrittura dei dati
  }