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High Voltage DC PWM Motor Voltage Measurement using AD8420



		

			

									
					    1Ironhorse
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Nov 6, 2015 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    I need to monitor the motor RPM to ensure that the driven device (propeller) is not over sped. I know some of the specifications for the drive motor and the propeller system:
    
 
    Motor Max RPM = 18,720 @ 36V (520 RPM / Volt) 
     Prop Max RPM = 7,500
     Drive Gear Ratio = 2.27:1
     Desired Motor RPM = 17,025 or 32.75 Volts (Approx 21A = 688W (.90HP))
    
 
    Since my max RPM is determined by the propeller, I need to monitor the RPM and adjust the throttle so that I do not exceed this design limit. Feedback to the shore based unit will display RPM to the operator.
    
 

    
 
     
    
 

    
 

    So, looking at the AD8420 Instrument Amp, with gain = 1, per the data sheet R1 = open and R2 = short (0W).  The device will be powered from a regulated +5V source (+Vs, -Vs).  
    
 
    The motor is powered with a PWM output from the Motor Controller.  Supply voltage is a 36V Li-Ion battery assembly with Max voltage of 42V and Min voltage of 30V.  Maximum desired voltage to the motor brushes is 32.75 volts.  The voltage divider circuit is a 1:10 ratio. 
    
 
    PWM average voltage range is -32.75 to +32.75 volts (the PWM controller is reversible). 
    
 
    Vin range is -3.275 to +3.275 volts
    
 
    Vout should follow Vin but always be positive (0-5V).
    
 
     The PWM output frequency can be set from 2.93 kHz to 12 MHz, I am considering 20kHz. The motor cannot respond fast enough to the 20kHz pulses, however the op-amps can.  Do I need to add an RC circuit to the inputs (+IN and –IN)?
    
 
    Is this the correct device for an application like the one I am using?  Does it come in a Through-Hole version?  I do not have expertise in soldering surface mount components.