adamreeve / npTDMS

Hey community,

thanks for creating such a helpful library. I use this often to work on tdms-files for voltage measurements.
To read the files, I use this kind of code:


from nptdms import TdmsFile
tdms_file = TdmsFile.read("C:/Users/Desktop/DDMS.tdms")
group = tdms_file.groups() 
channel = group[0].channels()

Q1 = channel[0][:]

When I try to read strain gauge files I get the following error:

Exception: Strain gauge configuration 10188 is not supported

Unfortunately, I could not find any solution so far. Did anyone else had a similar problem before?
Thanks for your help in advance.

Linking related discussion: #266

Currently only full bridge type I is implemented, support for half-bridge type I (10188) requires implementing the computation of strain from the measured voltage.

I added the half-bridge- configuration to the library. It works perfectly fine. Thanks again for the support. def scale(self, data): """ Convert voltage data to strain """ if self.configuration == StrainScaling.FULL_BRIDGE_1: if self.initial_bridge_voltage != 0.0: voltage_diff = data - self.initial_bridge_voltage else: voltage_diff = data return voltage_diff * (-self.gain_adjustment / (self.voltage_excitation * self.gage_factor)) if self.configuration == StrainScaling.HALF_BRIDGE_1: if self.initial_bridge_voltage != 0.0: voltage_diff = data - self.initial_bridge_voltage else: voltage_diff = data return voltage_diff * (((-4*self.gain_adjustment) / ((self.voltage_excitation*self.gage_factor)*((1+self.poisson_ratio)-2*(self.gain_adjustment-1))))*(1+(self.lead_wire_resistance/self.gage_resistance))) raise Exception("Strain gauge configuration %d is not supported" % self.configuration) FULL_BRIDGE_1 = 10183 FULL_BRIDGE_2 = 10184 FULL_BRIDGE_3 = 10185 HALF_BRIDGE_1 = 10188 HALF_BRIDGE_2 = 10189 QUARTER_BRIDGE_1 = 10271 QUARTER_BRIDGE_2 = 10272

Awesome, thanks Tom, I'll make sure to integrate this into the library.

Hi @BockiBock, I've now merged this to master and added the other strain gauge configurations as well. The equation I ended up with is slightly different to yours and more closely matches the output from LabView. The gain_adjustment only multiplies the whole strain value, and I've included the extra 2 (Vo / Vex) (1 - ν) term that makes the equation non-linear.

npTDMS/nptdms/scaling.py

Lines 243 to 249 in b4396c7

    
           # In the half bridge type I configuration: 
        
           # R1 = R2 = R0 
        
           # R3 = R0 (1 + ε ν G) 
        
           # R4 = R0 (1 + ε G) 
        
           # Which gives Vo = [(1 - ε ν G) / (2 + ε G - ε ν G) - 1/2] Vex 
        
           # Rearranging for strain: 
        
           # ε = -4 (Vo / Vex) / [G (1 + ν + 2 (Vo / Vex) (1 - ν))]

	# In the half bridge type I configuration:
	# R1 = R2 = R0
	# R3 = R0 (1 + ε ν G)
	# R4 = R0 (1 + ε G)
	# Which gives Vo = [(1 - ε ν G) / (2 + ε G - ε ν G) - 1/2] Vex
	# Rearranging for strain:
	# ε = -4 (Vo / Vex) / [G (1 + ν + 2 (Vo / Vex) (1 - ν))]

Support half-bridge type I (10188) strain gauge configuration