Hello everyone,
I'm a student currrently working on a project for my university / for my work and I think I need some help and some ideas.
What is the project about?
I have to design a load cell simulator. Imagine you are creating software for an industrial weighing system (like a SIWAREX module for example). Instead of testing your ideas in the field you try to analyze and verify the behaviour in the lab. Currently we are using a manual setup for those tests: You turn a potentiometer knob to change the weight, you press buttons for digital outputs etc.
My task is it to automate those tests. The software part is not the problem, I am currently stuck on the hardware, more precisely on the load cell voltage generation. So the voltage that is proportional to the weight.
Hardware prequesits:
I have to use a Raspberry Pi for the base of the simulator
More importantly: I have to use a second add-on board that generates the analog voltages that have to be converted into the load cell voltage, so those are fix
The task im currently stuck on:
I have to create a circuit that uses the add-on board volatages (0 - 10V analog, 2 channels) to generate the differential load cell voltage. The load cell voltage is dependant on a lot of factors, so i cannot use a simple voltage divider type circuit (i guess at least).
Wheatstone bridge loadcell working priciple:
As you can see: I have to generate a differential voltage between the SIG+ and SIG- lines of the loadcell. The DC voltage of the cell is half the exitation voltage (here EXC=5V). Those voltages are quiete small, and I have to mimic different types of load cells (1mV/V to 4mV/V). With possible EXC voltages from 5V to 10V the maximum differential voltage is 40mV.
What I have done so far:
I designed an opamp circuit to add 0 - 20mV on 5VDC based on the first 0 - 10V channel for the SIG+ line and to substract 0 - 20mV from 5VDC for the SIG- line to generate 0 - 40mV differential voltage between SIG+ and SIG-. You can see the circuit in the next picture.
VGT1 and VGT2 are the 5VDC, which is half of the (here for example 10V) EXC voltage. V1 and V5 are the two 0 - 10V analog channels. The upper part is an adder and an inverter, the lower part ia a subtractor.
The simulation looked promising, I made a PCB and assembled all the parts last week.
Immediately I noticed some problems with my circuit:
1. Noise: There is some noise on the output, however because there is a lot of filtering going on by the SIWAREX module I dont think that is a major problem as of right now
2. Offset: I noticed an extremly high offset of about 4mV when V1 and V5 are 0V. As i found out later, the circuit i designed is highly dependant on precise and stable resistor values. I used only 1% resistors, whis is quiete problematic i guess. However, I think i can get away with it by using a software based calibration and correction. I am working on that right now, but i do not have any results.
3. Temperature: Temperature changes have an extremly high influence on the differential voltage, because as i already mentioned the circuit is very dependant on stable reistors. The RaspberryPi emits a lot of heat causing temperature instability. After about 30min, when the whole device has warmed up, the output differential voltage fluctuates with about +-50uV, i think that is fine for my application
4. "Long" term drift. The output voltage drifts over the term of about 100uV / 30min. I dont think i can fix that problem without temperature controlling the device. Maybe a case would help, and i could do some software temperature correction, but i think i have to rely on the zeroing capabilities of the SIWAREX module.
My "simple" question:
Do you have any ideas on how i can further improve the circuit, to fix the problems i mentioned? Or should i consider redesigning the whole analog circuit?
Thanks in advance for any help / new circuit ideas (btw that was my first analog circuit design, so please be kind
)