PICOLOG MATH SERVO APPLICATIONS – HIGH PRECISION PROGRAMMABLE LABORATORY VOLTAGE REFERENCE
Picolog software in conjunction with a Data Logger can implement very useful Servo functions, once the potential of the software is understood and suitable interfaces are used.
Servo operation is possible because Pico Data Loggers have one or more alarm outputs that can actuate external hardware. Unfortunately servo applications are severely limited because the outputs sense the input channels alarms only: if the alarms of the calculated parameters could also be sensed, then the number of possible Servo configurations would be virtually infinite.
In this post we shall describe a Programmable Mathematical Servo which can supply any reference voltage with high precision and stability. This source can be very handy in any laboratory when assessing equipment or prototypes. I call it Math Servo because the value of its output voltage depends on data written into two Picolog Channels. One can prepare a few Picolog files, one for each useful voltage and once the DA is opened with that file, the corresponding precision reference voltage will be available.
The Servo can output any voltage with an absolute precision, once the DA calibration is checked, of better than 1 mV and no measurable temperature drift. The only limitations are:
- Minimum output level: 0.6 V. This limitation is caused by bipolar transistor saturation voltages and Operational Buffers. To go nearer to zero volt output the bipolars must be replaced with Mosfets and the Op.Amps. supplied with a negative bus as well.
- Above 2.5 V output a voltage divider must be placed ahead of the DA inputs to observe the input voltage limit of the device.
- Up to 4 V output the USB Bus + 5V can be used. Above this value an external PSU outputting higher voltage must be used.
Looking at the Blocks Diagram, capacitor C3 stores a charge at the required voltage level. If the voltage is too low, switch Q2 (LOW END) will be turned on. Once the required level is reached, Q2 will be turned off. If voltage across C3 is too high, switch Q4 will be turned on, bleeding part of the excess charge to ground. Once the required level is reached, Q4 will be turned off. The switching thresholds are set by Picolog and they can be as close as less than 1 mV, hence we have an output voltage which depends only on math levels written in the Picolog software.
In Part 2 the Servo details will be described and PLW files given.
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