Capacitive Sensors FAQ

1. What's the difference between "analog" output and "linear" output?

2. What is the LED Bar Graph on the probe driver telling me?

3. Does it matter into which driver I plug the probes?

4. What effect does the shape of my target have on my measurement?

5. Why are there different types of calibration?

6. What does the sensitivity selector do on the CPL290, DMT22 and ECD22?

7. Can I get different cable lengths?

8. What are the TMP190 and TMP72 Temperature Modules used for?

 

1. What's the difference between "analog" output and "linear" output?

Linear output has consistent sensitivity throughout the measurement range - a 1 micron gap change will produce the same output voltage change at the near end and far end of the measurement range. Analog (nonlinear) outputs have different sensitivity at different points within the measurement range - A 1 micron gap change at the near end of the measurement range will produce a larger output voltage change than it will at the far end. Linear output devices generally have less than 0.1% linearity error. Back to Top

Analog output circuitry does not produce a linear output. Drivers are not calibrated for linearity. They are only adjusted for sensitivity and zero. These adjustments are accessible to the operator and are generally used where the system is calibrated to a master part or condition, and then used to adjust or notify when the tested part or condition exceeds a trip point. High degrees of linearity are not necessary in these types of applications.Back to Top

2. What is the LED Bar Graph on the probe driver telling me?

The LED Bar Graph is internally calibrated to the first stage of the probe amplifier. It is adjusted to indicate where the probe is in terms of its calibrated range, not the driver output voltage.

The LED Bar Graph is unaffected by adjustments to the front panel zero control. Therefore, the LED Bar Graph indicates whether or not the probe is in its calibrated range, independent of the output voltage.

The zero control can adjust the output voltage to produce what appears to be a legitimate reading even when the probe is actually out of range. The LED Bar Graph warns the operator with red lights (or computer when monitoring the Out of Range signals), that the measurement may not be accurate. Back to Top

3. Does it matter into which driver I plug the probes?

Yes. Probes are calibrated to specific drivers. Even if it is the same model probe and driver with the same calibration, accuracy will be affected if the probes are not kept with their calibrated drivers. For this reason, in multi-channel systems probes and drivers are tagged with channel numbers. The calibration report that comes with the system will list the serial numbers of the matched driver and probe. Most systems include calibration stickers which list the serial numbers of the probe and driver that are calibrated together. Back to Top


4. What effect does the shape of my target have on my measurement?

Target shape is a major factor in measurement accuracy. Factory calibrations are done with flat targets. When other shape targets must be measured, custom calibrations can be done to the final target, or multipliers can be determined to correct the measurement. For more detail see the Target Shape section of our Capacitive Sensor Operation TechNote. Back to Top


5. Why are there different types of calibration?

Generally speaking, the resolution of a measurement system is a percentage of range. If range increases, the smallest measurement that can be made (resolution) also increases. Typical resolution values are 1 part in 10,000 or 1 part in 15,000. Different calibrations allow the user to optimize the combination of range and resolution. Back to Top

Available Calibrations:
Not all calibrations are available for all probe/driver combinations.
Extended Maximum measurement range
Standard Standard range
Fine Finer range, higher resolution (~ 10% of standard range)
Ultrafine Ultrafine range, highest resolution (~ 2% of standard range)



6. What does the sensitivity selector do on the CPL290, DMT22 and ECD22?

These drivers contain circuitry for two separate calibrations. This switch selects the calibration to be used. Typically, there are two calibrations for the same probe; one calibration may be a standard calibration (low sensitivity) while the other may be Fine or Ultrafine for maximum resolution (high sensitivity). Back to Top

7. Can I get different cable lengths?

Cable length affects the calibration. The probe must be calibrated with its final cable length. Uncalibrated extension cables would adversely affect the operation. Custom cable lengths can be ordered. At lengths over 10' (3m) the probe drive frequency may have to be reduced, lowering the measurement bandwidth. Back to Top

8. What are the TMP190 and TMP72 Temperature Modules used for?

The TMP190 and TMP72 are used with the Spindle Error Analyzer to monitor machine tool temperature during spindle error motion tests. They require a computer interface to select active sensors and gather the measurements. Back to Top

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