PnP Sensor+Actor System (PnPSA)

State of the art

While working and measuring with sensors in university's tool machine area, I miss development towards digital measurement and handy, interchangeable and easy-to-use devices. Even in 2012, the measurement technician has to deal with analog multi-channel data acquisition systems. Frequently, analog voltage (± 10 V) is used for representation of the measured value, besides those listed in this document.

Here is a summary:

Why it's so hard to move toward complete digitalization:

Today's scenario

Measurement is performed with a confusing amount of cables, connectors, adapters, tools and equipment, not to mention the weird software.

Suppose you want to measure a mechanical force with a strain-gauge load cell, you need a couple of devices as shown in the picture. For simplicity and cost-efficiency, I would use a NI USB DAQ device „NI USB-6008“ containing the ADS7870 chip. Despite the low performance of 12 bit accuracy and 10 kSa/s overall sample rate, this device is USB bus powered and therefore easy to use. With a self-made signal conditioner containing a DC/DC converter to get power from „NI USB-6008“ device, no extra power supply is needed, and the overall design is capable for in-field measurements. However, some efford is needed to get this, and you have to calibrate all! Too much to do for a routine job.

Moreover, a software must be built using LabVIEW (that's easy) and calibrated (that's a bit more complicated).
Tools
  • Screwdriver to connect wires
  • Multimeter to check voltages and connections, for troubleshooting
  • A mass unit to check whether the force sensor works properly

Every modern human outside who wants to measure a mechanical force thinks that such constellation is easily available and ready to use. Power is sorced from the (even more lightweight) netbook.

This is not available nowadays! But it should be. And is expected by non-professionals.

Suppose you want to measure mechanical force (strain-gauge load cell) and temperature (Pt100). For a cartoon picture too confusing, I decided to draw a block diagram for the devices needed. So suppose you have a „NI USB-6008“ for analog-digital conversion. You need two different signal conditioners, for industrial application always with its own power supply and calibration issues.

Probably a better, faster A/D converter is necessary; at least all the National Instruments devices need extra power too.

With a suitable and modular system like HBM Spider8, MGCplus, QuantumX or NI cDAQ-9172, it's easier to handle, but still you have to solder some wires and plugs, and the overall cost is overwhelming. Extra Power is also needed, and as all devices are made for mains supply, the power consumption is not suited for in-field battery powering. No one of those systems are powerable by a notebook.

Things get worse when you already have a “smart” sensor with digital output, like Micro-Epsilon µε optoNCDT2200. This Laser Triangulation Sensor outputs distances (± 10 mm) with 15 bit resolution (< 1 µm) at pretty fast 10 kSa/s. A PC built-in serial interface with its maximum 115.2 kBaud is too slow. So you need a special adapter for connecting. For USB as Do-It-Yourself device to keep costs in bounds.

The next problem is to prevent drifting of another A/D converter, i.e. synchronization. You need an extra Sync Wire. Really! But this imposes three assumptions: The second sensor (here a strain-gauge load cell) works as before, with signal conditioner. Moreover, some sort of a start trigger is needed (depending on latency and demands), so you may need another sync cable, not shown in the picture.

If you have two “smart” sensors with no synchronization between, you must do it later by resampling in software if necessary. Note that resampling is not measuring but interpolating values where no values were. Same applies for AC measurements (e.g. sound pressure, modal analysis) per sound card.

A digital solution is needed!

Thow away analog interfacing! Take advantages of a fully digitized solution!

I would prefer using USB. Advantages:

Why USB and not FireWire, Ethernet/EtherCat, Fieldbus or CAN should be now clear.

Disadvantages and their workarounds

Already invented?

Yes, there are some fully-digitized, PnP-able solutions, but every vendor makes his own, and it's never interchangeable:

Hurdles

Technicians don't use USB

To be translated

Foresee (what to implement)

Target

Plug-And-Play eben!

Implementation

Preparations

Im Vorfeld sollte folgendes abgeklopft werden:

A roadmap

A tree view to PnPSA

Why I and here?

You may wonder why this project should be part of tool-machine profession. But a young electronics engineeer cannot see the problems within machine measurement …

My references: