A Lab automation Overview#
Lab automation has become essential to perform complex laboratory experiments like for example high capacity fiber optics transmission experiments, where dozen of instruments have to be continuously adjusted during the measurements, that can take many hours to complete. Additionally, lab automation also helps freeing up operators from repetitive measurements, and can provide more consistent results by avoiding distractions that human operators are subject too. Finally, most modern measurement instruments provide ways to be controlled remotely, making lab automation even more compelling.
What instruments can we automate#
Essentially every instrument that has some computer interface can be automated and even instruments that do not have a computer interface can be automated if necessary, by e.g. using a stepper-motor that is controlled from a microcontroller to turn a control button. Similarly an instrument that can only be controlled via a proprietary graphical user interface can if necessary be controlled by programmatically controlling mouse movements. However, in general it is much easier if your instruments have a standard way of connecting to a computer. We highly recommend that the ability of accessing the instrument from a computer and an application programming interface (API) that is well documented should be important parameters for purchasing decisions.
There are many different interfaces and protocols that are used for connecting to instruments. The naming and use of names is often confusing as interfaces and protocols are often used interchangeably and devices are often compatible with several systems.
Serial#
The RS-232 standard, which is commonly referred to as serial connection, is a standard that was originally introduced in the 1960s. It is still commonly encountered in laboratory instruments and being used for connecting to microcontrollers. Most modern PCs do not have a easily accessible RS-232 ports and instead connections are made via USB-to-serial adapters. On the device side the connection is most commonly via the 9-pin DE-9 connector seen in DE-9 RS-232 connector (plug) source: wikipedia
The communication protocol can take many different forms, some instruments take SCPI commands, while others use home-grown protocols. With Python the PySerial module gives programmatic access to the serial bus. VISA-compliant devices can also addressed via PyVISA.
GPIB#
The GPIB (General Purpose Interface Bus) connector (see figure GPIP connector source: wikipedia is probably still the most commonly encountered connector on lab instruments. Originally developed in the 1960s it is still commonly found on many instruments such as oscilloscopes, optical spectrum analyzers or optical power meters. Technically, the hardware connector is defined via the IEEE-488.1 which also defines the basic protocol parameters. The later IEEE-488.2 standard further introduced common commands and protocols (however it does not technically require GPIB hardware connectors). The SCPI standard further extends IEEE-488 with instrument class specific control commands. The most common way to control GPIB instruments in Python is using PyVISA, however you can also use PyGPIB to talk with GPIB devices. Note that your system will need to have the appropriate drivers installed, in particular USB-to-GPIB adaptors often require specific drivers.
USB#
The USB test and measurement class (USBTMC) is a subclass of the USB interface for test and measurement instruments. It essentially specifies instruments that communicate using IEEE-488 messages over USB. Many more recent test and measurement instruments such as oscilloscopes, spectrum analyzers etc. will communicate over USBTMC. Commands and syntax are typically of the same format as GPIB instruments and often use SCPI type commands. In Python access to USBTMC instruments is most easily done via PyVISA or alternatively using the python-usbtmc module. The advantage of the python-usbtmc module over PyVISA is that it does no visa library is required.
Ethernet and VXI-11 and HiSLIP#
Ethernet connections to test and measurement equipment most commonly uses VXI-11 and HiSLIP protocols. They are specifications for communicating with instruments using the IEEE-488.2 command syntax over an ethernet/TCPIP connection. HiSLIP is the successor format to VXI-11 and primarily offers faster transfers via an asynchronous mode and shared and exclusive instrument locking. If available HiSLIP is always the better choice (see the PyVISA chapter) for a more detailed discussion of how to select between the protocols). Typically access to instruments would be via a VISA library and PyVISA in Python. There exists also a python implementation of the VXI-11 protocol in the python-vxi11 module.
SCPI#
The Standard Commands for Programmable Instruments (SCPI) standard is not a protocol or interface, but instead a standard for syntax and commands on top of the IEE-488 standard. It is commonly used by many test and measurement instruments from all the main manufacturers, independent of communication interface (USB, TCPIP, GPIB…).
Command syntax#
Commands are ASCII strings and are grouped in a tree structure, where different hierarchies are separated by a column. For example a measurement of the AC voltage would take the form of MEASure:VOLTage:AC?
while a measurement of the DC current would be MEASure:CURRent:DC?. Where queries (asking for a value) are denoted by a question mark ? and set commands contain the value to be set, without question mark.
Commands are typically specified in upper-lower case format, however commands are case-insensitive and lower-case simply specifies that these letters can be omitted.
VISA#
The Virtual Instrument Software Architecture (VISA) specification was defined in the mid 90s as an API for communication between test & measurement instruments and the computer. It is an industry standard implemented by several T&M companies including Keysight, Tektronix, National Instruments … VISA includes specifications for communicating with resources (mostly instruments) via I/O interfaces such as serial, GPIB, VXI, TCPIP (VXI-11), USBTMC and HiSLIP The idea behind VISA is to achieve interface independence, i.e. it does not matter how the instrument is connected. Thus it does not matter how an ASCII command string is send it is the same on each interface.
In Python VISA is supported by the PyVISA module which provides a “pythonic” interface to VISA commands. Importantly PyVISA does not implement the VISA protocol itself, but provides a wrapper around a VISA library installed on the system, such as the NI-VISA libaries, the tektronics VISA library, or the Keysights IO libraries. Additionally, there is also the PyVISA-py lightweight pure python VISA implementation available, that is particularly useful for linux based computers or smaller computers like for example on a raspberry pi or similar. PyVISA is discussed in more detail in the Instrument control with PyVISA chapter.
Other#
Some instruments might use other interfaces and protocols than the ones mentioned above. It is particularly common that some instruments only provide access to their
instrument via a vendor supplied dll or library. Accessing these devices is covered in the advanced short-course.
General Recommendation#
The best way to connect to your instruments can often depend on a number of factors. Nevertheless, there are some general recommendations when selecting new instruments:
Choose instruments with standard interfaces that are publicly documented
Choose instruments based on publicly documented protocols
Avoid instruments that require custom vendor supplied libraries, as they are often tied to a particular operating system version that can quickly become obsolete
Make sure that all functions needed for you experiments can be accessed by the supported remote control commands
VISA libraries offer a simple way to connect via several different interfaces by using PyVISA. If there is a good VISA library available for your computer and you use devices connected via several different interfaces, it is often easiest to use PyVISA and VISA libraries. The operating system support in VISA libraries is however somewhat limited, especially if you are using linux or OSX as operating system, where VISA libraries are often either unavailable or difficult to install. In that case PyVISA-py, the pure python VISA implementation is preferred, and often works quite well, but fails when no device drivers are available. This is the case for example for some of the USB-to-GPIB and GPIB-ethernet adapters (please check before acquiring new adapters). Additionally, if needed, instruments can be accessed without VISA by using protocol specific modules (pyserial, python-usbtmc). However, this will require learning and using multiple protocol specific modules instead of just a single unified interface.