Encoders are essential to accurate positioning in most motion systems, particularly where high precision is most needed. For instance, applications like semiconductor manufacturing with nanometer precision, or medical applications where there is virtually no room for error, because that could mean the difference between life and death.
Designers of medical equipment are able to use a plethora of new technologies. Motion control is an area where designers need to keep up to date with as technology increases in complexity. Medical applications such as robotic surgery, radiation treatment, and laboratory analysis such as DNA sequencing are all benefiting from advances in encoder technology.
Machines that treat patients for tumors with radiation inherently have complicated coordinated motion control. Guiding the radiation beams when the patient is on the table is critical, and any errors in positioning may damage healthy cells. Because of the radiation environment on the machine, communication between encoders and the control is of upmost importance and requires redundancy.
What caught the attention of medical machine designers is a little known ENDAT function called CRC. ENDAT is the high-speed serial interface from HEIDENHAIN that is RS-485 based and offers a fast bi-directional communication protocol. CRC is Cyclic Redundancy Check and is transmitted along with the position words from the encoder to the control. Basically, the CRC reports the number of bits transmitted to the control, and the control can then evaluate the number of bits and make a checksum. If the number of bits match, then everything is operating normally. However if the checksum does not match, then the ENDAT interface reports an error which the control can then handle and take the proper steps needed for reliable motion control.
Machines that are performing large amounts of chemical analysis like drug testing machines or those that sequence DNA are also reliant upon high quality encoder feedback. For drug testing machines, they typically use pipettes and a high number of wells per tray. Smooth reliable motion is critical here as the machines are usually left to operate overnight and sometimes 24 hours a day. When moving the pipette array or trays, a motion that is jerky or somehow unsmooth can destroy data, as liquids can spill from the pipettes into unwanted wells.
Repeatability is also critical. As the machine operates and heats up, the shape of the machine may change. Linear encoders that report the real position versus a rotary encoder/ ball screw set up are more accurate, the latter possibly causing repeatability issues that can even lead to the pipette array missing its targeted wells or causing a crash.
DNA sequencing machines are those that use flowcells, which are a “Lab on Chip” and may contain several billion micro wells. Colors need to be detected from these tiny wells and the motion system with cameras is designed for sub-micron repeatability. In these fine resolution applications, the interpolation error of the encoder (or sub-divisional error) is critically important. When the encoder signal periods are electronically subdivided to gain more resolution, errors can happen within that signal period and then eventually lead to errors in data collection from the camera. One encoder, the HEIDENHAIN LIP 200, has interpolation error at +/- 1 nanometer. This kind of encoder helps to ensure the stability of the image required to evaluate the colors of the wells on the flowcell.
Surgery robots that are surgeon-assist based are becoming more accepted in the medical world. In fact, over 80% of prostate surgeries done in the U.S. today are done with robotic assist surgery robots. Some surgery robots need to have redundancy for the motion control components, while others can drill and measure distances at the same time, making accuracy especially important. Here again, a reliable high-speed serial interface such as ENDAT as well as a full line of rotary and linear encoders that are Functionally Safe with various mechanical options can fulfill the need for these applications.
Filed Under: Motion Control Tips, Encoders • linear, Encoders (rotary) + resolvers
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