This issues articles: Graphene • Valve Control • Applying Analog Outputs • Motion Control of Treatment Chair • Feature video: Programming sensors for robotics • To view newsletter as webpage click here.

The Amazing Material Called Graphene

A mere summary follows. It’s a sheet of carbon atoms, that when isolated as a single atomic layer, has “mind blowing properties”. Most mechanically strong (100-300 times stronger than steel), lightest, most conductive (heat at room temp. and electricity), most transparent, 1st 2D material, impermeable, most elastic (20%) and stiffest as well as magnetizable. According to Prof. James Tour of Rice University, “it’s a sheet of atoms you can pick up!” For comparison, a sheet of aluminum foil is about 140,000 atoms thick. Potential or marketed uses: super capacitors, flexible displays, conductive ink, super solar cells and sensors.


Editor's note: this section will be covering output interface selection and applications of the most frequently used interfaces in coming newsletters, starting with analog in this issue.

Analog interfaces are an obvious choice for design engineers needing to connect a position or angle sensor with a pre-selected control device that accepts standard current and/or voltage inputs. These sensors are generally available with one or more of three analog outputs: percentage of supply, defined voltage and defined current outputs.

A voltage output option that is a percentage of supply voltage, say 10 to 90% of a 10 V supply would result in a sensor output range 1.0 to 9.0 V full range.

An example of a defined voltage range output, independent of a 24 V supply, could be stated as a 0 to 10 V output range representing 0 to full scale of stroke length (or angular range). The actual usable range must be a small minimum voltage e.g. 0.2 V, making the actual output range 0.2 to 10 V. This output is sometimes available with a decreasing value representing an increasing value in stroke or angle. In that case, +10 to 0 V could represent, for example, 0 to +100 mm change in stroke position.

There are generally two defined current output ranges. These are 0 to 20 mA and 4 to 20 mA. These current values represent a minimum to maximum stroke range or angular range.

Voltage output sensors are easy to implement as most PLCs have modules accepting a voltage input. Sensors with current outputs are used for longer cable runs and where electrical noise is present in the environment, as current is inherently unaffected by noise and its level is sustainable over a long cable run with a sufficiently sized conductor wire gauge.

Sensor current outputs of 4 to 20 mA have the additional benefit of making it possible to detect a broken or disconnected wire condition because the output would drop to 0 mA. Since this value is outside of the operating output range, it would indicate a fault condition.

Medical Treatment Chair and Table Positioning

To help the care giver provide the best treatment, healthcare providers often use a chair or table that optimally positions the patient. In medical practices many specialists need to perform procedures on patients to either help diagnose or treat a medical condition.

Moving a seat back or foot rest helps the physician move the patient to be in the best possible position for a procedure on a certain part of their anatomy.

Midmark designs and manufactures procedure tables and chairs that combine mechanical positioning mechanisms, motors, microprocessor-based electronic controls and sensors, and to move the sections. Knowing the position of the table or chair sections is critical. By knowing where the sections are positioned, Midmark tables and chairs become “smart tables” through factory programming to stop 2 or more inches off the floor, depending on the model, so they don’t touch the floor or run into a doctor’s foot.

Rotary position sensors selected for use a Midmark procedure table as well as several other power procedure chairs models had several requirements: ease of mounting, very long sensor life, resolution of <0.1°, customer support and repeatability of 0.04% that allowed Midmark to hold to a repeatability tolerance of ±0.5 degrees for each moving section of their tables.

Midmark’s control system for each of these tables and chairs monitors multiple rotary sensors – one on each moving axis the table moves about: table base vertical, seat tilt, foot tilt, and for tables, back section tilt . They are read with a rate defined through software to be able to precisely control moving the base, back and foot sections up or down as well as tilting the tables.

The rotary position sensors along with the control system allow medical practitioners to set up to four preset positions stored into memory and recalled at the push of a button and be able to precisely return to those same positions each time. This saves time when doing the same procedure on different patients. If there is a need to adjust a table from a preset position for a patient, they can be manually adjusted as well. Obviously, a lot of thought and design effort went into these products and they find daily use by medical practitioners.