How Cable Extension Transducers are used in patient positioning equipment
The Cable Extension Transducer (CET), sometimes referred to as a draw-wire, string-pot or cable actuated position sensor offers a compact, straightforward, and economic solution where medium- to long-stroke linear displacement measurement is required.
They are useful for space restricted installations and where the measurement component cannot be perfectly aligned parallel to the positioning axis. Although CETs are often associated with particularly long travel and heavy industrial linear measurement applications (sometimes more than 40m for areas such as crane and hoist, large-scale automation machinery, commercial and construction vehicles), smaller measuring devices with one or two metres displacement are extensively and increasingly being used on medical equipment applications. With the need for precise and repeatable patient treatment positioning, and with the ability to set up equipment quickly and efficiently for uncomplicated patient turnaround, CETs are gaining ground in the industry for beds, chairs, treatment tables and scanners used for MRI, oncology and a range of other clinical procedures and treatments.
The CET principle comprises a cable-driven and spring-loaded winding spool connected to a rotary sensor, all housed in a protected casing. With the cable fixed between the spool and the measured load, as the cable is extended or retracted the rotary sensor outputs a signal that is proportional to the cable’s travelled linear distance. Thanks to its misalignment tolerance the CET housing does not require precise positioning regarding the origin point of the measurement, and the thin diameter cable needs only to be secured to the moving load at its furthest point. For patient treatment equipment, CETs for each positioning axis may be easily concealed for maximum design aesthetics.
With a choice of stainless-steel, aluminium or polycarbonate housings, coated stainless-steel cables and optional IP67/68 protection, these devices also have high shock, vibration, and temperature resilience gained through well proven use in harsh industrial measurement tasks. Furthermore, a choice of voltage, current, digital, and even CANopen bus outputs offer maximum installation flexibility for the customer’s equipment. This makes the technology ideal for applications where LVDT, linear potentiometers, linear encoders and other medium travel linear measurement devices are too bulky or cannot offer the durability that the CET delivers.
A CET variant, widely used for medical treatment platform positioning and available with full support in the UK from Variohm EuroSensor, is the SG series from TE Sensors. The sensor has up to a 2-metre measurement range in a compact 75 mm x 75 m x 50 mm polycarbonate housing and is available in a choice of 0 to 10V DC, 4 20 mA. incremental encoder and CANopen or CAN J1939 outputs. With accuracy rated up to 0.35% of its measurement range the SG series is available with IP67 protection and is powered from a standard 30 VDC supply. A supplied mounting bracket allows vertical or horizontal installation. For more space restricted installations and for measurement lengths up to 1.27 metres, TE Sensors’ SP3 series has a 25 mm wide x 50 mm deep x 90 mm high envelope and a choice of 4-20 mA, 0-10 VDC and incremental encoder outputs.
Variohm EuroSensor, a member of The Variohm Group, supplies alternative technology linear position sensors as well as single and multiturn rotary encoders. Other components from the sensor specialist, used as part of medical equipment controls include inclinometers, load cells and temperature sensors.