1 Analog proximity sensors and magnetic position sensors in metalworking machineries

May 27, 2015

In metalworking industries, one of the most automated equipment family are the machine tools and metal forming machineries. Machines tools fabricate parts by grinding and cutting, while metal forming machines form parts by forging, pressing, punching, bending and shearing. Many types of sensors, such as ultrasonic sensors, vibration sensors, rotary encoders, photoelectric sensors, are deployed to manage, position, and control the automated tasks. One of the most widely used are the contactless inductive proximity and the magnetic cylinder sensors, whose key roles are to check motion limits, detect positions, feedback angular movements etc. 

This whitepaper discussed the possible applications of analog proximity sensors and magnetic position sensors in metalworking machineries

1.1 Analog Proximity Sensors simplifies machine control

Machine tools and metal forming machineries have rotary tasks in their automation processes. Most of the time, ro-tary encoders are used to decode information on the angular displacement. Sometime, several proximity sensors can be utilise in binary technique, e.g. 00, 01, 10, 11, to decode rotary shaft information if the application is not too concern on the precision. One other optimum way is to use an analog inductive sensors  to capture  higher precision angular position sensing and measurement.

 

The major advantage of using analog inductive sensor are its inherent abilities to provide a variable analog output voltage or current proportional to the distance of the target from sensor face. As this sensor is contactless, no actual mechanical coupling between the sensor and the rotary shaft is needed. This is very useful to engineers who design machine tools that demand precision position sensing, without the worries of mechanical coupling, or any long term degradation issues.

 

Typical applications are like detecting angular position of a machine tool shaft in a metalworking CNC machine. Traditionally, designers used the expensive, bulkier rotary encoder. Alternatively, engineers can machined a eccentric shape CAM, e.g. elliptical shape, to provide a variable distance between sensor face and the CAM faces at different angular position.

 

Analog proximity sensor for angular position measurement

 

Another advantage is that this compact analog inductive sensor is housed in diameter as small as 12, which is few time tinier than the rotary encoder. Besides, no mechanical coupling between rotary shaft and the physical sensor, is required. As such, the sensor’s compactness and mechanical independence mean straightforward implementation and easy replacement. All these benefits will groom the analog proximity sensor as an attractive candidate for rotary position feedback. 

 

Nevertheless, as with any data conversion, designers must also compensate for non-linearity of the output. Linearity error can caused a higher reading is expected in quadrant 2 (see graph above), and compromise the automation task accuracies. Consequently, designers should compensate the non-linearity error in their controller, by coding a simple lookup table to linearise the output across the whole dynamic output range. If lookup table is not desirable due to system resources constraint, then designers have to consider using only the most linear response curve of the entire dynamic range, such as those in quadrant 1 of the response curve shown below.

 

Other applications for analog proximity sensor can measuring the rate of product flow in any metalworking automation process. In the illustration below, the analog sensor is used to output analog data according to the “curvature” of the metal bodies in real-time, and the rate of changes on the output can be computed into accurate rate of product flow.

 

 

1.2 Magnetic positioning sensor increase productivity 

In some metal forming automation task, piston cylinders equipped with magnetic position sensors are gaining popularity. Unlike cylinders using standard magnetic sensor that detects only ON-OFF status position, the magnetic position sensor can provide provides information on the exact piston position along the complete cylinder stroke. As manufacturing processes get more demanding, ON-OFF status is no longer adequate. The system needs continuous feedback of pneumatic piston position for this sophisticated tasks realisation. 

 

Traditionally, designers use various types of linear measurement sensors. These sensors are mechanical parts that follow, and measure directly the piston stroke. Firstly, ingress protection (IP) can become an issue with the dis-placement sensors. Besides, as linear measurement sensors are often designed for a vast number of applications, they need special mounting brackets. Most important, linear sensors are mechanical moving parts which are sub-jected to operating wear and tear. 

 

Using magnetic position sensor in measuring piston linear displacement is gaining popularity due to the accuracy, simplicity, robustness and cost effectiveness. Robustness-wise, the MPS involves no moving parts, and therefore not subjected to mechanical wear and tear. Simplicity-wise, the MPS is designed to be mounted on T-slots cylin-ders. Other notable features are that the MPS are rated IP67, which means that it can be used in harsher environ-ment within the electronic industries.

 

Technology-wise, the MPS discussed here is an advanced microprocessor based sensors for pneumatic cylinders, electrical cylinders, grippers and slides. Inside the sensor housed a row of Hall-effect sensors which “monitors” the cylinder’s piston. The output is both analogue voltage output of 0…10V, and analogue current output of 4…20mA.

 

Application-wise, the MPS is able to measure the cylinder stroke ranging from 32mm to 256mm. It is can be mount-ed on the popular T-slot cylinders, and no additional adaptors are required. For other slots, mounting accessories are available. Site commissioning is straightforward as indicative LEDs are guiding during installation. The optional teaching functions are available to designers to calibrate the required operating range.

 

 

Applications for MPS are increasing. Some common task of magnetic position sensor system includes orientation and positioning, assembly, inspection. More and more automation task, like precision depth-controlled drilling ma-chineries, metal punch in metalworking, are starting to switch to magnetic position sensors.

Precision drilling machineries for metalworking

 

In conclusion

With increasing numbers of tasks in the metalworking industry demanding more rapid and precise inputs, both the analog proximity sensors and the magnetic position sensor are set to simplify and enable more sophisticated tasks at reduced cost. With analog proximity sensors, precise angular information can be extracted without the needs for encoder based design, thus simplifying machine tools design. Likewise, using the magnetic position sensor metal-working automation task like drilling, punching e.t.c, can reap the simpler and yet reduce cost benefits. 

 

 

Author :

Mr Chris Chan

Product Manager

Business Unit Sensors & Connectivity

Presence Detection