1.1 Introduction
Proximity sensors are the eyes and ears in factory automation processes. They can be photoelectric type, inductive type, magnetic type or capacitive type. Of all, inductive proximity sensors are one of the most popular choices among designers. They come in many shapes and sizes to fit different applications, and are distinguished by a long operating life and extreme robustness in harsh industrial environment.
Inductive proximity sensors are non-contactless switch triggered by the presence of metal object. The output signal is mostly ON-OFF type, connected to digital inputs of either PLC (programmable logic controller) or the distributed input-output (IO) units. The inductive proximity sensor is used either to (a) detect unidirectional motion (left picture), or (b) detect rotational motion (right picture).
In automation task, the PLC will poll the IO or sensor periodically. For unidirectional motion detection, an automated task will trigger mechanical movement of the metal target towards the sensor, and activate this sensor. For rotational motion detection, the inductive sensor generates ON-OFF pulses due to the rotating movement of the target object. The PLC counts the pulses to determine the speed and acceleration data, and decide on the next automation task. Rotational motion detection normally requires more PLC processing bandwidth.
Typically there can be several hundred or several thousands of inductive sensors in factory automation line. Each sensor may demand bandwidth from PLC. Hence, the PLC has to have enough MIPS to liaise with all equipment including sensors at the same time. Unfortunately, as automation line gets more advance where more equipment are demanding the PLC bandwidth, and it may not be able to cope.
1.2 Demands for intelligent sensor
In many factory automation processes, such as steel cable stranding machines, textile weaving, certain conveyor systems e.t.c, the work-piece quality is highly dependent on shaft rotary speed and acceleration. Any small deviation can result affect end product quality which result in higher scraps. For tighter control, each motor on the machine needs a sensor to capture rotational pulses information, and feed them to PLC.
Classical method is to use a simple inductive ON-OFF proximity sensor to measure the rotation of the toothed flywheel. The angular speed, acceleration-deceleration computation is dedicated to the PLC. The PLC “MIPS” has to be high enough to compute all the data simultaneously, and control the process tightly. Unfortunately this classic method hogs a lot of PLC resources. A typical steel cable stranding machine uses around forty motors and therefore requires forty inductive sensors. The PLC has to poll all the sensor and analyse the total number of pulses to determine the angular parameters. This is taxing to the PLC, which may not be fast enough to handle such tasks. PC-based controller may have to be use instead. Alternatively, IO modules such as programmable counters attached to the inductive sensor, can be used to offload the controller, but this can result in escalating system cost.
A more intelligent sensor is required, whereby the PLC can dedicate the task of computing the angular speed-acceleration data to this intelligent sensor. It can trigger the PLC when some abnormal conditions are detected.
1.3 Introducing SAM
The Speed-Acceleration Monitor (SAM) inductive sensor is designed to simplify the automation process, by offloading the tedious task of computing the speed and acceleration data from the PLC. Briefly, the SAM is a 4-pin inductive sensor, housed in cylindrical threaded housing. Two housing type are available, the Ф18 type which has sensing range of 7mm, and the Ф30 type whose sensing range is 10mm. SAM output is a simple discrete ON-OFF signal like any other inductive.
The SAM can be programmed using an IO-Link-enabled programming box and a Laptop-based user interface software. The SAM is connected to the programming box as shown in the diagram. (IO-Link is an input-output technology worldwide (IEC 61131-9) for the communication with sensors. Further information can be found at http://www.io-link.com). On the user-interface program, designers can setup a few threshold values to monitor. Once the SAM is setup, it is ready to go, where they can operate either as a stand-alone sensor, or operate as one unit in a IO-link environment.
Application wise, it is straightforward. One SAM inductive is mounted on each motor. Instead of transferring every single pulse to the PLC, the computation of the speed is directly done in the sensor itself. The SAM will output a single ON-OFF signal to the PLC once the motor speed or acceleration gets above the pre-set threshold values. This reduces the traffic and computing time of a PLC dramatically therefore is a real cost saver.
1.4 How it works?
The SAM inductive offers two main functions: (a) Speed monitoring, and (b) acceleration monitoring. In the speed monitoring mode, the SAM checks if the speed of the rotating target reached the upper threshold, and activate the output if this is true. The upper threshold and a lower threshold can be set from 6 pulses to 12,000 pulses per min.
Application-wise, the upper threshold can emulate the MUST_NOT_EXCEED speed of the motor, while the lower threshold as the desire operating speed.
In acceleration monitoring mode, the SAM can check for a deceleration of the system between the range of 0,1…2 pulses/sec². At constant rotating speed, there is no change in acceleration and output remains. If deceleration is detected, the output is toggled immediately.
Application-wise, the acceleration monitoring mode is perfect for monitoring any machines with regular speed changes e.g. due format changes in the production process. When using standard speed monitors, the designers have to adjust the thresholds in accordance with the speed after every format change, in order to detect motor breakdowns immediately. With the SAMs acceleration monitoring mode, this is not needed anymore. The acceleration is independent of the absolute speed value. By monitoring the acceleration ON-OFF data from SAM, the designer can check for motor breakdown in the automation process.
1.5 SAM Benefits
1.5.1 Simpler Process and lower cost of system
SAM inductive can simplify the whole automation process design. The example below shows an automated process with many cascading motors’ whose rotation speed and acceleration are coordinated closely to ensure a smooth process flow. Should one motor breakdown, while all the other motors are still running, crumpling of the work-piece can happen. To safeguard this, SAM can be used to monitor speed and acceleration characteristic of mission critical motors, and triggers an ON-OFF command to PLC who then executes recovery sequences.
With SAM inductives, the system does not need high-end PLC or PC-based controllers. The result is a simplified control system which is cheaper now.
Lastly, SAM inductive shortens commissioning time and effort. During process commissioning, the engineers need to teach the system when and how to react. When using classic method, the engineers need to measure the system characteristics. Next, they need to write comparison algorithm and lookup table for the controller.
With SAM, commissioning the system is much simpler. The engineers just setup the SAM’s threshold trigger via IO-Link. No physical information is required on shaft speed and acceleration. The SAM will process all data and interrupt the system controller should the actual shaft speed or acceleration falls out of SAM pre-set threshold. This saves a lot of commissioning time and resources.
1.6 Conclusion
The new Speed-Acceleration Monitoring (SAM) inductive sensor is designed to simplify factory automation process. With SAM inductive, the bandwidth consuming task of computing angular speed and acceleration data can be offloaded by system PLC to this sensor. This results in a simpler control system, which drives cost down. In addition, system commission and calibration is also made easier.
SICK is one of the world's leading manufacturers of sensors and sensor solutions for industrial applications. Founded in 1946 by Dr.-Ing. e. h. Erwin Sick, the company is headquartered in the German town of Waldkirch, in the Breisgau region near the city of Freiburg. It is a technology and market leader, maintaining a global presence with more than 50 subsidiaries and equity investments as well as numerous representative offices. In the 2014 fiscal year, SICK had around 7,000 employees worldwide and generated Group revenues of €1,099.8 million.
