“If you can’t measure it, you can’t control it”. The importance of measurement in Motion Control

Feeding raw materials, processing the material into a finished product, labelling, then packing the final product involves motion at every stage. Many of these processes have sub-processes that, in turn also rely on motion. By definition the distribution of a final product involves motion, but before a product is loaded into a lorry, train or plane, often automated warehouses load the product into storage bays for temporary holding before being picked for dispatch. It is fair to say then that at the heart of almost every production and distribution process is motion control.

“If you can’t measure it, you can’t control it”. This phrase is used in many arena's but in general society, it is probably most associated with the Management of people or processes. In the sphere of automation, measurement is an important part of a closed-loop control system. Measurement devices provide the intelligence to the control system, enabling decisions to constantly be made to ensure the correct functioning of the system. As a result, Sensor Intelligence is central to the optimum functioning of a closed-loop control system.

Fig1: Typical closed-loop control system

When it comes to Motion Control, parameters such as angle, position, speed and acceleration are key measurement values which need to be fed back for the reliable functioning of closed-loop motion control.

There are different types of sensor available to provide this sensor intelligence, but the most popular one is a rotary encoder. Rotary encoders are fitted to rotating shafts on machines, sometimes directly to an electric motor driving the machine, but often onto other parts of the mechanics of the machine. Their task is to provide electrical signals in relation to speed position, angle and distance.

Pic 1: SICK DBS36, DBS50, DBS60 Rotary Encoders

Different types of encoder

There are two main types of encoder.

First is the incremental encoder. An incremental encoder can be considered a pulse generator. As the encoder shaft is rotated it provides square wave electrical pulses at the output. Input cards on Drive Controllers or Programmable Logic Controllers (PLCs) monitor either the frequency of the pulses or count them. Monitoring the frequency of the pulses provides real-time tracking of speed and acceleration. By counting the pulses, the relative position can be tracked and distance measured. Input cards on controllers are designed to take either 5V signals (TTL) or 24V signals (HTL). It is important that the encoder selected has the appropriate signal for the input card. The number of pulses per revolution an encoder produces defines how precise the measurement is. The higher the number of pulses, the more accurate the encoder. In recent years the development of programmable incremental encoders means that there are options where a customer can tailor the electrical output and resolution of the encoder on site. An example of this is the SICK DFS60 encoder.

The second type of encoder is the absolute encoder. The absolute encoder can be considered a code generator. This type of encoder produces a unique digital code for each position of the shaft. These digital codes are transmitted at high speed back to the control system, normally a PLC, where the codes are interpreted as positional or angular values. The absolute encoder is a more sophisticated type of encoder but the big benefit is that it can immediately identify its position directly after a power cycle. The unique digital codes enable the control system to initialise in a few seconds and then the machine can start running. By contrast, an incremental encoder can only track relative position and therefore require a so-called initialisation run to initialise the absolute positional value. Absolute encoders are separated into single turn and multi-turn models. A single turn absolute encoder only produces angular information. After one complete rotation, the sequence of codes starts again. A multi-turn absolute encoder provides angular information but also provides unique codes over a number of rotations so that it can tell the control system where it is positioned in one turn but also which turn it is operating in. Single turn absolute encoders are normally used on rotary axes like turntables or Crankshafts whereas multi-turn encoders are normally used in applications for linear measurement such as winches or tracked vehicles. The way the PLC is set up to interpret digital codes from field devices will determine the interface type of the encoder. Examples include SSI, Profibus, Profinet, Ethernet IP and more recently IO-Link.

Mechanical connection

Before the operation, a rotary encoder needs to be mechanically attached to the source of the motion. To facilitate this there is a wide range of mechanical solutions available. These include so-called solid shaft encoders which are often applied using flexible couplings or measuring wheels. Hollow shaft encoders are designed to fit directly onto an existing shaft. Wire draw mechanisms are used to directly convert the linear motion of a machine into rotary motion of the encoder.

Over the past 12 months, the team at SICK Sensor Intelligence have worked closely with the product management team at RS to extend the available solutions of Motion Control Sensors to RS customers. The solutions detailed above can be selected based on key attributes via the following link.

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