Continuous measuring equipment

Continuous measuring equipment is a specialized tool used to precisely measure linear or angular position, displacement, and sometimes speed and other parameters in a mechatronics system. These measuring tools continuously provide real-time position information of a moving object in the form of an electrical signal to the controller. Measuring equipment can be classified into two primary groups: rotary measuring tools (such as encoders, resolvers, and tacho generators) and linear measuring tools (such as Linear Scales and Inductosyns). The basic function of these measuring devices is to create a constant electronic signal and transmit it to the controller. The following are some commonly used measuring devices or equipment found in various mechatronics systems.

Optical Encoder

The optical encoder is the most commonly used rotary measuring device in mechatronics systems. It is used to measure the position and speed of various elements, such as the location of an axis or spindle speed feedback in CNC machines. Typically, the encoder is integrated inside a servomotor and connected to the motor shaft through a coupling. Alternatively, it can be attached separately to a ball screw or coupled to a spindle unit using a timing belt. Inside the optical encoder, a graduated glass disc is attached to the shaft, which is fixed to the encoder body flange with preloaded ball bearings. The glass disc can rotate freely along with the shaft. The disc is designed with opaque and transparent sections. Photo-electric cells and light sources are positioned on either side of the glass disc in a way that allows the light to pass through and strike the photo-electric cells.

As the servo motor shaft or ball screw rotates, the glass disc attached to the encoder's shaft also rotates. This is because the encoder shaft is affixed to the motor shaft or ball screw. The transparent and opaque sections of the glass disc cause the emitted light from the light source to intermittently reach or miss the photo-electric cells. The output signal from the photo-electric cell is then passed through an electronic circuit, which converts the sinusoidal signal into a rectangular waveform. This waveform is transmitted to the controller via a signal cable. There are two types of optical encoders commonly used in mechatronics systems: the incremental rotary encoder and the absolute rotary encoder. The image below depicts an optical encoder that is widely used in mechatronics systems.

Incremental rotary encoder

The concept of incremental measurement involves computation through counting. In the case of an incremental rotary encoder, its output signal is directed to an electronic counter located inside the Controller. This counter keeps track of each increment in the encoder's output signal, providing a comprehensive measurement. The following diagram illustrates the functioning of an incremental rotary encoder.

The primary distinction between an incremental rotary encoder and an absolute rotary encoder lies in the structure of the graduated glass disk contained within the encoder. As depicted in the preceding diagram, the graduations on the glass disk of an incremental encoder are marked radially, ranging from 200 to 18000 PPR (parts per revolution). The PPR value signifies the number of pulses generated by the encoder as the shaft completes a 360-degree rotation, depending on the markings on the glass disk. Additionally, a reference mark is present in a specific location on the graduated glass disk, serving as the starting point for measurement counting.

Absolute rotary encoder

The working principle of an absolute rotary encoder is similar to that of an incremental rotary encoder. However, the construction of the glass disc differs. In an absolute encoder, multiple tracks are formed on the glass disc instead of a single track as in the incremental encoder. Each track has transparent and opaque sections arranged in a unique pattern. The signals received from the photo-electric cells create a unique pattern for each specific position of the encoder shaft. The signals from different positions of the encoder shaft are commonly expressed using Binary or Gray code. The image below illustrates the structure of a glass disc in an absolute rotary encoder.

Magnetic Encoder

In situations where using an optical encoder with a glass disc is inconvenient, such as in environments with vibration, extreme heat, or humidity, a magnetic encoder is used. The magnetic encoder operates using Hall Effect technology and provides accurate feedback even in harsh conditions. Both rotary and linear types of magnetic encoders are used in mechatronics systems. The images below depict two types of magnetic encoders.

A magnetic rotary encoder consists of three main components: a magnetic disk, a sensor, and a conditioning circuit. The encoder disk is designed with tiny magnetic poles arranged alternately along the circumference. When the disk rotates, a sensor inside the sensing head detects changes in the magnetic field and converts them into a sine wave signal. The sensor used is typically a Hall Effect device or sometimes a Magneto-resistive element. The signals from the sensor pass through a conditioning circuit to make them understandable to the mechatronics controller. The sensor and conditioning circuit are usually housed inside the sensing head and connected to the controller with a signal cable. The working principle of a linear magnetic encoder is similar to that of a rotary magnetic encoder, but a magnetic tape is used instead of a magnetic disk. The magnetic tape is magnetized with alternating poles along its length.

Linear Scale

A linear scale, also known as a linear encoder, precisely measures the linear displacement of an object, such as the linear axis movements of a CNC machine. In CNC machines, a linear scale is typically installed along with the machine's slide to provide accurate and precise position measurement compared to an encoder, which is usually fitted with a motor encoder. Backlash is always present when there is a conversion from linear displacement to rotary movements, such as with a motor. Therefore, a linear scale offers better accuracy than an encoder. The image below illustrates a linear scale and its internal components.

A linear scale consists of two separate parts: the glass scale and the reading head. One of these components (either the glass scale or the reading head) is affixed to a moving body, while the other remains stationary. The glass scale features alternate gratings, similar to an encoder's glass scale, with transparent and opaque sections. The reading head, like an encoder, contains a light source, lens, scanning reticle, and photoelectric cells assembled inside it (as seen in the previous image). When the reading head moves over the glass scale, the transparent and opaque grating parts on the scale align alternately with a scanning reticle index. This alignment causes the light passing through the lens to reach the scanning reticle and glass scale, ultimately reaching the photoelectric cells. Consequently, the light fluctuates alternately over the photoelectric cells, generating a sinusoidal signal based on these fluctuations. The sinusoidal signal output from the photoelectric cells is converted into a rectangular waveform using an electronic circuit inside the reading head. Finally, it is transmitted to the controller through a flexible cable.

Resolver

A resolver is a rotary measuring device that is attached to a motor shaft. It provides position and velocity feedback for a rotating device. A resolver consists of two stator windings and a rotor winding. The stator windings are typically wound in a manner that creates a 90° phase shift between them. The rotor winding and stator windings are integrated within the resolver, functioning as primary and secondary windings of a transformer. If a sinusoidal signal is passed through the stator windings (with a 90-degree phase shift), a sinusoidal signal is also induced in the rotor winding. As the rotor shaft rotates, the output signal changes based on the reference signal, and the magnitude of the output signal depends on the extent of rotation of the resolver shaft. The phase of the output signal changes from 0° to 360° when the resolver shaft rotates continuously. Since controllers only recognize digital information, the resolver's output signal is usually converted into a digital signal before being sent to the controller. The image below illustrates a resolver.  

Tachogenerator

A tachogenerator is a rotary measuring device used to obtain speed feedback from a rotating element. It was commonly used in earlier mechatronic systems, but with advancements in technology, it has become almost obsolete, as optical encoders now provide both speed and positional feedback. A tachogenerator is a simple permanent magnet DC generator typically installed on the same shaft as a servomotor or a rotating object. It generates a DC voltage, which serves as the signal output. The analog DC voltage generated varies with the motor's RPM, and a controller retrieves information about the motor's speed or RPM by measuring the analog voltage from the tachogenerator. The image below displays a simple DC tachogenerator.

Inductosyn

Inductosyn is an analog type of precision measuring equipment and can be considered one of the world's most accurate position-measuring devices. It comes in two main types: linear and rotary. Both types consist of two non-contacting elements. In the case of linear Inductosyn, these elements are a scale and a slider, while for the rotary type, they are a rotor and a stator. Inductosyn is commonly used for high-accuracy measurement and functions reliably even in harsh environments. One significant advantage of using a linear Inductosyn over a linear scale is that the scale can be easily expanded and is suitable for measuring long distances. The following images illustrate linear and rotary Inductosyn devices.

The operating principle of an Inductosyn is similar to that of a multi-pole wire-wound resolver. A resolver operates as a rotating transformer with two windings, while in a rotary Inductosyn, the windings are in the form of printed circuit patterns on the rotor and the stator. An AC signal is applied as excitation to the winding on the rotor element, and the current flowing through the rotor induces a current in the stator winding. The output amplitude varies cyclically as the rotor rotates relative to the stator, producing the signal output. The output signal is then amplified and transmitted to an analog-to-digital converter circuit to obtain a suitable signal for the controller.

A linear Inductosyn can be considered a resolver that has been unwound onto a flat surface. In this analogy, the stator of a resolver corresponds to the scale of a linear Inductosyn, while the rotor corresponds to the slider. The linear Inductosyn scale is typically attached to the machine bed, and the slider is fitted to the moving element. The slider moves over the scale, maintaining a small gap (usually 200 microns). Similar to a resolver, the slider of an Inductosyn contains two windings, and a sinusoidal voltage is applied to these windings with a 90-degree phase difference. This voltage induces a voltage in the winding across the slider. The induced voltage within the slider changes with the slider's movements, which is considered as the signal. The induced voltage has a very low value (microvolts), so a suitable pre-amplifier is always used with an Inductosyn to amplify this voltage. This allows the signals from the Inductosyn to be directly interfaced with the controller.