An optical encoder measures positions, lengths, and displacements without contact. They are often used as a feedback loop for positioning in machine control. This position sensor operates based on light and a ruler with a reflection pattern. They position with low resolution, accurate to within 1.2 nm! Furthermore, this technology has low power consumption, making the optical encoder suitable for applications in vacuum environments.
You want to move a machine. With an encoder, you know where the object is and what distance it needs to travel to reach the desired position. Does your application require position control with low resolution and high speeds?
Even in a vacuum environment, the optical encoder can be the solution. Read all about the possibilities of this position sensor.
How does an optical encoder work?
Similar to magnetic and inductive encoders, optical encoders also work with a ruler. In optical technology, the ruler consists of a pattern of reflectors, or the light-sensitive layer. As an object moves, the encoder is moved across the ruler. These rulers are available in both straight and circular designs.
Peter Verstappen, Account Manager at Sentech explains: “The transmitter of an optical encoder is a light source. Often this is a VCSEL, a piece of silicon that emits light. As soon as the ruler is illuminated by the transmitter, the pattern of reflectors reflects the light back to the receiver. This creates a waveform, an amplitude. This movement is converted into a sine and a cosine, which are translated into a signal. This signal tells the motor's controller the actual position of an object.”
Positioning feedback loop
Encoders are often used as a feedback loop for positioning in a machine. What does such a position control look like?
“A motor is tasked with moving an object from A to B. To know where the object is during the movement between A and B, you need a measurement. This measurement is the feedback loop to the motor's control. If the motor's control knows where the object is, the motor can supply the correct amount of power to bring the object to the desired position,” according to Verstappen.
Incremental or absolute
Optical encoders are available as incremental and absolute systems. What is the difference between these two types of encoders?
Incremental encoders
An incremental encoder measures step by step. This is because they measure the change and direction of movement. When starting up, an incremental system must ‘home’ to find the index – or the zero position.
Absolute encoders
An absolute encoder directly indicates the actual position. This encoder type does not need to ‘home.’ The system sees every position as a unique signal.
Compared to incremental encoders, absolute encoders are more complex and have higher latency values, resulting in a delay in data transmission.
If component size plays a significant role, precise placement is of great importance. Placement speed is also crucial here. This allows you to offer end products at competitive prices.
5 benefits of an optical encoder
- Very accurateWith a resolution of up to 1.2 nm, optical encoders are among the most accurate measuring systems for positioning.
- Small building formDue to its small form factor, the optical encoder is easy to integrate into compact machines. Housings are available as small as 9 x 7 x 1.2 mm.
- High speedsThanks to their advanced technology, optical encoders are suitable for high-speed applications. From 3 meters per second in entry-level systems to 10 meters per second in advanced systems.
- Suitable for vacuumBecause the light source of the optical encoder is a VCSEL, they require less power, making them generate significantly less heat. This also makes this technology suitable for vacuum environments.
- Immune to electromagnetic interferenceThey are immune to external electronic interference. Optical systems work with a balanced A and B signal. If this signal is disturbed, the differences between the A and B signals remain. This results in a good signal.
When do you use an encoder?
Before you select a specific type of encoder, it's important to know what an encoder does. With an encoder, you measure positions, lengths, and displacements. They are often used to control a position, in other words, as a feedback loop for positioning in the control of a motor or machine.
Depending on the requirements and environmental factors, you can determine which type of encoder best fits your application. For example, an inductive encoder or magnetic encoder is more suitable for a contaminated environment.
Optical encoders from Celera Motion with MicroE-technology are built from materials suitable for vacuum environments.
Position control in vacuum
In vacuum environments, there are no air molecules. Without air molecules, electronic systems can lose their heat poorly or not at all. “For optical encoders with a lens, this means they overheat, causing them to break down quickly. Because the MicroE encoders contain a VCSEL, they consume less power and generate much less heat. This makes these encoders suitable for vacuum environments,” Verstappen explains.
Do you want to use a system that overheats in a vacuum space? Connect the system to a conductive material. This is the only way to lose heat in a vacuum.
Measure displacements with extreme accuracy
An optical encoder measures displacements with an accuracy of up to 1.2 nm. However, the actual accuracy is determined by various factors, which also affects the system's price. Therefore, it's important to know what level of accuracy your application truly requires.
Absolute and repeat accuracy
Encoder systems are built from two factors that determine the precision of your measurement. Namely, absolute accuracy and repeat accuracy. When selecting a position sensor, it's good to know the extent to which these factors are important for your application.
Absolute accuracy is the actual position in space, without prior calibration or calling a reference signal. With repeat accuracy, an index signal is present. When the machine starts, the encoder always moves to this preset point first. That point is always the same. All movements are then made from the index.
Explanation:
With absolute accuracy, you move 10,000 mm, but how accurate is this 10,000 mm in reality? That 10,000 mm could actually be 10,004 mm. Repeatability is when you send the machine to the same position ten times. How much this position deviates from that position is the repeatability.
Tolerance field
Both absolute accuracy and repeatability have gradations in precision. The smaller the tolerance field, the more precise the measurement. If the absolute accuracy is 10 mu, and the repeatability is 1 mu, then the encoder may deviate by a maximum of 1 mu from the reference point. This margin may deviate by 10 mu from the actual (absolute) position.
The degree of absolute accuracy has a significant impact on the price of an encoder system. If repeatability is particularly important, you can usually find a more affordable system. Often, high repeatability is sufficient for reliable measurement.
Ruler material
Every material has its own coefficient of thermal expansion. For example, a metal ruler expands at high temperatures and with temperature changes. In contrast, glass has no coefficient of thermal expansion, making a glass ruler more reliable at higher temperatures.
Interpolation: An Even Lower Resolution
Do you want a lower resolution? You can do that! By interpolating the signal, you can measure more steps per second. The original signal is divided into even smaller steps. How much you can interpolate depends on the capacity of your controller: the input frequency must be able to handle the pulse train. Ensure that the input frequency of the controller is higher than the output frequency of the encoder.
How to prevent a dirty encoder
Because optical systems work with light, dust and dirt are fatal to the measurement results. To limit contamination, you can take this into account when integrating the encoder.
The Account Manager explains: “The design includes options that will make the system less prone to getting dirty. For example, you can mount it upside down or place it under a cover. Also, consider fingerprints. In certain positions, the ruler will come into contact with fingers more quickly. Furthermore, optical systems are suitable for cleaning. It's also advisable to reserve space for this in the design.”
How to select the right optical encoder?
Among optical systems, various options are available. To select the right optical encoder, it is important to know which requirements and environmental factors are relevant. It is important to be specific here.
For example, if you know that low latency is important for your application, you will also want to know how precise your system needs to be. And whether your system's control capabilities can handle that.
Map out which specifications are truly needed for your application with our sensor expert.
