Linear encoders by their very design overcome various mechanical machine limitations by providing a direct source of positional feedback. They can monitor most applications, including hostile, oily environments or those that require total submergence. Some IP67-rated encoders provide resolution to 0.1µm or even absolute positional feedback. But there are limiting factors in speed-extreme applications. “For both low and high-speed applications, the stiffness of encoder dynamic response is crucial,” explains Mark Hudman, Ph.D., technical director at Newall Electronics Inc., Columbus.
At high speed, it is important that linear encoders provide near-real-time positional information to the control system. The bottleneck lies in transferring positional values or changes from the encoder to the control at the required resolution. For example, to achieve 0.1-µm resolution at 2 ms, a quadrature rate of 20 MHz is required for a RS422-style encoder — input beyond what many controllers can accept. “Using analogue 1-Vpp signals of a 20-µm period is one way of overcoming this problem,” says Hudman, “but it puts emphasis on decoding the exact position on the control.” Absolute protocols such as synchronous serial interfaces or SSI, an absolute encoder protocol that uses a clock signal from controls to transfer absolute position on data lines, are not quick enough by themselves for high-speed closed-loop positioning, as data transfer rates are too slow, resulting in latency issues.
As such, for speed-extreme applications, absolute encoders generally include an incremental positional signal for dynamic control as well.
Mechanical and physical considerations of the encoder must also be considered. Here, encoders with moving parts, such as traditional glass-based systems, suffer from acceleration and deceleration effects, as they rely on guides and bearings to maintain the integrity between the sensor and scale grating. “But encoders with no mechanical elements do not falter at high speed,” says Hudman.
For low-speed applications, latency is far less of an issue, because the rate of displacement is small for a given control update rate. Here, depending on the application, the key criteria are generally stability and resolution.
Addressing limitations
At high speed, any encoder that relies on mechanical components for its operation will suffer acceleration and deceleration effects. Essentially, the motion subjects encoders to impulse, throwing shockwaves through its parts. In optical systems this threatens to knock sensor components into the glass grating track and cause physical damage — in addition to degrading position. Too, running mechanical bearings at speed for prolonged periods leads to wear and sometimes even mechanical failure. So, encoders that have no such mechanical components have no such limitations.
Hudman says, “In one application, noncontact encoders work in specialized molding lines that require 600 g of acceleration force. This is far beyond most technologies and certainly beyond traditional glass encoder systems.”
Low-speed applications have challenges of their own. For example, it can be difficult to determine the difference between real movement and instability due to external factors such as vibration. “To overcome this issue, a highly stable encoder is required — and in some cases, additional positional filtering must be performed.”
Your role as designer
Designers can optimize electromechanical systems to make linear displacement measurement easier. “Measurement is becoming more and more critical in all forms of systems. For example, while we all appreciate that machine tools need measuring systems for component accuracy requirements, we may not appreciate that machines cutting salami slices for pizza may require precision measurement as well,” says Hudman. “However, some do — because by accurately cutting these slices, waste is reduced, resulting in substantial monetary savings over years of production.”
Designers need to look at their machines, whatever they are, and envision what measurement can do for them. “Too often, the addition of positional feedback systems is late in a machine design, taking on the appearance of a retrofit. Instead, linear measurement should be a principle part of machine design, and the location and nature of feedback chosen specifically for the environment and application,” Hudman explains. For starters, encoders should always be mounted as close as possible to the axis of travel, to reduce offset errors and ensure that all mountings are as stiff as possible.
For more information, visit newall.com or search “displacement sensors” at motionsystemdesign.com.