How to Measure Linear Distance with Rotary Encoders

Rotary encoders inherently monitor the displacement or position but can be used to measure linear distance by calculating the number of pulses compared to the known number of pulses per arc length and designed into a system that returns linear feedback.

There are three basic approaches:

• Direct measurement of load using a draw-wire encoder
• Direct measurement of load using a follower-wheel encoder
• Indirect measurement of load using an encoder on the motor shaft

For a detailed discussion of distance measurement using draw-wire encoders, see the article How to Measure Distance with Draw Wire Encoders. Here, we focus on length-measuring applications, reviewing best practices for specifying and implementing these systems.

Direct Measurement Using a Measuring Wheel Encoder

A measuring wheel or follower-wheel encoder consists of an encoder wheel mounted on the shaft of an encoder (or vice versa). The wheel interfaces directly with the surface of the material being measured. As the material moves relative to the encoder, the wheel turns, rotating the encoder code disc and generating a signal.

To calculate the length traveled L (inches) using the output from an incremental encoder, we start by calculating the number of pulses per 1 in. arc length (PPI):

where PPR is pulses per revolution and D is follower wheel diameter (inches). Then length L is given by:

Depending on the application, the encoder measuring wheel may be fixed, as for a paper measuring system in a converting line or a printing line. Alternatively, the follower wheel may be in motion while the material being evaluated remains fixed. Examples of this case include roadways for paving applications or the side of a hoistway in an elevator shaft.

How to Maintain Measurement Accuracy with Wheel Encoders

The most important aspect of measuring length with a measuring wheel encoder is avoiding slippage. The encoder wheel needs an adequate coefficient of friction relative to the material being evaluated. The coefficient of friction can be adjusted by a combination of the structure of the wheel surface and its materials. In the case of measuring lengths of silk fabric in a large industrial cloth-cutting operation, the wheel might be fitted with soft rubber fingers. In a hay baler, the device would use wheels fitted with long metal teeth. The spikes catch in the hay, turning to measure out the length of a bale so that the machine knows when to cut it and band it. On a machine for applying asphalt sealant to roadways, the follower wheel may simply be finished with a treaded O-ring.

Another way to prevent slippage is by adding a preload. The preload needs to be carefully balanced, however. If it is too low, the wheel might skip or slip, resulting in lost motion. If the preload is too high, it can damage the product being measured. It can also apply excess load to the bearing, resulting in increased wear and, potentially, premature failure.

Avoiding Overhung Loads

Overhung loads can pose significant risks in follower-wheel applications, potentially leading to premature bearing failure. For example, in a belt and pulley system, a toothed follower wheel engages with the belt. If the belt is too loose, the wheel can slip; if it's too tight, it can damage the encoder. Similarly, tracking the movement of a gantry involves mounting an encoder on a trolley with the wheel pressed against the frame, where excessive overhung load can damage the encoder.

Rack and pinion designs also require careful handling to avoid equipment damage and measurement compromise. Machine vibration can transfer to the encoder via the pinion, introducing artifacts into the data. In the railroad industry, maintenance vehicles use follower-wheel encoders to measure track length. Here, the encoder shaft should not bear the weight of the assembly, as this can apply high force loads.

To mitigate these issues, consider using dual-shaft encoders with a follower wheel on either side, or hollow-bore encoders for applications with higher shock and vibration. In this setup, an extended shaft of a fifth or idler wheel is added to the vehicle, and the encoder is mounted on the shaft, allowing the wheel bearing to take the load. In cases where no workaround is possible, harsh-duty encoders provide the best solutions.

Indirect Measurement via Motor Shaft Monitoring

In many applications, directly measuring the load isn't feasible. For example, in destructive testing, grippers pull apart a test piece until it fails. Here, an encoder mounted on the motor shaft tracks rotations, which can be used with specific equations to determine the displacement of the grippers.

Mounting the encoder on the motor shaft is a common and simpler approach compared to using a follower wheel, reducing potential mounting errors. However, mechanical elements like gearboxes and couplers can introduce issues such as compliance, backlash, and hysteresis. Actuators can also cause mechanical slop and practical problems. Screw-type actuators with finer thread pitches have a high number of turns per inch, posing challenges even for multi-turn absolute encoders.

Length measurement has diverse use cases, each with specific requirements. By leveraging different implementations and a variety of encoders, engineers and maintenance staff can meet application needs. Pay attention to these tips and follow best practices for encoder wiring and installation to ensure your system operates as desired.