An Introduction to Feedback Devices for VFD and Servo Applications

Feedback devices are the backbone of precision control in modern industrial applications, so understanding the right encoder choice is crucial. When designing VFD-based closed-loop applications, one question often comes up: which type of feedback encoder is best suited for VFD motors? 

This article will expand on the different types of feedback devices in VFD (Variable Frequency Drives) and servo applications.

KEB’s S6 servo drive and F6 VFD have multi-function encoder cards that can support many different types of feedback from one standard hardware.

What Is a Feedback Encoder?

A feedback encoder is a device that converts a motor’s mechanical motion into electrical signals interpreted by a VFD or servo drive. In closed-loop systems, the encoder provides information on the motor’s position, speed, and direction. This allows the drive to compare the motor’s actual behavior with the commanded value and make adjustments as needed.

With encoder feedback, drives may maintain precise speeds while holding positions under load, delivering stable performance. Without encoder feedback, the drive operates in an open-loop configuration. It lacks a reference to correct potential speed or position deviations.

How Does a Feedback Encoder Work with a VFD?

In a VFD system, an encoder provides closed-loop feedback, enabling the VFD to adjust motor speed and torque in response to real-time motor behavior. This system continuously monitors the encoder’s position data and compares it to the target motor speed. If any discrepancy is detected, the VFD adjusts its output frequency to maintain the desired speed and torque.

Encoders work with VFDs by generating digital pulses or signals that indicate motor shaft position and speed. As the motor rotates, the encoder counts these pulses, enabling the VFD to make high-precision adjustments.

With encoder feedback, VFDs can support applications that require precise motor positioning, stability under load, and accurate speed control, such as conveyor systems, robotics, and servo motors in stainless steel environments.

What Is Motor Feedback?

This process of sending real-time information via the feedback encoder is known as motor feedback. To put it simply, with a feedback encoder, you can know what the motor is doing as well as what it should be doing.

In open-loop systems, VFDs send a speed command and adjust output frequency to match the needed speed. This approach works well for simpler applications, but the drive has no way to verify whether the motor is actually running at the intended speed. Any load change, slip, or a locked rotor can cause the motor to drift without the drive knowing.

An open-loop control diagram would look like this:

Adding motor feedback closes the loop. This can improve speed regulation performance and even add torque control and positioning functionality. The feedback goes back to the VFD, where the actual speed is compared to the command speed. The drive looks at the difference between these signals and tries to reduce the error to 0 by adjusting its speed controller.

A basic closed-loop control diagram looks something like this:

Types of Motor Feedback

Commonly used motor feedback devices can be grouped into several common categories.

Analog Feedback

Analog feedback channels use sine-wave signals, with the analog voltage representing shaft position. Evaluating the shaft position over time will give velocity and direction information.

Resolvers: Suitable for Environments with Extreme Vibrations and Temps

A resolver is a robust electromagnetic feedback device that detects motor shaft position and speed using sine and cosine voltage signals.

This creates a type of rotational transformer when excited with a carrier frequency. This rotation induces two voltage signals on the stator windings 90° out of phase with each other (sine and cosine).

A KEB VFD can read these signals to determine the position of the motor shaft.

Resolvers are “old tech” but are often preferred because they are very robust. Resolvers are tried and true and can be used in many servo motor applications. The inductors are epoxied into the housing, so they tolerate wide temperature ranges and extreme vibration. They require no extra electronics or onboard signal processing.

These are often used in applications where high-precision control is not critical, but monitoring general motor behavior is still necessary, such as in basic conveyor systems or pumps.

Example applications: basic conveyor systems, pumps, and heavy-duty industrial settings

Sin/Cos Encoders: Best for High-Precision Tasks like Robotics and CNC Machines

Sin/cos encoders are precise analog feedback devices that provide two signals: a sine wave track and a cosine wave track. Similar to incremental encoders, they are commonly provided with 1024 or 2048 ppr.

These tracks provide position and direction information in the form of 1 Volt peak-to-peak (1Vpp) analog sine waves (typically referred to as “A” and “B”) in quadrature. The sine and cosine tracks can be sampled at high frequency, providing much more information than their incremental counterparts.

The high ppr and ability to sample the signal means that Sin/Cos encoders can provide over a million unique positions in one motor shaft revolution. For this reason, encoders in this family are preferred for precision applications.

• Sin/cos encoders are typically used on servomotors, where the higher feedback resolution benefits both the velocity and position loops.
• They are available in single-turn and multi-turn absolute variants, making them a common option for absolute position applications.

Example applications: robotics, CNC machines, and multi-axis motion control

Incremental Motor Feedback

An incremental encoder provides a digital pulse for each pre-determined angular shaft rotation. The resolution of incremental encoders varies widely. There are often two offset signal channels that help to establish the direction of shaft rotation.

Incremental Encoders (TTL & HTL): Suitable for General Speed and Position Control

Incremental encoders typically have a glass disc with a black/clear etching pattern for through-beam LEDs that become on/off pulses as the disc rotates.

In one encoder rotation, an incremental encoder delivers a specific number of pulses, which enables the movement value to be deduced along with the speed.

Common voltage levels are 5V (TTL) and 24V (HTL). The signals consist of three tracks: tracks A, B, and Z (Zero signal). The A and B tracks have a 90° phase shift to indicate the rotation direction, while the zero signal (Z) track gives the number of revolutions and is useful for homing routines.

Its resolution is the max number of pulses that it sends per revolution.

• Incremental encoders provide position feedback, but the absolute position is not retained when the drive/encoder is powered down.

• VFDs can evaluate the rising and falling edge of the signals to determine the direction of rotation. Monitoring the rising and falling edges effectively doubles the position information for each of the 2 tracks. Therefore, these are often called quadrature encoders.

• Incremental encoders are typically used on induction motors with indexing

• Glass discs are especially vulnerable in high-vibration settings, such as assembly lines or heavy machinery. Steel disc options are commonly available as an alternative. Choosing steel alternatives can mitigate this issue.

Example applications: assembly line motors, basic automation systems, and indexing conveyors

Absolute Position Feedback

Some feedback devices offer absolute position feedback in single or multi-turn variants.
Absolute encoders provide a unique position value for each rotation, representing the “absolute” position of the encoder. When you switch the drive on, the absolute encoder can tell you the exact position of the shaft it measures.

If your machine were to lose power, absolute feedback would tell the system where it is without having to re-home the machine. This is especially important in critical applications where re-homing procedures might take too long or lead to expensive scrap material.

Single-Turn Absolute Encoders

Single-turn absolute encoders measure position within one full 360° revolution. The position value repeats every revolution. So, they’re ideal for applications that only require precise monitoring within a single turn of the shaft.

Multi-Turn Absolute Encoders

Multi-turn absolute encoders track the shaft’s position within a single revolution as well as the number of complete revolutions. That way, the system will maintain an accurate position count over long distances or continuous cycles (even after power loss).

Example applications: packaging machines, gantry systems, wind turbines, and factory automation

Serial Feedback Devices

Serial feedback devices measure shaft position with analog or incremental means but then transmit the information to the VFD via a serial connection. This can reduce conductor wires in the encoder cable and susceptibility to electrical noise.

Furthermore, serial feedback devices can transmit much more information, including OEM encoder/motor parameters, error codes, and encoder diagnostic information.

BiSS Encoders: Ideal for Advanced Motion Control with Diagnostics

BiSS encoders are an open-source feedback interface for digital absolute encoders commonly used in VFD and servo applications. Designed and developed by IC-Haus in Germany, it is capable of transmitting the encoder’s position value and reading or updating the information stored in the encoder.

Like other digital feedback encoders (Hiperface, EnDat), the BiSS comms link can carry information other than position value. Additional information, such as encoder resolution, manufacturer information, and temperature, can be stored in a non-volatile memory area in the encoder.

KEB drives can read and write to the encoder memory without interrupting real-time operations. Due to their high transfer rate, BiSS encoders can be limited on the permissible encoder cable length.

Expert Tip: BiSS encoders do not require additional license fees, providing a good price and performance value.

Example applications: robotic manufacturing and advanced motion control

Hiperface Encoders: Designed for Applications Needing Hybrid Feedback with Both Incremental and Absolute Position Data

Hiperface encoders are a proprietary hybrid feedback interface SICK Stegmann developed in 1996. It consists of a bidirectional interface for absolute encoders, combining a digital channel for absolute position information and an analog channel for incremental position and speed feedback.

The encoder also features a memory area, which is read or written to by KEB drives through a communication channel. Asynchronous serial transmission requires only two lines to transmit the encoder position data. The serial link requires terminal resistors to operate, along with pull-up and pull-down resistors to increase interference immunity.

Example applications: servo applications, robotics, and synchronized multi-axis motion systems

EnDat Encoders: Favored in Systems Requiring Absolute Positioning and Error Diagnostics

EnDat encoders are a proprietary feedback interface developed by the Heidenhain Company in the 1990s. It consists of a bidirectional digital interface for absolute encoders. It can transmit position values from incremental and absolute feedback. It can also transmit or update information stored in the encoder and save new data.

EnDat 2.2 offers serial transmission. Only four lines are necessary for transmitting the encoder position data synchronously with the clock delivered by the electronics. The type of transmission (position values, parameters, diagnostics, etc.) is determined by mode commands sent to the encoder.

BiSS, Hiperface, and EnDat encoders are all available in single and multi-turn absolute formats, making them ideal for applications that require absolute positioning. Typical uses of these encoders include applications such as robotic manufacturing, motion control with multiple axes, lifts, and CNC machines.

When paired with a KEB drive, safety functionality with SIL3 standards is possible in applications for speed and position.

Example applications: CNC machinery, robotic arms, elevators, and other critical motion-control devices

Choosing the Right Feedback Device for Your Application

Choosing the right feedback device comes down to a few factors. That includes your environment, your precision needs, and the motor type.

• For high-vibration settings, resolver or steel-disc incremental encoders should provide the durability you need.

• Should your application require tight position control (robotics or CNC work, for example, you may want high-resolution sin/cos or serial encoders.

• Standard incremental encoders are typically sufficient for simpler speed-regulated systems.

Other factors to consider in your device search are cable length, noise immunity and protocol-driven compatibility. Matching encoder capability with your control goals ensures reliable closed-loop performance.

How Feedback Devices Integrate with KEB’s VFDs and Servo Drives

KEB’s VFDs and servo drives support a wide range of encoder types. They include flexible multi-function feedback interfaces, enabling closed-loop speed and position control.

Our drives ingest real-time feedback directly for precise motor regulation and motion control. This simplifies setup and improves performance across applications.

Contact a KEB application engineer today to discover how we can help elevate your next project with our feedback solutions.