FEATURE Drives, Controls & Motors
Feedback devices for VFDs and servo applications
One question that often arises when designing closed-loop applications is which type of feedback to use on the motors. In this article, Jeff Kardell of KEB America describes the different types of feedback devices in variable frequency drives (VFDs) and servo applications, as well as some examples of where they can be used
W
hat is motor feedback? To answer this question, we should begin by considering an open-loop system that
has no feedback. The VFD is given a speed command and attempts to run the motor at that speed by adjusting its output frequency. The advantage of this system is its low complexity and low cost; see Figure 1. The problem here is that the drive has no way of knowing if the motor’s actual speed is deviating from the intended speed. This can easily happen as the motor is loaded or if the rotor becomes locked. We can improve speed regulation performance and even add torque control and positioning functionality by adding feedback to the motor shaft. The feedback goes back to the VFD, where the actual speed is compared to the command speed. The drive looks at the diff erence between these signals and tries to reduce the error to zero by adjusting its speed controller. A basic closed-loop control diagram is shown in Figure 2.
Types of motor feedback
Commonly used motor feedback devices can be grouped into several categories: Analogue feedback
Analogue feedback channels are sine-wave feedback signals where the analogue voltage represents the shaft position. Evaluating the shaft position over time will give velocity and direction information. Resolvers
out of phase with each other (sine and cosine). These signals can be read by a VFD to determine the position of the motor shaft. Resolvers are now considered old technology but they are often preferred
38 April 2022 | Automation
A resolver is an electromagnetic transducer found in a wide range of position and velocity feedback applications; see Figure 3. The main design of a resolver has two windings: in the stator (non-rotating part) and rotor (attached sto the motor shaft and rotates), which creates a type of rotational transformer when excited with a carrier frequency. This rotation induces two voltage signals on the stator windings 90o
because they are very robust. They are tried and tested and proven highly capable in many servo motor applications. The inductors used are epoxied into the housing so they are very tolerant to wide temperature ranges and extreme vibration. They require no extra electronics or onboard signal processing. Sin/cos encoders Sin/cos encoders are analogue feedback devices that provide two signals: a sine wave track and a cosine wave track. Similar to incremental encoders, they are commonly provided with 1,024 or 2,048ppr. These tracks provide position and direction information in the form of 1V peak-to-peak analogue sine waves (typically referred to as ‘A’ and ‘B’) in quadrature; see Figure 4. The sine and cosine tracks can be sampled at high frequency, which means they provide much more information than their incremental counterparts.
The high ppr and ability to sample the signal means that sin/cos encoders can
provide over one million unique positions in one revolution of the motor shaft – this is why they are preferred for high-precision applications.
Sin/cos encoders are typically used on servo motors, where the higher feedback resolution is a benefi t for both the velocity and position loops. They are available in both single-turn and multi-turn absolute variants, making them a common option for absolute position applications.
Incremental motor feedback An incremental encoder provides a digital pulse for each pre-determined angular rotation of the shaft. The resolution of incremental encoders varies widely and there are often two off set signal channels that help to establish the direction of shaft rotation. Incremental encoders (TTL and HTL)
Incremental encoders typically have a glass disc with a black/clear etching pattern for through-beam LEDs that become the on/off
automationmagazine.co.uk
Figure 1: Open-loop control
Figure 2: Closed-loop control
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