Resolvers and encoders - essential components of servo systems
March 2002
Maintenance, Test & Measurement, Calibration
Whenever a servo system is considered for an application, the question of feedback is often raised. To deal with this issue, Lenze motors can be supplied with resolvers, incremental encoders and SinCos encoders. This article looks at what these feedback systems are, why they are used and which is the correct selection for your application.
An incremental rotary encoder generates a pulse for a given increment of shaft rotation. The total angular rotation of a shaft is determined by counting the encoder output pulses. Encoders have different specifications such as 2048 or 4096 pulses per revolution and TTL (5 V) or HTL (15-30 V). Lenze uses the TTL standard.
Standard encoders come with two channels, A, A', B, B' and a zero pulse per revolution Z, Z'. The two channels are offset by 90° and each channel has a complementary signal which assists in eliminating electronic noise.
Absolute SinCos encoders use two sinusoidal signals shifted by 90° with 512 periods per revolution. Absolute positioning within one or 4096 revolutions. Resolvers are in principle rotating transformers consisting of two stator windings shifted by 90° and a rotor winding with a transformer winding.
The induced voltages result in two offset sine curves. The displacement of one curve relative to the other at a particular point in the cycle is unique to that position. This information is converted to a digital form with resolutions up to 65 536 counts per revolution. The advantage of the resolver is that it is more robust than an encoder - and has no internal electronics. As it is also the most economical, it is most commonly used with servomotors.
Why are feedback systems needed with servo controllers?
Servo controllers are current controllers and can accurately control motor speed as well as torque. To achieve this, the controller must manage the field relative to the rotor position. The rotor position is fed back using a resolver or encoder. Without this feedback, it is not possible to run a servo system. The advantage is that the controller has the information to control speed, torque and position.
If the basis of selection is price, the encoder is approximately twice the price of a resolver, the SinCos encoder two-and-a-half times and absolute encoder four to six times the price. For phase accuracy - the ability to maintain a phase angle - the resolver is accurate to 15 arc min (60 min per degree), the encoder 2 arc min and the SinCos encoder 0,5 min. In applications that require very high synchronisation accuracies, such as printing, SinCos encoders are used. If you need to know the position of the rotor when switching on a system, absolute encoders are essential. Experience has shown that the resolver resolution is adequate for most applications. It is the cheapest and most robust solution with the advantage that they can be supplied on all Lenze motors. Lenze products are distributed locally by SA Power Services.
SA Power Services
(011) 787 1801
[email protected]
www.sapower.co.za
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