ncoders are the typical choice when feedback on the position or speed of a servo system is required due to the
high resolution they provide. However, sensor technology commonly integrated within a motor, rather than added to it during design, can achieve a more compact and lower cost application. While Hall-effect sensors are typically used to manage commutation, they can provide a useful addition to incremental encoder technology. In certain applications, typically based on general torque control, these can also achieve the required feedback when used alone. Alternatively, resolver technology can give
precise feedback similar to that of an encoder. Being of a durable design, they are an ideal choice over optical encoder designs for applications set in rugged environments.
When a current-carrying conductor is exposed to amagnetic field, a voltage is generated at right angles to the current: this is the Hall effect, the basis of simple feedback technology for brushless servo systems. Hall-effect sensors are mounted inside the motor near the stator windings, with one sensor for each of the motor’s three phases. The signals of the Hall-effect sensor provide
feedback on the position of the rotor every 60˚, based on six commutation states with two phases conducting at any one time. Formotors with more than one pole pair, the mechanical resolution can be increased but compared to an encoder, which can sense multiple positions per degree of rotation, Hall sensor feedback is relatively coarse. With this six-step commutation system,
also known as block commutation, the current waveform is trapezoidal, advancing every 60˚. Unlike sinusoidal control, with a
24
smooth waveform based on electrical angle resolution finer than one degree, trapezoidal commutation delivers relatively coarse current and torque control.
Unlike DC systems, brushless servo motors require feedback and control electronics for commutation. Unless sensorless control methods are used, a Hall-effect sensor is a simple and low-cost method of commutation. As a result, they are often integrated into frameless brushless motor kits, like the Kollmorgen KBM Series frameless motors, allowing commutation in prototyping from the outset, even if precision encoders are required at a later stage of development. While the Hall-effect sensor is relied on
for commutation, it can be combined with an incremental encoder, which then takes over control to provide fine speed and position resolution. This combination is known as the comcoder: the commutating encoder. This approach removes the start-up challenge inherent to an incremental encoder, particularly if any movement of the motor shaft is undesirable, or if the application is starting under load. Although a Hall-effect sensor’s feedback
resolution and current control isn’t sufficient for purposes that demand high precision and smooth torque delivery, for applications such as power tools or general tightening requirements, where only torque delivery is sufficient, Hall- effect sensors can be used on their own.
Resolvers can achieve a level of feedback precision similar to an encoder. Also commonly
integrated with the motor, a resolver is a rotary transformer, with a primary winding and two secondary windings arranged 90˚ apart. As the rotor turns, voltages in the secondary windings vary as a result of the shaft angle, meaning that any specific, angular position is represented by the signal pair. These signals are analogue, so control electronics or software are required to convert them into a digital position value. A standard two-pole resolver can achieve
absolute position measurement within one motor revolution, and with digital interpolation, a resolver provides resolution similar to high- count incremental encoders. While resolvers can also handle motor speeds in excess of 10,000 rpm, resolutionmay be limited by motor speed because of amaximum frequency ceiling. Resolvers are commonly rated at 155˚C, with
special models able to withstand 230˚C, and they can even be radiation-hardened. They are also highly resistant to shock and vibration, and are less affected by EMI noise. Thismakes resolvers well-suited to applications such as aerospace and defence, as well as the electric vehiclemarket.
Unlike encoders, which have many various interface types, resolver designs are relatively standard, and they are often required for special purpose applications that benefit from the protection of a robust housing. As a result, resolvers are commonly integrated with a motor, rather than supplied separately. The drawback of resolvers is that they require additional electronics for analogue-to-digital signal conversion, adding size and mass, as well as complexity of design. Moreover, while they can offer up to 16-bit resolution, the most precise encoder technology, particularly optical versions, still achieve the optimum feedback. To help select the right feedback technology, speak to the experts at INMOCO.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56
