Feedback Sensors Keep Servomotors on Target
- Tuesday, 01 October 2013
Linear encoders contain a linear track and a read head, and are usually used with systems that track linear movement, such as X-Y stages, and position tables. The linear track can range in length from a few inches to several feet. It is etched with graduations that are scanned by the read head as motion components move. The read head detects multiple channels to provide position and direction data. Encoders with sinusoidal outputs use additional interpolation circuitry to electronically improve resolution.
For equipment that requires particularly high resolution, linear encoders are the best choice in their class. Resolutions to 0.1 microns are common, with some systems offering resolution to 20 nanometers. Accuracy, typically 20 microns per meter, may decrease linearly over the travel distance of the track. However, this can be compensated for with slope error correction to bring any error below 5 microns per meter. Machines operating at high speeds use linear encoders for feedback because these devices typically operate at higher speeds than other feedback devices.
Optical rotary encoders consist of a light source, rotating code disc, and a light detector. The disc has either slits or graduations that divide it into equally spaced areas of dark and light. These markings often are referred to as lines — hence, the unit of measure of lines per revolution (LPR). This measurement indicates an encoder’s resolution or granularity. Accuracy for encoders is defined as plus (+) or minus (-) lines or counts. It is important to note that accuracy and resolution are different attributes, although they are often related. With encoders, accuracy typically increases with resolution since accuracy is defined as +/- so many counts. As count resolution increases, so does accuracy. With resolvers, however, increasing resolution by more interpolation — 16 bits vs. 12 bits, for example — does not increase the accuracy. It is quite common for resolver systems to have 100 times lesser accuracy than resolution.
As the connected components rotate, the light detector registers the on-off pattern of the light passing through the disc. The detector converts this on-off pattern into an electronic, digital signal that looks like square waves. Typically two rows of slits or markings are offset by one half of their width or one quarter of a complete cycle (90 electrical degrees), generating two electrical signals known as Channel A and Channel B. This offset lets the control determine the direction of the shaft rotation, an important piece of information for the drive during start up, and essential for servo systems providing bi-directional motion.
Instead of using only two channels, some encoders use additional channels to track shaft position or help with noise immunity. These channels include what is referred to as the Index and Compliment channels. Another means of tracking shaft position is to add a commutation, or Hall equivalent channel. They represent alignment to the Aphase, B-phase, and C-phase back EMF of the motor.
Depending on how the encoder counts the A and B channels, resolution can increase four-fold. This will arise when the counting circuit tracks both falling and rising edges of both signals, also referred to as quadrature detection. Increasing resolution will increase system repeatability. High resolution also enables higher gain for position and velocity loops, ensuring superior system stiffness. Encoder resolutions of 50 to 5,000 lines per revolution are standard among most vendors, but line counts to 100,000 are available. In high-accuracy applications, system accuracy is affected by errors from other sources such as lead-screw cumulative error, thermal expansion, or nut backlash. Linear encoders can overcome these challenges.
Sine encoders are at the high-cost, high-accuracy, and precision end of the feedback device spectrum. They are similar to incremental encoders, except that the A and B data channels are sent to the controller, typically as one-volt, peak-to-peak sine waves instead of square waves. The benefit is that these devices can interpolate each complete sine wave, increasing system resolution and giving more information to the velocity controller. This reduces truncation and quantization errors, allowing higher loop gains. Sine encoders can achieve over 2 million counts per revolution, or about 0.62 arc-sec of resolution. Such capability is well suited to applications that require high precision or have high inertia loads.
Like other encoders, sine encoders also may have commutation tracks, Hall emulation tracks, or auxiliary sinusoidal channels called C and D, which provide absolute position within one revolution. The C and D channels are similar to the Sine and Cosine signals used in resolvers.
A variation of sine encoders is the multiple-turn sine encoder. Multi-turn versions are implemented using an internal mechanical gearbox. This provides absolute positioning over many revolutions of the device. These encoders can provide up to 8192 steps per revolution and up to 8192 shaft revolutions, providing a total of 26 bits of absolute resolution before interpolation. Sine encoders offer high precision, resolution, and accuracy.
Feedback devices can output electrical or optical signals. One advantage of using optical transmission lines for feedback signals is that they are immune to high noise or EMI/RFI environments. High noise levels interfere with clean signals and distort data sent to the drive, compromising a drive’s ability to provide high-quality position, speed, and torque control. When sending signals electrically, amplifiers or signal conditioning devices may be needed to modify noisy signals. Newer feedback devices use IC chips to convert and interpolate signals to more robust waveforms that aren’t corrupted by noise, and that won’t diminish as they propagate through the cable to the drive.
This article was written by Gene Matthews, Product Manager at Kollmorgen, Radford, VA. For more information, visit http://info.hotims.com/45608-321.