Modern motors such as ac, dc and brushless dc servos are all different in construction but have some performance characteristics. However, each design has particular strengths and weaknesses. A dc motor typically has a bit more starting torque than an ac motor, but brush-style dc motors may require maintenance to replace the brushes. The brush action can also cause electrical electromagnetic (EM) and radio frequency (RF) noise. Having a good model of the mechanical requirements will make choosing the correct motor torque, speed and power easier. There are many free motor-sizing tools available on the Internet to make the selection easier.
Both the National Electrical Manufacturers Association (NEMA) and the International Electrotechnical Commission (IEC) have standard motor size specifications. Using a standard-sized motor allows different manufacturers’ equipment to be used interchangeably. If a motor must be changed from one brand to another, consider also changing the drive or amplifier to the same brand, especially with servo motors. If technical issues arise when different motors and drives are employed, many manufacturers offer little assistance and tend to blame each other instead of trying to solve the problem.
Manufacturers often offer many options when ordering a motor. Brakes, shaft seals, connector style, smooth shafts or keyways and connector styles are a few. Having many options makes picking the perfect product for your application easier. A downside to the many options is the availability of a replacement part. Distributors and warehouses may not keep one part of each possible combination in stock. Choosing a motor that is normally stocked, even if it has options that will not be used, may better serve your customer. One machine builder I have worked with would list all of the possible motor combinations that could be used directly on the electrical prints.
Some motor manufacturers offer special or custom designs. These should be utilized only as a last resort in manufacturing equipment. Be aware of the lead time required if motor replacement becomes necessary at some point. Plant managers are usually not very understanding when machines are idle waiting on some special part to be built.
In a closed-loop servo motion control system, the motor’s actual position is compared with its commanded position. Typical feedback devices include resolvers, incremental encoders, absolute encoders and Hall effect sensors. Although not typically used in an industrial system, analog tachometers and rotary potentiometers may also serve as feedback devices.
Resolvers are simple rotary transformers. An analog excitation wave is supplied to a single primary winding. Two secondary windings are typically mechanically spaced 90° apart. As the resolver’s primary winding is rotated, the secondary winding generates sine and cosine waveforms that represent the rotational angle of the primary. Resolvers provide an absolute position whenever they are under power. However, electromagnetic interference (EMI) and radio frequency interference (RFI) may influence the analog output signals. Proper shielding is essential. Resolvers are rugged and can operate in the harshest environments.
Incremental encoders produce one to three pulsing signals. A single-phase encoder supplies x number of square wave pulses per rotation. A quadrature encoder outputs two signals that are 90° apart. With this information, the controller can determine rotational direction. A third square wave pulse, called the z phase, is output once every revolution. If power is interrupted for the encoder or the controller, a system using an incremental encoder would need to be “homed” before use.
An absolute encoder outputs a binary count, depending on the angular position of the encoder. Typical count values are 255 (8 bit), 1,023 (10 bit) and 4,095 (12 bit). An advantage of an absolute encoder is the encoder supplies its rotational angle upon power application. Mechanical and electrical multi-turn options can provide direct positional information to remove the need to home a system. A considerable price difference once existed between incremental and absolute encoders. This difference is becoming less and less.
Feedback signals from an encoder may be transmitted to the controller in several ways. The oldest, and perhaps the simplest based upon electronic component count, is a direct wire connection between the motor-mounted encoder and the system controller. A disadvantage of this approach is the low voltage susceptibility to EMI; and, if the encoder is an absolute style, increasing the resolution increases the signal wire count.
Manufacturers have developed proprietary serial communication protocols as an improvement over the parallel method. Some manufacturers have allowed their systems to be open standard, free to be used by anyone else. They include these three.
- Synchronous serial interface (SSI) was developed in 1984 by Max Stegmann, now Sick Stegmann. The SSI interface is based upon RS-422 standards and removed the need for multi-conductor cables and replaced them with two or three twisted-pair communication cabling.
- Biss, developed by iC-Haus, included concepts from SSI and Interbus. Multi-slave bidirectional communications using RS-485 uses one twisted pair and, optionally, one set of power wire. Additional registers in the encoder allow motor information and additional sensor readings, such as motor temperature, to be retrieved from the encoder at will.
- A rather recent development by Sick is Hiperface DSL. This open technology incorporates the motor supply wiring with the position feedback signals in a single cable. Additionally, it can provide motor information such as temperature, mechanical revolutions, diode current, motor specifications and serial numbers. Hiperface uses eight wires for communications. Two for RS-485 communications, two for power and four for 1-V peak-to-peak sine/cosine.
According to an informal, nonscientific poll of some maintenance personnel I’ve worked with over the years, the single most common source of failure in a servo system is the motor connection cable. This part should be the easiest to build. Some of these failures may be a misapplication of nonrobotic cables in a system where the motor movement causes the cable to flex. However, most maintenance people have told me, “If the servo stops, change the cable first.” I would suggest that, if you have an option to buy higher quality cable when the system is built, spend the extra money. The people who keep your machine running may never know, but they will not curse you for it.