By Steve Meyer
Steve Meyer is the President Solid Technologies
Strictly speaking, any control element that employs feedback is a servo. Therefore, what does the term “servo rated” mean when it is used in conjunction with a motor? And if used with a gearbox, does that mean that the gearbox must have an internal feedback device?
As shown in the diagram, lost motion in the gearbox cannot be measured or controlled from the encoder on the motor. Typically, an alternative control structure is used, often called “axis and a half” because the same axis uses two feedback devices, one for the motor/amplifier and one at the load. The load position is then based on the actual position as measured by the second feedback device, the “half axis.”
In its 187-page standards document, Industrial Control and Systems: Motion/Position Control Motors, Controls and Feedback Devices, the National Electrical Manufacturer’s Association defines a servo motor as “an electric motor that employs feedback with the purpose of producing mechanical power to perform the desired motion of the servo mechanism.” This definition is consistent with the earlier one relating to the use of feedback.
A slightly different view would be to look at all the aspects of motion control as “behavior.” Will a Position behavior or Velocity behavior create the desired motion? The NEMA definition hints at this in referring to the “desired motion of the servo mechanism.”
If the desired motion requires close coordination of two or more axes, as in the draw-a-circle application, then the motion profile requires both velocity and position behaviors. Accuracy of the position feedback, velocity feedback in the motor/amplifier, and the refresh rate of the control system will define how precisely the circular path can be generated.
Therefore, we can define a “servo rated” motor as one that uses feedback to more precisely govern its velocity or position. Other properties of the motor design, such as high pole-count, become important as they affect the accuracy of the motor at many levels. High pole count means that for a given motor diameter there are more magnetic poles in the circle.
Step motors have 50 magnetic poles on their rotors and 4 electromagnetic poles in the stator, which interact to create 200 step positions per revolution, making them very accurate.
In combination with a 5:1 pitch lead screw, the step motor can produce 0.001 in. linear motion without feedback, which makes them easy and inexpensive to use in many precision
As machining throughput increases, more-parts-per-minute (or hour) also translates into a need for higher performance from the motors and electrical system. A common metric for understanding the needed responsiveness is bandwidth. In such applications bandwidth is the ability of a system to respond to changing conditions. A speed control bandwidth of 10 Hz, common for most open loop ac drives, means that changes in load can be compensated for in 1/10th of a second. Bandwidth ratings for servo motors are typically 500 to 2000 Hz.
The “draw a circle” requirement may be fairly simple with small loads and lots of time. But as either speeds or loads increase, the demands on the equipment increase exponentially.
Combining servo rated with gearing
With respect to gearboxes, the term “servo rated” implies that a gearbox product is suitable for use with a servo motor. If the desired behavior of the system is position, then it makes sense that gearbox accuracy, defined as lost motion or backlash, becomes an important criterion of performance. But the more precise the gearbox, the more expensive it is. Therefore, you must balance the precision carefully against the cost of the components. Accuracy is always a component in motion control, whether it’s controlling tenths of an inch in a large hydraulic actuator or millionths of an inch in a semiconductor process.
Interestingly, the American Gear Manufacturer’s Association has no standards or guidelines that recognize “servo rated” terminology or gear designs intended for use with servo motors. Even though precision planetary gearboxes and other designs are “servo rated,” the AGMA ratings for gear design derive strictly from geometry of the gear tooth itself, whether spur, helical, spiral bevel or other. AGMA doesn’t provide guidelines for defining a “servo rated” gear design.
AGMA and most gearbox suppliers use a traditional rating method called “service factor” to evaluate the operating conditions that a gearbox will be subjected to in order to insure long operating life. Service factor relates to both the number of times a motor will be started and stopped, and the number of hours of operation in a twenty-four hour period. Heat and shock load work to limit the life of the gearbox and the service factor helps adjust for environmental conditions. The rating system, however, is limited to gear reducers designed for use with the traditional constant speed ac motor design.
Gearboxes have evolved from their primary role of speed matching, which was sometimes done with belts and pulleys in the early days of the industrial revolution. This hints at the most important issue when evaluating gearboxes for servo applications; input motor speed. Traditional ac motors run at speeds that are multiples of the line frequency, 60 Hz. The most common motors are 1800 RPM. Most gearboxes, therefore, are designed to run at these input speeds. Possibly because of this, early dc motors, which could have been designed for any base speed desired, were also designed for speeds of 2000 RPM so that they would be compatible with existing gearbox designs.
As servo motor designs evolved with higher torque density, smaller high-pole count motors at more RPMs could do the work of larger motors at fewer RPMs. This is exactly analogous to the evolution in automotive engine design. We run higher RPM smaller engines for more fuel efficiency, instead of the “old school” large engines at lower RPM. The connection here is the measure of work— horsepower— which we will deal with in detail in a future article.
Horsepower is an RPM dependent measure of work, so specifying equipment based solely on horsepower can lead to problems. Putting a 6000 RPM servo motor into a 2000 RPM rated gearbox is going to cause trouble. Newer gearbox designs, however, are rated for the higher input speeds. Thus, the term “servo rated” in connection with gearbox design would have some specific meaning, most likely that gearboxes will run at high input RPM.
The servo-rated term would also mean that the gearbox is more accurate; that is, it shows little backlash. A common behavior of a servo motor is positioning. Accuracy in gearboxes is based on measurement of the output shaft angle. Conventional gearboxes may have backlash of a few degrees at the output shaft, but since most applications are uni-directional (conveyor belts, etc.) the backlash is taken up at startup and is not a consideration. Since we are talking about high precision devices, the angular measurement is usually arc-minutes (1/60th of a degree) and sometimes arc-seconds (1/60th of an arc-minute).
Servo motors continue to be engineered for high speeds, putting more demand on gearbox manufacturers for better performance. This phenomenon is sometimes driven by the need to reduce size and weight in the target design. Some applications will continue to drive ever improving power-to-size requirements, so we can expect this trend to continue.
Some material for this article was contributed by Mitch Machelski, Cone Drive Textron; Charlie Fischer at AGMA; and John Mazurkiewicz, Baldor.
Quick evaluation checklist
Here are several factors to consider when evaluating motors and other mechanical systems for your application.
Check your supplier’s ratings. The information is not always on the front page of the catalog, so ask specifically what the rating basis is for the parts you are considering. If you are using a sizing program to evaluate parts, make sure the ratings and duty cycle requirements are clearly defined.
Motors are transducers of electrical energy input to mechanical energy output. Heat is always the by-product. It takes at least four hours of actual run time to reach thermal equilibrium. Make sure that your servo motor supplier has incorporated thermal performance in the sizing software.
When comparing several vendors, be aware that every manufacturer has different rating techniques. Use the vendor supplied sizing software from each supplier with the same application data to make a level comparison. This will insure that you don’t inadvertently misapply one vendor’s ratings with another.
With respect to gearboxes, the same rules apply. Check the vendor for input speed ratings and make sure the motor contemplated for the application does not exceed the rating. Call the vendor in case they do. Sometimes catalog data is very conservative and, if asked, a vendor may assure performance beyond the printed specifications.
The mechanical devices attached to the output of a motor and gearbox combination should also be evaluated for the same properties. Lead screws have very specific acceleration limits called “wind up” which restricts how quickly a load can be accelerated.
Belt drive actuators may also have speed or life expectancy limits based on the diameter of the pulley being used and the bend radius created in the belt. Belt vendors have developed many ingenious ways to improve belt performance, such as fiber composites in the belting material, so check with suppliers and see what they have that may affect your requirements.
Always evaluate in terms of duty cycle and life expectancy. In many situations the external business issues that determine the success or failure of a new piece of equipment include how many hours of operation before maintenance is needed. Duty cycle calculations that were done by the supplier as part of their sizing software help insure that the design will operate reliably and provide lasting value to your customer.
Filed Under: Gears • gearheads • speed reducers, Motion control • motor controls, Motors • servo