Robots: clearing up confusion about servo drives.

All so-called 'servos' are not equal in cost or capabilities. Some suppliers use the term more broadly than others. Don't get tripped up by loose terminology.

Despite costing more than pneumatics, servos are often hailed as the drive of choice for sophisticated and very high-speed automated parts handling. Yet, even as robot suppliers have eagerly added servo-driven models to their product lines to fill this niche, processors often lack enough detailed knowledge about servos to make informed buying decisions.

For example, a single supplier may offer a choice of servo, frequency servo, and CNC servo models. This can make it difficult for a molder to judge the cost-effectiveness of an electric versus pneumatic drive on either one, two, or three axes--especially if robot suppliers use different servo terminology.

The textbook definition of "servo" is straightforward. It is a means of achieving closed-loop control of position, velocity, and acceleration through the use of a feedback device. However, a host of available types of "servo" controls make important differences in a robot's capability and ease of use. And the loose terminology that often characterizes this technology has clouded the boundary between servos and other types of electric drives.

ELECTRIC MOTOR TYPES

Electric drives on robots can be servo or a-c variable-frequency varieties, the latter commonly referred to as "frequency drives." Although they share some capabilities, there are also important differences. Both servo motors and frequency-drive motors are capable of ramping up and down in speed, and both are "teachable" when the control is configured for that purpose.

Yet servo motors are much more efficient than a-c frequency motors. "True" servo drives, in the view of some suppliers, use high-performance servo motors and are capable of higher top speeds and faster acceleration and deceleration than a-c frequency motors. This is an advantage when complex motions inside and outside the mold must be accomplished during short cycles, according to John Campbell, national sales manager of Ranger Automation Systems, Shrewesbury, Mass.

Servo motors can be of the d-c brush type but are more typically a-c or brushless d-c. The latter two are essentially the same: Both are electronically commutated to move the magnetic field in the motor windings, and both avoid the maintenance requirements and wear problems of brushes in classic d-c motors.

One major difference between servo motors and frequency-drive electric motors is torque. Servo motors are variable-torque motors and will maintain a given torque under different motor loads. For example, if a servo motor is programmed to carry a 10-lb load at 20 in./sec, it will maintain the same acceleration and speed for a 20-lb load. A servo motor will also maintain some torque even in a static power situation; this gives it high positioning accuracy and allows it to hold its position even without the use of a friction brake.

Frequency drives control the speed of standard induction motors by varying the a-c frequency to the motor. Frequency-drive motors are constant-torque motors and so cannot maintain the same acceleration rates or top speeds if the load changes. Unlike true servo motors, a frequency motor requires a friction brake to maintain its position at zero velocity.

FEEDBACK DEVICES

Both servo motors and frequency-drive motors require some sort of position-feedback device, which can range from simple limit switches to encoders and resolvers. Position-feedback devices are one common source of misunderstanding, according to Tony Mass, v.p. of sales for AEC/Application Automation, Wood Dale, Ill. Basic feedback devices such as limit switches can only indicate position and are not programmable, Mass explains. "Some companies say they supply a servo system, but they are only using switches for feedback positioning. They can't do much more with that than they can with a pneumatic cylinder."

Encoders and resolvers are feedback devices that tell the controller where the robot is at any point in time. Both have the important advantage of programmability from a remote device. Encoders are rotary position transducers that operate by counting light or magnetic pulses. A resolver is a rotary transformer that puts out a variable voltage signal and is typically more expensive than an encoder. However, a resolver may allow robots to achieve greater speeds with higher accuracy, according to Mark Tetreault, engineer at Conair Martin in Agawam, Mass. Conair is one supplier that's moving from encoders to resolvers on its robots. Not all suppliers agree that there is a meaningful difference between the two choices.

Encoders may be either "absolute" or "incremental." Most resolvers are absolute. The difference is that an absolute device "remembers" its position after the power is turned off. An incremental encoder goes through a homing sequence when it is turned on, but cannot remember its position after power is turned off.

CONTROLLERS

The common PLC-based control has the advantages of being simple to and programmable through familiar ladder logic. One limitation of the PLC, according to AEC's Mass, is that it typically can control only a sequence of operations, not speed or acceleration.

Historically, logic controllers and motion controllers have been separate devices, according to AEC application engineering manager James Schmitz. Recently, however, he has seen a trend toward combining the two functions, in which servo motion-control cards are plugged into a slot in the PLC rack. "It allows you to use the PLC to control the motion," he says.

One robot supplier that uses this approach on its servo robots is Husky Injection Molding Systems Ltd. of Bolton, Ont. Husky's a-c servo motor is equipped with an absolute optical encoder, and basic control is provided by a PLC with a servo motion-control card. On top of this, Husky adds an operator interface that allows control of the robot without resorting to a complex programming language, according to product manager James Rodrigues. One advantage of this approach, he notes, is that the PLC easily integrates robot control with the company's injection molding machine control.

Some robot manufacturers supply dedicated microprocessors on their robot controls that have many of the same functions as a PLC. According to Campbell of Ranger Automation, "These are not PLCs; they are a microchip that has been pre-programmed." These controls often come in the form of a hand-held pendant or other operator interface that offers a standard selection of sequences or positions. They lack the full programmability of PLCs.

A true servo controller TABULAR DATA OMITTED is fully programmable for sequence, position, and speed, says Mass of AEC. Basic servo controllers (using PLCs or dedicated microprocessors) can control more than one axis, so the axes can operate at the same time. Each axis, however, runs independently of the other, so this type of control is not capable of coordinating movement between the axes.

Most suppliers consider the highest level of controller to be a CNC servo controller. CNC controllers integrate motion and logic controls, which permits the use of a single controller for sophisticated sequences. The sophistication of CNC controls gives the ability to link motions together, says Mass. A CNC servo control can program x and y axes, for example, to follow a definite path or curve in space and finish at the same time, even though one is traveling a greater distance than the other. (Controllers can be configured to coordinate simultaneous movement of any number of axes.) "This is where you can really start linking motions together," Mass says. "If I am pulling a part and have a tricky curve to pull it out on, I can do that. This is not the case with just a plain servo controller."

One drawback of CNC servo controls is that they typically use complex programming languages. However, many suppliers of CNC controllers offer some sort of "teach" pendant to simplify the robot's operation. The user simply leads the robot through a sequence of motions at low speed, and the pendant's built-in intelligence translates the moves into programming commands. Such a "lead-through" technique makes the CNC programming invisible to the user.

Robot makers see no tradeoff between ease of use and flexibility. "A good robot is freely programmable and easy to use. It shouldn't cost more to get those features," says Kenneth Heyse, sales manager of Wittmann Robot & Automation Systems in Torrington, Conn.

A SYSTEMS APPROACH

Suppliers agree that much of the confusion surrounding servo robots stems from the many ways the various basic components--motor, feedback device, and controller--can be configured together. According to Ranger's Campbell, "Servo defines a combination of motor and controls, consisting of an actual servomotor and an encoder in a closed-loop feedback system. It gives a robot the ability to repeatably find a position it has been taught to seek."

Several suppliers consider it a misconception to view a-c frequency drives that are equipped with an encoder as a servo system. According to Husky's Rodrigues, "People call it an a-c frequency-controlled servo, but there really is no such animal. It's not a servo control; it's closed-loop control on position and open-loop on speed and acceleration."

Typical a-c frequency drives are limited to point-to-point movement and are not capable of complex motions, says AEC's Schmitz. "The drives cannot talk to each other, so you can't necessarily coordinate the motions on the axes. Because the motors can increase and decrease torque, true CNC servo robots can tell the axes to start at one point and go to another point. The axes continually communicate back and forth to stay on a coordinated path," he adds.

Campbell expresses a similar view. He says the key capabilities of a true servo robot are high speed, teachability, and simultaneous motion. Most servo robots are equipped with individual motor drives capable of communicating with the controller to provide simultaneous motion. By contrast, he says, a-c frequency drives on robots generally are not equipped with individual motor drives, and are not capable of simultaneous motions.

However, a-c frequency drives with feedback via encoder--termed a "frequency servo" by some--have an advantage in cost. Pat Moran, product manager at Sterltech Div. of Sterling Inc. in Milwaukee, notes that frequency drive with encoder on all three axes today can cost 40% less than CNC servo drive on all three axes--and only 10-12% more than CNC servo on the traverse and pneumatic drive on the other two axes. Several suppliers agree that frequency drives can suffice in a large proportion of today's applications--i.e., where programmable positioning is more important than maximum speed or accuracy in velocity and acceleration. Rodrigues says Husky has gotten away from frequency drives in favor of CNC servo because the trend appears to be in the direction of more demanding robot performance requirements. He also sees the cost difference declining--perhaps by half.

Norton Kaplan, product marketing manager of Automated Assemblies Corp., Clinton, Mass., says his company's new Robot/Workcell Controller, introduced at NPE '94 in June, offers flexibility equal to or greater than that of CNC controls. This multi-microprocessor servo controller provides multi-axis coordinated motion, as well as programmability and teachability. It can also be data-driven--e.g., the robot can "track" a moving mold or part by means of an external encoder.