Robots for the '90s: why servo is the way to go.

Throughout the 1980s, injection molders in North America have been warming up to the idea of automated parts removal with pick-and-place robots. Nonetheless, the acceptance of robotics in domestic plastics manufacturing still has a long way to go. Estimates are that only 10-20% of injection molding shops in North America are using robots. In Japan, 90% of molding plants already are robotized to some degree, as are 60-70% in Western Europe.

In these more mature robot markets there is a trend toward servo-driven robots with flexible controls. Molders in these markets have already gone through the stage of employing simple pick-and-place pneumatic robots and then asking themselves, what else can be done with a robot? This market pressure has led to the development of servo robots. In fact, servo control combined with automatic tool change has been used to create total manufacturing flexibility.

Prior to its introduction on injection molding robots, servomotor control technology was prohibitively expensive. Servomotors have been used extensively in general industrial robots for applications such as spray painting and welding in the automotive industry. It was the development of these robots that created a multitude of affordable motion-control products which are now used in top- and side-entry robots for injection molding.

Many molders in North America that do not use robots now have an opportunity to step back and look at the progression to servo robotics occurring in Japan and Western Europe. In an effort to remain globally competitive, domestic molders should consider making the jump directly to servo robots instead of following the same pattern that has already taken place in those other markets. Why go through the same progression, only to find that in five years you will still be five years behind in technology?

To start out by purchasing servo robots, the inexperienced user must consider aspects such as what is a servomotor, the advantages of servo drives, ways to use controls, determining the right application, automation objectives, and the modularity of the vendor's robot product line. Given the basic fact that servos cost more than pneumatics for robot actuation, the above factors must be considered in order to arrive at a cost-effective choice.

WHAT IS A SERVO?

Within the past year there has been a tremendous proliferation of robot manufacturers offering new servo-driven models. "Servo" has become the latest buzzword in the industry, and as is often the case with new trends, some people have little or no knowledge of what a servomotor is.

First, the term "servo" does not refer to the type of motor used but the control system for the motor. A servo system is a motion-control system comprising an amplifier, a motor, and a feedback device. This feedback device is used to supply performance information back from the motor to the amplifier. Thus, a servo system is a self-contained closed-loop control system.

The accompanying schematic shows the components of a basic servo system. A command signal from an external control device is applied to the input of the drive amplifier. The amplified power output is applied to the motor to cause the desired motion or rotation. The feedback device, normally an encoder internal to the motor, then informs the amplifier of the actual performance so that changes can be made in power output to the motor to achieve the desired motion. A second feedback device, normally a motor-mounted encoder, is shown in the figure at right as external to the servo system. This allows position and velocity information to be supplied to the controller, enabling it to modify the servo-system input signal as required.

TYPES OF

SERVOS

Servomotors can be of three types: conventional DC motor, AC servomotor, or AC induction motor. Of the three motor types, the most commonly used is the AC servomotor, because of its superior performance advantages. AC servo-motors are sometimes, but less often, referred to as brushless DC motors. The reason for the dual name is that the motor construction resembles an AC motor design but is powered by modulated DC current. The robot purchaser should make sure the servo robot being considered is equipped with this motor type. The advantages of this motor design over the others are:

* Reduced size.

* Low maintenance.

* High-torque performance via rare-earth magnets and magnetic ceramics used in the rotor construction.

* Higher acceleration rates due to the lower rotor mass.

The only disadvantages is the higher cost of the AC servomotor.

In spite of the perception that servo drives are complex and a maintenance liability, AC servomotors are extremely reliable. There is no mechanical commutation in the motor and so there are no brushes to wear out (a disadvantage of conventional DC drives). The motor is sealed and the bearings are lubricated for the life of the motor.

Some robot vendors may use a less costly AC induction motor as part of a servo system and market the robot as a "servo robot." Technically it is a servo robot, but because of the induction motor's inferior acceleration, torque, and accuracy performance relative to the AC servomotor, the description is misleading.

WHEN TO USE SERVO

A servo drive should be specified if the motion required must be very fast, infinitely programmable, and/or extremely accurate. Robots equipped with servomotors have many performance advantages. Though most servo robots are similar in design to pneumatic robots, the flexibility afforded by the servo's programmability makes it possible to design extremely versatile robots such as turret-style top-entry robots, the flexibility afforded by the servo's programmability makes it possible to design extremely versatile robots such as turret-style top-entry robots that can remove parts to either side of the injection molding machine (see photo on previous page). By combining motions of different servo axes, the robot can perform many different operations next to the machine, such as palletizing. This flexibility can potentially eliminate some downstream handling equipment that would be necessary if only a simple pneumatic pick-and-place unit were used.

High-speed take-out applications are ideal for servo robots because the AC servomotor is capable of producing maximum torque over virtually its entire speed range and providing controlled deceleration, which is another essential element in high-speed operation. These motors, because of their permanent-magnet rotor construction, have very low inertia for a given torque and horsepower performance. This makes them very responsive and capable of very high acceleration rates.

The same torque and horsepower characteristics that make servos ideal for high-speed operation make them well suited for robots designed for high payloads. Robots have been designed for maximum end-of-arm tool payload capacity of 550 lb for removal and cooling of injection molded PET preforms from high-cavitation molds. This type of robot would be useful for other applications, such as lost-core injection molding where heavy metal insert must be accurately placed inside the mold.

The precise positioning accuracy of the AC servomotor is useful for some specialized insert molding applications. As an example, insert molding of automotive multicolored lenses can require that the insert be placed in the mold within [+ or -] 0.0005 in. Traditionally, this kind of insertion was done by a person who could hold the part and hunt for the location where the insert would fall into place. The use of servo robots in this application can save up to 20 sec in cycle time and reduce scrap caused by misplaced inserts.

'FRIENDLY' CONTROLS ARE KEY

Most pneumatic robots have relatively simple controls, which make them more attractive to first-time users, as compared with some of the more sophisticated servo robot controllers. Simple-to-use controls that do not require a high skill level to operate are essential for all servo robot users, regardless of experience. If the robot controls can only be operated by a skilled electrician on the day shift, then a job changeover on the third shift may turn out to be a shutdown instead.

Fully integrated controls can provide the highest degree of user friendliness. "Fully integrated" controls describes a system where the robot runs off the injection molding machine microprocessor. The machine operator interface CRT has between one and three robot screens for operation and troubleshooting (see photo at left). This level of integration is only possible if the robot and machine are from the same supplier, which has the added benefit of a single-source responsibility. The supplier can review your robot requirements to determine all of the possible variations, present and future, and design the robot screens around this information.

The end result is similar to machine setup screens. When you set up a machine for a particular mold, you do not have to actually program the machine; rather, all you have to do is put appropriate values into input fields (a "fill-in-the-blanks" technique) and make the appropriate option selections. The same can be done for the robot with fully integrated control. This is unlike some robot controllers, which actually require you to program the robot sequence, positions, and functions, making them more difficult to use. The latter controllers also require that an additional robot interface be added to the injection molding machine.

A further benefit of a fully integrated control scheme is the reduction of robot cycle time. By having the same PLC control the robot and the injection molding machine, it is possible to coordinate their motions. A servo robot will have an encoder feedback for position, and the clamp on the machine will have a linear position transducer. By knowing where the robot and clamp are at all times, it is possible to begin robot entry prior to reaching the full mold-open position and begin mold close prior to the robot reaching the full out position. With a freestanding robot controller, this type of operation is not usually possible, because there are two controllers with a very simple interface between them. The machine controller can only communicate discrete signals to the robot controller and vice versa. As a result, for safety reasons, it is only possible to start robot entry when the mold reaches the full open position, and to start mold close when the robot is known to be in the full out position.

SERVO COST JUSTIFICATION

When selecting a robot application, why just pick-and-place to a conveyor, which then feeds the part to an operator or another robot for post-mold-operations? Have the robot on the machine do it all. If you are at the point where you have justified the purchase of a pneumatic pick-and-place robot based on a consistent cycle, part quality, and, in some cases, cycle reduction, then justifying a servo robot will not be difficult if it will eliminate downstream automation equipment and/or manual operations.

In general, a full three-axis servo robot will cost about 50% more than its pneumatic counterpart. One with two servo axes and one pneumatic will cost somewhere in between. Here's one example of an application where a three-axis servo robot will pay for itself. Consider the situation where a pneumatic robot places the molded part onto a conveyor, which then moves it to a waiting downstream robot, where it is degated, hot stamped, and stacked. A single servo robot would be capable of taking the part out of the machine and either cutting the sprue in the end-of-arm tool or on a floor-mounted cutter, placing the part in a hot-stamping machine,

SAMPLE COST JUSTIFICATION
(7000 HR/YEAR)
Robot Drive                   Penumatic   Servo
Capital Cost:
Press-Mounted
 Robot                        $50,000     $80,000
Conveyor to
  Next Robot                   10,000     --
Downstream Robot               30,000     --
Total Capital                 $90,000     $80,000
Operating Costs:
Robot In/Out
 Time(a)                      3 sec       1.5 sec
Average Molding
 Cycle                        20 sec      18.5 sec
Molding Machine
 Rate                         $50/hr      $50/hr
Cycle Savings                 --          $26,250
Robot Down-
 Time(b)                      3%          1%
Downtime Savings              --          $7000
Total Savings
 Per Year                     --          $33,250
  (a) Including part pick-up.
  (b) Downtime for robot setup during
mold changes (not unscheduled downtime).

and then stacking the part of an indexing conveyor. This conveyor could inventory stacks of parts to allow a half hour of unattended production before the operator would be required for inspection and packing.

The operator might have to spend five minutes at the machine every half hour. This allows the operator to spend time at other machines. For an additional investment it could be possible to have an automated caron indexing conveyor to allow the robot to pack the parts, as well.

In this example, the servo robot justification would be based on direct labor savings, increased production time, and reduced cycle time relative to the pneumatic robot. The additional production time would be a result of reducing the total job changeover time when making mold changes. For frequent mold changes, servo robots have the advantage of permitting setup information to be stored in the micro-processor controller for instant, automatic recall. By contrast, a pneumatic robot requires manual setup of stop positions and perhaps also the robot operating sequence.

A sample cost justification for the application described above is shown in the table at left. This scenario is based on 7000 hr/yr operation.

HOW MUCH SERVO

DO YOU NEED?

As a robot purchaser, you should look for a vendor with a modular set of specifications, allowing you to purchase only as many servo axes as you need. With this flexibility in specifying the type of robot drive, you need to evaluate the application axis by axis. For example, after studying your particular application, you might determine that you simply need a servo on the vertical take-out stroke for high speed and part stacking. The traverse and strip stroke could use point-to-point pneumatic or electric drive, because speed, accuracy, and flexibility are not important after the part is out of the mold in this application. In this situation, it would not be cost-effective to purchase a robot with three servo drives just for the sake of one of the axes' requirements. This will keep your investment down without giving up robot performance for the application.

You may question whether this approach might limit your future operating flexibility. If a servo drive is not required today on a particular axis, and a robot is purchased for that specific requirement, what happens two years from now when another servo axis is needed? This is a valid concern, but if the vendor's robot line is of modular design, it could be possible to retrofit a servo drive for an axis that was originally penumatic. In this way it is possible to amortize equipment capital cost over the part production that actually requires the servo drive on the robot.

This again raises the question, when do you need a servo drive? If there is a need for either very high speed, high accuracy, programmability of position, or a short overall cycle time (which increases motor heating), then a servo drive is a wise investment.