A labelling applicating head operates by taking a stationary strip of material with labels on it, accelerating it to a given speed, and moving it till the edge of the label is detected, and then decelerating the strip to a stop. This acceleration and deceleration sometimes needs to be from zero to 200 feet per minute in a mere 10 msec. (this is about three times faster than an AA fuel dragster's acceleration or the ability to reach 230MPH in a second or a 1000MPH 1/4 mile).
The consistency of a label placement on a moving product is dependent upon how consistently the label accelerates with each firing of the head. If the acceleration of the label is slightly different each time then the position on the product will be considerably different. It is the point on the product that the label first strikes that controls the placement of the label.
Furthermore, labelling wipedown and placement accuracy is irrevocably dependent upon the speed of the label matching the speed of the product. Only a very small amount of slippage between label and product can be tolerated. This means that slow speed applicators can not apply labels to fast products. As well, if the acceleration of the label time is greater than the length of the label, the label placement accuracy suffers greatly. The end result is that a short label to be applied to a fast moving product (>100fpm) is impossible for most labelling equipment. (these general statements on the performance of labelling heads is based on years of experience working with many makes and models of applicators.) The reasons these problems exist will be compared and explained below.
Label applicating heads have had a variety of mechanisms tried for driving a pressure sensitive label. The two most common of these are:
- CLUTCH AND CLUTCH/BRAKE
- STEPPER MOTOR
Clutch/brake systems have maintained the industry for at least the last 30 years. Clutch/brakes are composed of plates and friction surfaces that are pressed together to provide either drive or braking. (the clutches and brakes of a labelling head are not that different in operation than those of your car). These are most commonly electro-magnetically controlled. They may be electrically controlled in differing ways but they all operate on the basis of friction between rotating parts being used to create acceleration and deceleration. Mechanical friction is the source of most of the problems with clutch brake systems:
- friction creates heat and being deep inside a housing it is not dissipated well. We have done experiments on taking clutch/brake systems to their limit and discolouration of the internal components meant that the clutch parts and motor shaft reached between 500 and 570 degrees Fahrenheit;
- the amount of friction generated affects acceleration and the friction is a function of temperature. As the temperature changes the placement changes;
- friction creates wear on the friction surfaces and hence any clutch/brake system requires regular servicing;
- the surface finish of the parts and the air gap between the friction surfaces are critical to the performance of the clutch brake, for these reasons there is an expertize level that confounds many packaging maintenance mechanics;
- the friction coefficient of the clutch is very susceptible to contamination by oils (common from failed bearings in the system or failed gearbox seals).
Another range of problems is encountered in the response of the clutch/brake. The clutch/brake parts are moved and held in contact for drive by electro-magnetic coils. There is a rise time that exist as these coils come up to full strength and then a time for the parts to mechanically move and slip till they are lock together to provide either drive or braking. Typically this time is measured as the time to reach 90% of the rated torque. A typical labeller clutch or brake will take 30 milliseconds to reach this value (this may sound quick but at 100fpm this is 5/8" of travel). During this slipping phase of 30 milliseconds. the clutch/brake is very susceptible to changes in load. These changes drastically affect the amount of travel of the label and thus the placement on the product or the stopping position of the label in the applicator.
Now let us consider this slip phase of 30msec. at 200 fpm. The product will travel 1 1/4" while the label is trying to accelerate, if the label is only 1" long then it is doubtful that the label will even reach 200fpm. We then have to stop it, another 30msec. It can be quickly seen that there becomes a point where the speed is either not reached or the response of the system is so slow that only long labels can be used.
Furthermore, the friction versus the heat dissipation rate puts an extreme limit on speed and cycling rate (heat increases with the square of speed).
The combination of these issues and high service requirements have lead to industry to seek alternatives. The costs and simplicity of stepper motor drives has lead many label applicating companies to go that route. We have not wasted our time following this path but instead we have gone directly to using brushless servo technology.
A brushless servo motor is based on the concept that a micro processor controller can energize the windings of a motor in the same way as the carbon brushes do to a DC motor (through the copper lands of a commutator). In the DC motor, brushes pass from commutator land to land, sequentially energizing the motor windings. In the servo, the "commutation" is done by a microprocessor which sequences the energizing of the coils which are in continuous connection with the controller. This continuous connection means servos can run at much higher RPMs than DC motors. Also, higher currents can be used because the breaking contact of a carbon brush on a commutator land arcs when the current is too high. Being an electronic motor, the commutation is done on outside coils. Commutation is the area with the most heat, so it is a much shorter path to dissipate.
To create the "intelligent commutation", the controller needs to know exactly where the motor armature is in its rotation. For this purpose, an encoder or resolver is built into the motor to provide the feedback on armature position to several thousandths of a revolution or as small as 1/,000,000 of a rotation.
A signal going into a servo motor looks like a 3 Phase AC motor signal except that the offset between each of the phases is controlled to control the torque and position of the motor and the frequency is changed to control speed.
A stepper motor energizes coils to move the armature one "step" at a time. Most stepper motors are built so that there are 200 steps per revolution. By creating a steady train of pulses, the motor turns at a fixed speed. this makes for a very coarse motion so techniques have been developed where more than one set of coils is turned on at a time to create partial steps. The problem is that the creation of the pulses requires considerable current and this current is a strong function of frequency. This limits stepper motors - one must trade off speed and resolution to keep the current to reasonable levels.
What stops the armature at the end of the step is the turning off of the coil, the remaining coils brake the armature. If the inertia of the load is too high then the armature overshoots. Similarly, if the load is too high then the motor skips on start-up and locks-up. This means that it is difficult to get consistent start and stop action over a wide speed range with a stepper motor. It is necessary to tune the controller for a desired speed range or excessively oversize the motor. It is a typical rule of thumb to size a stepper motor to three times the required torque. Occurrence of over-shooting or over-loading easily results in the pulse train being out of synchronization with the actual armature position.
Furthermore, steppers do not normally have feedback to tell the controller where they are or whether they are turning as expected so compensating for loads is done blindly. (In contrast, a servo sees minute errors and compensates. Stopping is active, so much so that if required, the "commutation" is shifted so that the motor is instantaneously in the equivalent of being driven three times the speed in reverse to get the armature to stop where it is suppose to).
- capable of high speed (100 - 350fpm) (system inertia 10:1)
- motors function well when sized to equal load
- positioning accuracy to minutes or seconds of arc
- torque substantially constant over operating range
- resolution and waveform allow smooth motion at all speeds
- motion maintained over wide speed range
- systems designed to maintain motion profiles
- motor maintains desired motion despite changing load
- active stopping allows very high cycling rates
- controllers more complicated and larger
- large number of wires required (encoder - speed matching devices mostly)
- costs substantially higher than all electric drive units
- top speed less and top end achievable only at lower resolutions, loss of torque (difficulties begin at 100fpm limit near 200fpm)
- motor must be sized three time torque required for job
- accurate to 1.8 degrees or tens of minutes of arc
- torque drops off with increasing speed
- motion is a series of high speed jumps thus tends to be vibratory, smoother with micro-stepping at low speeds but microstepping done at expense of torque
- certain combinations of load, acceleration and speed create harmonic problems, loss of synchronization and rotor lock-up
- designed for accurate point to point movement
- motor changes acceleration or locks up under changing load
- stopping passive (by lack of signal) limits cycling rate
- controllers simpler and smaller
- only 3 to 5 leads required
- costs only slightly higher than DC PM if not equal
- 100 fpm normal limit but 250 fpm has been reached if one is willing to rebuild clutches every few weeks
- typically DC brushed motors are used and are actually undersized for the task because of the high rotor inertia of a constantly spinning rotor
- accuracy a function of speed and consistency of load
- torque a function of clutch condition
- motion is a function of clutch response smoothed by slippage
- motion profile a function of speed and clutch condition
- motion profile a function of speed and clutch condition
- changing load during acceleration and deceleration causes considerable variation. Clutch will maintain drive up to breakaway torque but speed is a function of motor characteristics
- active braking but accuracy a function of speed and consistency of load
- control can be as simple as a power supply, a latching relay and a motor drive
- 3 wires for clutch brake plus whatever is required by motor
- cost is a function of clutch enclosure design
