A reliable, autoloading, streaming half-inch tape drive - technical

A Reliable, Autoloading, Streaming Half-Inch Tape Drive

HP's NEWEST HALF-INCH TAPE DRIVE, the HP 7980A (Fig. 1), is an autoloading, reel-to-reel, horizaontally mounted, streaming drive that reads and writes in two standard nin-track formats: 6250 GCR and 1600 PE. This maintains compatibility with previous drives and tapes, and important feature because half-inch tape is still a significant standard in the computer industry for backing up, archiving, and interchanging computer data.

The HP 7980A provides computer system users with a reliable, low-cost, backup device for large amounts of online disc memory. It does this with higher performance and reliability and lower cost than its predecessor, the HP 7978B, which it replaces. IT reads and writes at 125 inches per second, roughly 60 percent faster than the HP 7978B. The HP 7980A can rewind a 2400-foot tape in less than 90 seconds, reducing overall data transfer times significantly. The HP 7980A is 40 percent more reliable, a result of the increased use of VLSI components to reduce parts counts even further than was achieved on the HP 7978B. The monthly maintenance cost of the HP 7980A is half that of the HP 7978B. Horizontally mounted in a standard-width rack cabinit, the HP 7980A is 8.75 inches high, a third the size of the HP 7978B. This saves valuable floor space and allows better use of rack cabinets.

The horizontal mounting means that the user normally cannot access the tape path to load the tape manually. Hence, an autoloading feature was designed into the HP 7980A. The operator simply places the tape reel in the door opening and closes the door. The autoloading sequence starts automatically once the door is closed, leaving the operator free to do other tasks while waiting for the tape to load nominal time of half a minute. There is no need for an EZ-LOAD cartridge around the tape reel, which is required for autoloading on the HP 7976A Tape Drive. The HP 7976A autoloads only 10.5-inch tape reels; the HP 7980A autoloads all standard half-inch tape reels from six to 10.5 inches in diameter. The earlier HP 7978A/B Tape Drive must be loaded manually and cannot autoload tapes.

In addition, the HP 7980A supports the use of 3600-foot half-inch tape, under certain guidelines, which the HP 7978B doesn't. This increase data capacity by 50 percent over standard-length tape reels.

The HP 7980A was developed and released in 40 percent less time than the previous tape drive. This was mainly a result of two factors. The first was keeping together an experienced core group of engineers from the HP 7978A/B development team to architect and design the HP 7980A drive. The second was concentrating on a core system development of the HP 7980A, that is, having a well-defined product and not adding additional features and configurations along the way. Such added features tend to prolong development cycles. This does not mean that these additional features are not eventually added, but they are worked on according to need after the core or base system is released.

Tape Path

The HP 7980A is a totally integrated tape drive, simultaneously incorporating a small form factor, an autoload feature, design-for-assembly concepts, low cost, and high reliability. The major design objective established to accomplish these goals was design simplication.

The HP 7980A has a very simple tape path as shown in Fig. 2. There are only two rolling elements: the speed sensor and teh buffer arm roller. There is only one additional fixed tape guide. The oxide side of the tape contacts only the tape cleaner, the magnetic tape head, and the tape displacement unit. The tape displacement unit, located between the tape head and the tape cleaner, contacts the oxide side of the tape only during respositioning and while the tape is stopped. The tape displacement unit pushes the tape off the very smooth surfaces of the head and the tape cleaner to prevent the tape's sticking to these smooth surfaces during high temperature and humidity conditions.

The buffer arm assembly (buffer arm, spring, and roller) helps take up slack in the tape during servo starts and stops. It also establishes the tension on the tape. The buffer arm roller and fixed guide, along with the speed sensor, guide the tape in a precise manner over the head. The speed sensor measures the velocity of the tape and feeds it back to the servo system.

The half-inch tape reel is centered, seated, and locked by the supply hub. The autoload blower forces air through the door. The louvers in the door direct the air onto the tape reel to lift the end of the tape. The tape end is then carried by the air flow around the buffer arm roller and the fixed guide. If then goes over the tape cleaner and the magnetic tape head. The tape is finally sucked onto the take-up reel after passing around the speed sensor.j

The drive motors are located directly beneath and are attached to the supply and take-up hubs. The blower is located between the two drive motors.

Integrated Autoload and Tape Path Design

The HP 7980A tape path is very simple. This greatly reduces costs and assists the design of the autoload mechanism. It simplifies the task of blowing the tape end off the supply reel and threading it through the tape path and onto the take-up reel.

The casting is an integral part of the tape path and autoload mechanism. The walls of the casting help determine the way the tape is blown around by the air flow created by the blower. The surface roughness of the casting is very important. The surface of the casting around the supply reel area must provide enough firction to permit the supply hub to autocenter the smaller tape reels. Conversely, the casting surface near the buffer arm assembly must be smooth enough so that the tape doesn't catch on any surface features while autoloading. The buffer arm shape is critical to autoload and servo success. The design of the air deflector on the speed sensor is crucial to how well the tape end moves around the speed sensor roller and attaches to the take-up reel.

The autoload success rate is greatly affected by the design of the door ramp and the door louvers. These are designed so the HP 7980A can autoload all standard half-inch reels with varying amounts of tape on each reel.

Holes in the top cover eliminate the need for gaskets or tight tolerances on the door-to-bezel fit and the top cover-to-casting fit. They do this by eliminating air flow reversal, which can occur when autoload air flows out the door-to-bezel and top-cover-to-casting cracks, rather than down the tape path. The ribs on the top cover fit inside the casting to reduce air leakage and allow loosening of the top-cover-to-casting fit.

The speed sensor is placed in the tape path for maximum tape wrap on the roller. This prevents the tape from slipping on the roller during tape acceleration and deceleration.

The buffer arm and its spring are designed and placed so that tension variations caused by the tape coming off the supply reel at different angeles relative to the arm are minimized. An additional roller could have been used to feed tape at a constant angle to the buffer arm, but was not put in for several reasons: cost, simplicity, and ease of autoloading the tape.

Air System Design. The autoload air system was designed into the deck casting and the tape path from the beginning. Reduction of air ductwork was a major consideration. The less the air has to pass through winding passages, the lower the air pressure and volume requirements. For this reason, the tape path needs to be simple. Reducing the air pressure and volume reduces the autoload air blower's size, noise, and cost.

The blower is placed between the two drive motors since that provides the shortest air passage. The air is sucked into the take-up reel, into the blower, and then blown out of the blower into the door area to repeat the path (see Fig. 3).

The door is used as an air passage for several reasons. No space is taken up by air ducting to blow air into the tape path, thus allowing the maximum amount of width for the card cage. In addition, using the door as an air duct allows the air to blow across the entire front of the tape reel for maximum design flexibility. The door louvers were then designed to blow air to autoload all standard reel sizes.

The air velocity required to move the tape down the tape path was calculated from momentum and Bernouli equations. The pressure drop was calculated in a similar manner. The blower was then selected based on these calculations and the tape path, with the blower, was measured for pressure drops and velocities. The measurements were fairly close to the theoretically calculated pressure drops and air volumes.

Autoload Algorithm. The HP 7980A autoload algorithms are designed to load all sizes of half-inch tape reels in a minimum amount of time, while making sure that the user's tape is treated with as much care as possible. The autoload process is a combined effort of firmware, mechanical design, and electronic sensors. Four sensors are used by the autoload process to monitor its progress and to indicate when an error condition is present. The first of these sensors is the door sensor, which consists of two microswitches connected in series that detect whether the front door or top cover is open. When this sensor detects a door closure, the autoload process begins. Opening the top cover at any point halts the autoload. Under the supply hub is the reel encoder sensor. This optical sensor detects the three reel encoder flags attached to the supply hub. These flags interrupt the sensor beam when a reel is properly seated on the supply hub. The tape-in-path sensor is an optical sensor which is positioned across the tape path preceding the head. The tape leader will interrupt this sensor beam when it enters the tape path. The final sensor is the optically encoded speed sensor. This sensor provides two quadrature pulse trains which are decoded to give position and velocity information.

Once a door closure is sensed, the autoloading operation begins. The first task is to detect whether a reel is already threaded through the tape path, a condition that is most likely to be present after a power failure. This condition is detected by the tape-in-path sensor and by turning both the supply and take-up motors in opposing directions at a low rate. If a tape is alrady threaded, activity will be seen on the speed sensor. In this case, the tape does not need to be automatically threaded and the servo loops can be closed.

If a tape is not already threaded, the load fan (blower) is turned on and the supply hub is slowly rotated in a counterclockwise direction to center and seat the reel. At this point, the reel encoder sensor is checked continuously for evidence that the reel is seating itself on the hub properly. When the reel is seated correctly, all three reel encoder flags will interrupt the sensor beam once per revolution. Time interval measurements are made to ensure that all three flags (not just one or two) are present, and that they have the correct relationship. If all three flags are not detected, the supply hub is shaken back and forth quickly in an attempt to get the reel to locate itself properly on the hub. A reel that will not properly seat after shaking a second time will be rejected and a MISLOAD will be reported.

The reel encoder sensor and flags are also used to regulate the speed of the supply hub during these open-loop operations. The dc motors must turn at relatively low speeds when centering a reel and feeding the tape. These speeds require relatively low voltages at the motors. Because of voltage offsets, motor constants, and temperature changes, the rotational speed could not be adequately controlled by a single voltage command. Hence, the voltage command is constantly adjusted based on the time measured between each reel encoder flag pulse. This provides rather gross, but sufficient control over the rotational speed of the motor in its open-loop mode.

After the reel is found to be seated properly on the hub, it is ready to be locked into place. First, however, the tape-in-path sensor is polled to determine if the tape is being seen continuously in the tape path. If this is the case, the reel has been loaded upside down. When the reel is spinning counterclockwise, the tape should only be momentarily flopping into the tape path. If the reel is found to be upside-down, the supply hub is turned in the clockwise direction until the tape clears the path. The door is then opened and the INVERT message is reported.

If the reel is not inverted, the hub locking routine can proceed. The supply hub motion is stopped and the hub lock solenoid is engaged. The hub is then rotated clockwise by applying a voltage ramp. As the hub rotates, three locking feet come up out of the hub and grab the tape reel, holding it securely to the hub. During this locking operation, the reel encoder flags and sensor serve as a check on whether the lock is a success. If the reel is properly locked on the hub, one of the encoder flags will interrupt the sensor beam as the voltage ramp is applied. After a successful lock, the hub lock solenoid is disengaged and the supply hub is free to spin again. This locking cycle is described in more detail later.

The supply hub is now rotated again in the counterclockwise direction in preparation for the inertia check. The inertia check is used to determine the size of the reel being loaded. This information is used to set up some autoload and servo parameters that can be optimized according to the reel size. The inertia check is performed by applying a step voltage to the supply motor at the instant a reel encoder flag breaks the sensor beam. The time it takes for the next reel encoder flag to come around and break the sensor beam is proportional to the inertia of the reel.

As the hub continues spinning counterclockwise, some additional error conditions are checked. The check for an upside-down reel is repeated. A check for the tape leader being stuck to the reel is done. In the same manner that tape continuously in the tape path signals an inverted reel, a tape that never breaks the tape-in-path sensor beam indicates that the leader is stuck to the reel. This can often occur because of static electricity or because the leader is jammed under the reel flange. If the tape leader is indeed stuck, the supply reel is spun counterclockwise at high speed in an attempt to free the tape end. Failure to free the tape leader at this point will abort the load process.

Next, the tape is ready to be threaded. The supply hub continues to spin in the counterclockwise direction, and the tape-in-path sensor is monitored for the tape leader. Once the tape leader is sensed in the tape path, the supply reel continues to spin for another half second (to pull the end of the tape back to the beginning of the tape path), at which point it reverses direction and starts feeding the tape down the tape path assisted by the air flow. The tape-in-path sensor is checked continuously to make sure the tape stays in the path during the feeding operation. If the sensor fails to detect the tape in the path, an error condition is flagged and the load is aborted.

As the threading proceeds, the speed sensor is monitored for activity. If the tape has been correctly fed down the tape path and has caught onto the take-up reel, the speed sensor begins to spin. The speed sensor is then used to calculate the amount of tape that is wrapping around the take-up reel. If activity is not seen at the speed sensor within a certain period, or the required number of wraps are not completed within another certain period, the tape will be pulled back out of the tape path by the supply reel and the autoload will be retried. The HP 7980A will attempt to load the tape in this manner five times before reporting failure to do so.

After it has been determined tha the tape is properly threaded, the load fan can be turned off and the servo loops closed. The take-up and supply motors are driven slowly in opposition to tension the tape. The tension arm position is monitored by an analog-to-digital converter (ADC) and, once it has reached the center (0V) position, the tension loop is closed. The tension integrator is turned on and the loop is given about a half second to stabilize. Next, the microprocessor closes the velocity loop and digitally controls its operation. The tension shutdown circuitry is enabled to prevent the drive from damaging itself or the tape during some sort of catastrophic failure. Any failure to establish tension during this process will cause the load operation to be aborted and a MISLOAD reported.

The autoload process takes 30 seconds if all goes well. Then the drive can be put on-line and all normal reading and writing operations can be performed.

Hub Lock Mechanism

The first major objective in the autoload process is to determine the presence of, seat, and secure reels of tape. This process is complicated by the availability of four different sizes of half-inch-tape reels ranging in diameter from six inches to 10.5 inches, the latter being the most common size. The tape reel is secured onto a supply hub which suspends the reel at the proper height, holds it secure, and rotates the reel.

When the door of the tape drive is closed, the autoload algorithm starts immediately. The supply hub rotates in a clockwise direction and an off-center protrusion atop the hub contacts the inner race of the reel of tape. The rotational motion drags the reel in an inward spiral pattern until the reel eventually drops onto the hub's mounting plane. Three small plastic springs (the reel encoder flags mentioned earlier) mounted around the mounting plane are pushed down by the weight of the reel and trip the reel encoder optical sensor. The use of three springs assures planar contact of the reel on the hub, which helps prevent tape edge damage by keeping the reel as parallel as possible to the tape path. A fourth spring may be deflected by the write enable ring on the reel of tape to relay that status as well.

At this point, the reel is secured to the hub by means of three feet that rotate up and out of the hub to clamp the inner race of the reel. The feet are driven by a four-bar linkage that translates the rotational motion of the hub into a vertical rotation of the feet. At the end of their travel, the feet are locked into position in much the same manner as a toggle switch with an overcenter design.

The mechanism that drives the feet up and into place has four basic parts [Fig. 4]. The first is the foot itself, which pivots about an axle swaged into the hub base. The foot is driven by a barbell-shaped tie rod which translates the horizontal rotation of the hub into the vertical rotation of the foot. The barbell, in turn, is driven by a sleeve-shaped part that fits around the base of the hub and is held in place by a spring and retaining ring. In normal operation, the sleeve rotates with the hub. During the reel locking stage, a solenoid engages an arm that stops the motion of the sleeve, and while the hub is rotated, causes the sleeve to rotate relative to the hub. The foot is pushed up and into place, depressing the wave spring when the foot contacts the tape reel. As the hub rotation continues, the barbell pops over center and starts to retreat. The relative motion at this point is ended by stops in the hub that limit the motion of the sleeve. The lock cycle is double-checked by the reel encoder optical sensors. Sensor placement is such that one of the plastic springs always interrupts the sensor at the moment the sleeve hits its stops. A neoprene pad embedded in the surface of the foot ensures that the reel does not slip relative to the hub. The hub is unlocked by simply reversing the process. In the case of a power failure, the entire lock or unlock cycle can be performed by manually turnign the reel and engaging the arm.

Many obstacles had to be overcome to ensure a successful design. The first of these was materials selection. The material for the plastic springs must have a high endurance limit, low creep at moderately high temperatures, and a consistent and high spring constant. The material chosen was ULTEM 1000 by General Electric. Material for the sliding parts on the hub was tailored to minimize wear and friction, and to resist deformation under load. The second obstacle was cost. We made extensive use of design for manufacturability to decrease part count and minimize assembly time. The result is a hub with no screws that is completely assembled from one side.

Integrated Tape Path

All the tape path components, except for the reel motors, are mounted onto a precision head plate which is mounted on the deck casting. The magnetic tape head and the tape cleaner are permanently mounted onto the head plate in a manner similar to that used for the earlier HP 7978A Tape Drive. This greatly reduces the number of tight tolerances required on the deck casting because of tape path requirments. The buffer arm and speed sensor assemblies are removable. This allows for easy replacement of the head plate, speed sensor, and buffer arm assemblies. Each of these assemblies is designed to be modular and interchangeable. There are no service adjustments on the HP 7980A because any adjustments required are done at the factory. No routine maintenance is required, except for regular cleaning of the tape path.

The tape path components perform several tasks in guiding the tape over the magnetic tape head. They make sure the tape is wrapped around the head properly and consistently. They also guide the tape past the head at a precise angle [skew] and at a proper height [tracking] so that the written tape is interchangeable with other tape drives. Normally, there are two precision stationary or fixed guides that perform these functions. The HP 7980A tape path incorporates one precision guide into the buffer arm assembly and the other guide is integrated with the speed sensor roller.

Speed Sensor

The speed sensor integrates the functions of an optical encoder and a precision tape guide. The particular challenge was to create a rolling element, needed for transmitting tape speed to the encoder, that would guide the tape with the accuracy of a fixed guide.

Fundamentally, a rolling is simply a vertical cylinder with flanges on the top and bottom. The upper and lower flanges restrict the vertical limits of the tape's travel. According to the ANSI standard, the width of a half-inch tape must be between 0.496 and 0.500 inch. Clearly, the distance between the roller flanges must be at least 0.500 inch not to damage maximum-width tapes, but this allows a 0.496-inch-wide tape to wander 0.004 inch between flanges. However, in the simplified tape path the speed sensor must also guide the tape for proper skew and tracking across the head. This makes 0.004 inch of vertical wander unacceptable.

A traditional solution of this problem is to put a fixed guide between the roller and the head. A fixed guide is a nonrotating cylinder with a top flange set to a precise height. A spring-loaded washer pushes the buttom edge of the tape so that the top reference edge of the tape stays in contact with the top flange. This eliminates tape wander since the distance between the washer and the top flange can vary with tape width. The HP 7980A speed sensor combines features of both rolling and fixed guides. Fig. 5 shows a cross section of the speed sensor. The roller, which includes a top flange, spins with the tape and transmits tape speed (through the shaft) to the optical encoder. The roller has no bottom flange, but there is a spring-loaded ceramic washer below the roller. The washer is restrained from rotating, but can move vertically to push the tape against the fixed height of the roller's upper flange. Thus, the roller is fixed vertically, but moves rotaitonally, while the washer is fixed rotationally and moves vertically.

To keep the bottom of the spinning roller from rubbing on the non-spinning washer, the roller width is slightly smaller than the minimum tape width. Thus, the semicircular arc of tape wrapped around the roller pushes the ceramic washer down and away from the roller. The spring is placed under the center of gravity of this semicircular arc of tape. This balances the forces on the washer so that it remains horizontal and does not damage the bottom edge of the tape.

Buffer Arm Assembly

The design of the buffer arm assembly presented several special challenges. The buffer arm assembly must provide tape buffering, proper tape tension, a servo position signal, proper tape guidance, and an overtension shutdown signal. The buffer must be lightweight enough to maintain servo bandwidth, but strong enough to hold the tape height within a tenth of a millimeter. Because the HP 7980A is an autoloading drive, the buffer arm must also act as an air dam to load the tape correctly. To achieve these requirements, a thin-wall aluminum die-cast part is used. Its flexibility allows an air baffle, a spring post, a stop, and a slot for overtension shutdown to be incorporated into a single part.

During testing, itwas discovered that the tape resonant frequency dropped when the drive repositioned frequently. It was determined that during such reposition cycles more air became entrapped in the tape stack. As a result, the effective tape length was increased, decreasing its spring constant. To overcome this problem, a Coulomb damper is used. A cantilever spring is placed between the buffer arm and fixed guide so that it produces approximately four inch-ounces of frictional torque. This dissipates enough energy from the system to allow servo stability without affecting other parameters.

In a typical tape drive, the tape, after leaving the buffer assembly, enters a fixed guide for proper skew alignment. To minimize space requirements, simplify service, and reduce parts count, the fixed guide is combined with the buffer assembly. The buffer arm pivots about the center of the fixed guide as can be seen in Fig. 6. The fixed guide, made of a stainless-steel ring and two ceramic washers, is bonded on the buffer base to a 0.015-mm tolerance. This allows the buffer assembly to provide all the tape guidance in the front half of the tape path, but still be removed by loosening only three screws.

Another feature is the way the position of the buffer arm is sensed. the sensing assembly had to be small enough to pass through a 40-mm-diameter hole in the head plate, but require no adjustment if the buffer is replaced. To satisfy these requirements, a small ceramic magnet is mounted on the buffer shaft. Changes in the rotating magnetic field are sensed by a linear Hall-effect IC. The plastic magnet holder allows each magnet to be bonded in its correct calibrated position. With this arrangement a signal that is linear within [plus or minus] 8% over the buffer arm range is achieved, requiring no further calibration for interchange.

Design for Manufacturability

The HP-7980A tape drive is designed to be built easily. The major improvement over the previous HP 7978B drive is its size. The HP 7980A is about a third the size and half the weight of the HP 7978B. this makes it much easier to build and handle.

All of the major components are attached to the main deck casting. The main deck casting is moved down the manufacturing line as major components and assemblies are attached to it. It is then flipped over to add components onto the top and tested. Most of the components and assemblies are attached to the casting in a downward movement and fastened with a common self-tapping screw.

Reduction of parts is another major improvement. The number of mechanical parts was reduced from approximately 610 on the HP 7978B to 340 on the HP 7980A. the number of unique mechanical parts was reduced from approximately 260 to 140. All these factors help substantially reduce the time to build the HP 7980A. It is, therefore, a much more manufacturable machine than its predecessor, the HP 7978B.

Acknowledgments

Thanks to Hoyle Curtis and Don DiTommaso for obtaining the necessary funding for the success of the HP 7980A and for fending off additional features and configurations so the core system could be delivered on time.

There were many difficult mechanical issues during the design of the HP 7980A Tape Drive. Thanks to Tom Bendon for managing the resolutions to these issues. The many inputs from our manufacturing team were invaluable and contributed greatly to the manufacturability of the HP 7980A. Thanks to John Meredith, Gregg Schmidtke, Mel Crane, and Lee Devlin, our manufacturing team. Our tooling engineers, Dave Halbert and Jesse Gerrard, provided much needed input into the tooling and design of the many metal and molded Lundgren did the early product design and Jim Dow did the industrial design. Dave Jones did the rack design and the HP 7980A rack slides. Many thanks to Dan Dauner for finishing the latter stage of the product design, which is the most difficult phase, and for wrapping it up for manufacturing.

To those not specifically mentioned, thank you for your contribution to the success of the HP 7980A.

COPYRIGHT 1988 Hewlett Packard Company
COPYRIGHT 2004 Gale Group