Commercial recorders have fairly complex parts and require considerable precision in manufacture. Hence they cost more than most amateurs are willing to pay. Thomas W. Maskell of Baltimore has developed a somewhat unconventional recorder that can be built inexpensively by an amateur. The instrument can not only make graphs but also record a series of events and measure the interval of time in which an event occurs. In addition it can feed information back to the source, thus functioning as a servomechanism. In its present form the instrument is best suited for the observation of events that extend over periods of minutes or longer rather than fractions of a second. Maskell hopes that other amateurs will improve the device.

Maskell writes that a graphic recorder must perform three functions. "The phenomenon to be recorded must be sensed, the information must be changed into a form acceptable to the recorder and the record must be written. Systems for performing these functions vary in complexity and capability.

"One of the simplest is the waterstage recorder, in which a pen linked indirectly to a float writes on a drum rotated by a clockwork. As the float rises and falls with the water level the pen moves across the rotating chart. In the resulting graph the water level is plotted against time. The float acts as the sensing element. The linkage-a rope and a system of pulleys-acts as the signal-conditioner, reducing the distance traveled by the float to a proportional motion of the pen that is appropriate to the width of the paper-covered drum.

"In more complex systems the sensing devices are strain gauges, microphones, photocells, thermocouples, scintillation counters and a host of other devices. The majority of modern signal-conditioners are electronic. Most pen motors have as their key element a rugged ammeter to measure an electric current that varies in proportion to the phenomenon under observation.

"Largely for diversion, I undertook about five years ago a review of available instrumentation to discover a combination of electromechanical elements that would yield the most recorder-controller system per dollar of investment. The results are embodied in the device I shall describe here. I call it a 'scanner-servo-recorder' to suggest its several functions.

"As a scanner it can make direct observations. As a servo its pen arm can be 'slaved,' or linked, to the phenomenon under observation; it thus closes a feedback loop so that the phenomenon becomes self-controlling. As a recorder it can write a record in any of several ways. I built the instrument for $18.20, excluding the cost of scrap materials. "The device has three major units: the scanner, the servo-recorder and the paper transport. The scanner consists essentially of a motor-driven lever that moves back and forth horizontally through an arc of about 45 degrees. At its outer end is a simple optical system consisting of a lens and a photocell. The arrangement is shown schematically in the accompanying illustration [right]. In a typical application the photocell 'watches' a miniature lamp bulb attached to the pointer of a dial. The photocell functions as an on-off device.

"A similar lever that also oscillates continuously in the horizontal plane forms one element of the separate servo-recorder unit. The oscillating levers of both the scanner and the servo-recorder units are driven through cranks by synchronous motors. When the levers are started together in phase, they continue to oscillate in phase.

"The servo-recorder contains a second lever mounted below the oscillating lever. Both levers turn freely on the same fixed shaft. The second lever can be equipped with a pen. I call it the pen arm. These two levers can be coupled and uncoupled by means of an electromechanical latch operated by a solenoid. When they are latched, the oscillating lever drives the pen arm. In the schematic illustration the latching mechanism is represented by a finger that extends from the end of the oscillating lever; the finger engages the armature of the solenoid when the solenoid is not energized. The latch can be disengaged by energizing the solenoid.

"The electrical circuit includes a relay wired to remain locked down after it has been energized by a pulse of current. The pulse can be generated by a push button (as is shown schematically in the illustration) or by a photocell (as is actually the case). A reset switch associated with the oscillating lever breaks the circuit and unlocks the relay at each extreme of the lever's travel. When it is appropriately directed by the push button, the oscillating lever will push the pen arm in either direction from any point to any point within the limits of its excursion. In effect the push button issues either of two orders to the oscillating arm: 'Move the pen arm' or 'Drop the pen arm.'

"The two units are combined into a scanner-servo-recorder system by replacing the push button portion of the schematic circuit with the photocell. The system is put in operation by applying power to all circuits and starting the oscillating levers in phase. Assume that the system has been started and that the miniature lamp being watched by the photocell is midway between the limits of its excursion. The solenoid is not energized because the reset switch has broken the circuit. The armature has therefore dropped to its lowered position for engagement with the finger of the oscillating lever.

"The pen arm is pushed along as the oscillating lever moves toward its distant limit. When the photocell encounters the miniature lamp, light initiates a signal that actuates the relay. The solenoid operates arm. The pen arm now occupies the same relative position as the miniature lamp. The oscillating levers continue to the distant limit of their excursion, where the reset switch unlocks the relay and drops the solenoid to the latching position. The oscillating levers now return. During the transit, if the position of the lamp has not changed, the photocell again issues an unlatching command on reaching the light; the pen arm then remains undisturbed.

"Now assume that the position of the lamp has shifted somewhat toward the distant limit. (Remember that the oscillating lever is moving toward the near limit.) In this case the solenoid will operate when the photocell reaches the miniature lamp and the oscillating lever of the servo-recorder will ignore the pen-arm. On reaching the near end of its excursion the lever will operate the reset switch and release the solenoid for engagement with the pen arm during the next outward transit. On that transit, therefore, the pen arm will be pushed to the position of the lamp and then dropped.

"Similar analysis will demonstrate that the pen arm is slaved to the movement of the light under all conditions. A periodic record of the position of the lamp can be written by attaching a pen to an extension of the pen arm and placing it in contact with a moving sheet of paper. A feedback loop can be closed by linking the pen arm to the input of the system, in this case by appropriately coupling the pen arm to the driving mechanism that alters the position of the lamp.

"The actual construction includes a number of refinements that add to the accuracy and dependability of the instrument. For example, the relay is replaced by a silicon-controlled rectifier that in effect can be locked down by either a pulse of current or the momentary interruption of an established current. Such a response occurs when the photocell encounters either a bright object against a dark background or a dark object against a light background, such as the pointer of a meter.

"The arrangement of the latching mechanism also differs from that depicted in the simplified schematic illustration. The outer half of the pen arm actually consists of a leaf spring that terminates in a rectangular finger about an eighth of an inch wide. When the leaf spring is straight, this finger engages a projection of the oscillating lever and is thereby pushed in one direction or the other.

"If both engaging projections were rigidly attached to their respective levers, the pen arm would be moved too far by an amount equal to half of the combined widths of the fingers each time the direction of the movement was reversed. The required compensation can be achieved by what is called a lost-motion pendulum. It is made by attaching one of the fingers to a pendulum lever free to swing through a distance equal to the width of the two fingers [see Figure 4]. The pendulum is attached to the end of the oscillating lever and its excursion is restricted to the distance between a pair of eccentric washers that act as adjustable stops. The edges of the movable finger, which serves as the lost-motion mechanism, are cut at an angle so that the faces of the two fingers make flat contact; in this way they disengage readily.

"The leaf spring of the pen arm can be bent upward by a long cam that extends completely across the traversed path. When the pen arm is thus bent, it is not engaged by the finger of the lost-motion mechanism. The cam, which is rotated by a solenoid, consists of a slender shaft fitted with a stiff wire in the form of an elongated C. The cam turns in bearings supported by a pair of posts. In addition to bending the leaf spring and thus unlatching the mechanism, the cam serves as a friction brake for locking the pen arm in the position to which it has been moved.

"The oscillating lever of the servo-recorder makes mechanical contact with a limit switch at each end of its traverse. The switch opens the solenoid circuit and 'unlocks' the silicon-controlled rectifier. The leaf spring then assumes its straight position for engagement with the lost-motion pendulum.

"The pen arm is actually pivoted in the middle, the leaf spring being carried by one end and the pen by the other. In the middle is a 'torque takeoff' fitting for coupling feedback devices to the instrument. Both the scanner and the recorder are supported by four mounting posts at the corners of a base plate of sheet metal. The length of the mounting posts was chosen to place the lens of the scanner at optimum height above the object to be scanned and to place the pen in light contact with the writing platform of the paper transport. I have used both a weighted ball-point pen and an unweighted Esterbrook Feltwriter.

"For economy of operation I designed the paper transport to accept adding-machine paper 3 1/2 inches wide. The writing table, made of 24-gauge brass, is four inches long, including a sloping apron one inch long. The supply roll is carried by a fixed shaft. I did not equip the device with a take-up; the paper collects in a basket.

"The paper is pulled through the transport by a rubber roller. The roller is a heavy-walled length of rubber tubing slipped over a quarter-inch shaft that is supported at each end by a synchronous motor. Since the output shafts of the motors face in opposite directions, one shaft must turn clockwise and the other counterclockwise so that the rubber roller will turn in the same direction regardless of which motor is energized. One motor turns at 10 times the speed of the other and thus the arrangement provides a choice of two paper speeds. When one motor is energized, the other idles. The disadvantage of using unruled adding-machine paper for the graphs can be overcome by scoring the calibration of the instrument on a sheet of thin, clear plastic and filling the grooves with color from a wax pencil. By this means the dimensions of the graph can be read directly by placing the scored sheet over the paper.

"I used synchronous-timing motors in all positions. They are available in speeds ranging from one revolution per second to one revolution per hour. Such motors can be obtained on the surplus market for about $1.50 each. Aluminum plate 1/8 inch thick was used for the base, and the sides were made of 1/16inch brass sheet. The reciprocating levers and the pen arm were made of one-inch by 1/16-inch extruded half-hard brass to avoid the labor of shearing and sawing. Steel or aluminum could be used in cases requiring greater strength or where weight must be minimized. Brass is a good compromise, since it is dependable bearing material that is easy to solder. The reciprocating levers might well be made of aluminum or magnesium in instruments designed to operate at speeds in excess of one sweep per second. The crank assemblies were made of steel.

"The photoelectric scanning head is only one of several available sensors. It is the most versatile, responding to the position of anything that can be seen-from bands of adhesive tape on the stalk of a growing plant to the hairline pointer of a galvanometer. Normally my scanner is used to follow the pointer of a direct-current meter that has a sensitivity of 10 millionths of an ampere at full-scale deflection.

"I use a Clairex CL-904 photocell, the active area of which is confined to a thin line. The cell is rotated so that this line is parallel to the pointer of the meter. An image of the pointer is focused on the cell by a lens of approximately 2 3/4 inches focal length. I bought the lens at a novelty store. A friendly optician ground the edge of the lens for a sliding fit with an aluminum tube about 3/4 inch in diameter. A preground lens of the same size is available from the Edmund Scientific Co. of Barrington, N.J.

"The lens is supported at one end of the tube by a split retaining ring. The photocell at the other end is mounted in a wooden dowel. I determined the focal length of the lens by clamping the glass in a wire fixture that made a sliding fit with a wooden yardstick. A similarly supported cardboard screen was clamped behind the lens. A light baffle placed around the lens prevented scattered light from reaching the image plane. The focal length was then measured by adjusting the separation of the screen and lens until a distant source (the sun) came to sharp focus. When the cell is placed at twice this separation from the lens, an object also placed at twice the separation on the other side of the lens will be in sharp focus on the active area of the cell.

"Photocells of designs that differ from the Clairex 900 series can be used. I selected this type solely because the active area is confined to a thin rectangle, a configuration that develops a maximum signal in response to the image of a slender dark object such as the pointer of a meter. Cells of other configurations would be more appropriate for images of other shapes.

"When the image of a white surface lighted by a pair of self-focusing miniature bulbs falls on the photocell, the resistance of the active material is lowered to about 20,000 ohms. At this resistance the voltage applied to the gate terminal of the silicon-controlled rectifier is more nearly equal to that of the cathode than that of the anode. At the gate potential the rectifier acts as an open switch and no current flows through the solenoid. When the light is reduced, as by the image of the black pointer, the resistance of the cell increases to many hundreds of thousands of ohms. This change has the effect of making the potential of the gate more nearly equal to that of the anode.

"When the change in the gate potential reaches a critical value, the rectifier conducts; it continues to conduct until the circuit is broken, even though the intensity of the light is restored. By interchanging the positions occupied by the photocell and the 500,000-ohm potentiometer in the circuit the same action can be triggered by a comparable increase in light. In each of these modes of operation the 100,000-ohm potentiometer is adjusted to select the critical intensity of light at which the rectifier conducts.

"Writing devices other than pens can be mounted on the pen arm. For example, it is possible to couple to the armature of the solenoid an inking device that would make a dot on the graph when energized by a pulse of current. The range of applications can be increased by making other obvious modifications. The scanning lever, for instance, can be equipped with an insulated electrode for making sliding contact with a resistor or a commutated surface. The relative opaqueness of translucent solids or solutions can be plotted by interposing an optical gray scale between the photoelectric scanning head and the light source to be measured. Such a determination can be made either by scanning the fixed gray scale or by replacing the photocell scanning head by the gray scale and sweeping the scale past the photocell.

"The magnetic state of a scanned surface can be evaluated by equipping the scanning lever with a reed switch positioned at selected distances above the surface. When it is equipped with the photoelectric head, the scanner can also replace stepping switches for periodically monitoring a variety of events. In this application each event to be charted could cause a miniature lamp to light. The lamps would be arranged in an arc below the scanning head. With this arrangement an impulse pen could then print out the sequence of events against time.

"Numerous comparable applications as well as refinements in construction will suggest themselves to experimenters who enjoy innovation. The present design is submitted in the belief that amateurs would find it useful to have not only an instrument that illustrates where a recording system begins and ends but also a design that accommodates the craftsman who, like myself, is endowed with an abnormal quota of thumbs."

Bibliography

FUNDAMENTALS OF AUTOMATIC CONTROL. G. H. Farrington. John Wiley & Sons, Inc., 1951.

PRINCIPLES OF MECHANISM. F. Dyson. Oxford University Press, 1951.

PRINCIPLES OF SERVOMECHANISMS. Gordon S. Brown and Donald P. Campbell. John Wiley & Sons, Inc., 1948.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.