Photoelectric device for sensing the height of a pile of paper sheets

The invention is a photoelectric sensing device, in the body of which is rotatably mounted a photoelectric sensing element, the sight opening of which is arranged in a plane with the front stops and bears on the vertical front plane of the pile of paper sheets. The body of the photoelectric sensing device can be mounted on the carrier rod of the front stops. The sensing device provides an advantage in that the device works with daylight and also with artificial light so that a special source of light is not necessary for operation of the sensing device. As a result of arranging the photoelectric sensing element in the plane of the front or lateral stops, the sight opening glass is continually wiped by falling paper sheets and is not obscured by dust. The photoelectric sensing device is simple to manufacture and is reliable.

The invention relates to a photoelectric device for sensing the height of a 
pile of paper sheets on a delivery table of a printing machine. 
The object of the invention is to provide a photoelectric device for 
sensing the height of a pile of paper sheets on a delivery table, where 
the height of the delivery table is controlled by the device and lowered 
during the printing operation in such a way, that the top of the pile of 
sheets remains at a constant height. 
One of the known devices for the control of the level of a pile of paper 
sheets on a delivery table of a printing machine comprises a microswitch 
feeler device which is arranged on a rigid or on a movable aligning 
element for the pile of sheets. When the height of the pile of sheets 
increases, a microswitch is activated and provides a signal to a further 
mechanism which controls the lowering of the delivery table with the pile 
of paper sheets. 
A disadvantage of this device is that a power supply is necessary for the 
control of the feeler device which results in difficulties when thin paper 
sheets with a weight of about 20-40 g/m.sup.2 are printed, because these 
paper sheets are often creased particularly at the beginning of the 
printing process. 
Another known device for the control of the height of a pile of paper 
sheets comprises a reflection receiving photoelectric sensing element. 
This photoelectric sensing device is activated by the light of an electric 
bulb reflected from the pile of paper sheets which falls on a sensing 
element comprising a phototransistor or a photodiode. By means of circuits 
connected to the sensing element, the delivery table with the pile of 
paper sheets is lowered as the height of the pile of paper sheets 
increases. 
A disadvantage of this device is coating of the sensing element with dust 
which results in failure of the operation of the device. Most types of 
offset sheet printing machines use devices for providing the printed 
sheets with a coating of fine powder to improve drying. The powder coats 
the sensing element and also reduces the life of the electric bulb source 
of the reflected light beam. 
Another known device for the control of the height of a pile of paper 
sheets is a capacitor sensing device. This device works on the capacitance 
principle where the pile of paper sheets located on the delivery table 
comprises the dielectric. These capacitor feeler devices are highly 
reliable in operation and from a technical point of view, the most 
advantageous sensing devices. 
The capacitor sensing device is expensive to manufacture which eliminates 
the use of such devices on lower price printing machines, particularly on 
printing machines for smaller sheet sizes. Also, the repair of capacitor 
sensing devices is complex and requires the servicing by an electronic 
specialist. 
The above-mentioned disadvantages are avoided by a photoelectric sensing 
device for the control of the height of a pile of paper sheets of the 
present invention. According to the present invention, in a body of a 
photoelectric sensing device, is turnably arranged a support element on 
which is eccentrically arranged a photoelectric element, the sight opening 
of which is arranged so that it can be located in the plane of the front 
or lateral stops and bears on a vertical plane of the pile of paper 
sheets. The body of the photoelectric sensing element can be mounted on a 
carrier rod of the stops. The photoelectric element is eccentrically 
mounted on a turnable element in the body of the photoelectric sensing 
device. The sight opening of the photoelectric sensing element is located 
in the plane of the front stops or the lateral stops and bears on the pile 
of sheets. The sight opening of the photoelectric element is located in 
the plane of the stops and bears on the side of the pile of sheets. The 
sight opening is covered by a light transmitting material which protects 
the photoelectric element. The photoelectric element is provided with 
connectors for connection with means which adjust the vertical position of 
the delivery table in response to the signal from the photoelectric 
element. 
An advantage of the device of the present invention is in that the 
photoelectric sensing device works with artificial light sources or with 
natural light from the environs so that a special source of light is not 
required. The photoelectric sensing device can be arranged in the plane of 
the front or lateral stops of the delivery device where the sight opening 
of its photoelectric element is continually contacted by the falling 
delivered paper sheets so that coating of the photoelectric sensing 
element with dust is not possible. 
Further advantages of the photoelectric device of the present invention are 
in the simple structure, low manufacturing costs and reliable operation of 
the device.

The photoelectric sensing device for sensing the height of a pile of paper 
sheets according to the invention consists of a body 4, in which is 
turnably mounted support element 5 which can rotate over a portion of a 
circle on which is eccentrically attached a photoelectric element 6. The 
photoelectric element 6 can be a diode, a photoresistor or a 
phototransistor. The photoelectric element 6 is provided with a light 
transmitting sight opening 3, which can be made from glass, transparent 
inorganic crystals or transparent organic polymers. 
The turnable element 5 is rigidly connected with a shaft 14 which is 
provided with a control knob 7. The turnable element 5 is provided with a 
cut out 9, which engages a pin 10 which is fixed in the body 4 of the 
photoelectric sensing device. The terminals of the photoelectric element 6 
are attached to connectors 13 provided on the turnable element 5. The body 
4 of the photoelectric sensing device is mounted on a carrier rod 11 and 
is secured by a securing element 12. The carrier rod 11 is turnably 
mounted on the side walls of the printing machine. The stacking surfaces 
20 of front stops 2, the sight opening 3 of the photoelectric element 6 
and the vertical front plane of the pile 8 of sheets are in a common plane 
1. An alternatively arranged device has the sight opening 3 of the 
photoelectric element 6 arranged in the plane of the lateral stop elements 
of the delivery table so that it bears on the side of the pile 8 of paper 
sheets in its vertical lateral plane. The photoelectric element 6 can be 
mounted either on the front stop 2 or on the lateral stop element which is 
not illustrated in the drawings. 
The photoelectric element 6 can be rigidly mounted on the body 4 (not 
shown). When the photoelectric element is rigidly mounted in the body, the 
position of the body must be changed to adjust the height of the pile of 
paper sheets. 
The photoelectric sensing device works as follows: 
When a pile 8 of paper sheets is set up on the delivery table by delivered 
printed sheets and the pile 8 reaches a height, where sufficient light 
cannot enter the sight opening 3 and fall on the photoelectric element 6 
of the photoelectric sensing device, the photoelectric element 6 gives, 
via an electronic relay 15 and an electromagnet 16, an impulse for 
lowering the delivery table until the pile 8 of sheets reaches the 
required level. 
By means of control knob 7, the rotatable element 5 can be turned together 
with the photoelectric element 6 through an angle about 120.degree.. Since 
the photoelectric element 6 is eccentrically mounted on rotatable element 
5, the photoelectric element 6 is shifted in a vertical direction relative 
to the top of the pile 8 of sheets from the mean position. Usually a 
vertical shift of about .+-.15 mm is suitable. The shifting of the 
photoelectric element 6 is advantageous as it enables the adjustment of 
the means values of the height and top level of the pile of paper sheets. 
The limits for the rotation of the turnable element 5 are set by the cut 
out 9 which is provided in the element 5, which is engaged by the pin 10 
fixed in the body 4 of the photoelectric sensing device. 
The photoelectric sensing device according to the invention can be used 
where automatic control of constant height and top level of a pile of 
paper sheets is needed.