The dot timer is a tool for measuring short time.If the paper tape driven by the moving object passes through the dotting timer, the points struck on the paper tape will record the movement time of the object, and the points on the paper tape will correspondingly indicate the positions of the moving object at different times.The motion frequency of objects can be analyzed by studying the spacing between points on the paper tape.
Electromagnetic dotting timerIt is a kind of useAC power supplyOfChronograph, whichworking voltage Below 6V, generally between 4~6V, it needs to be controlled byStudent power supplypower supply[2]The frequency of the power supply is 50Hz, and it hits a dot every 0.02s.That is, 50 points per second.Electric spark ignition timerIs usingSpark dischargeA timing instrument that prints ink on a paper tape and displays dots, using 220VAC voltage, when the frequency is 50Hz, it makes a dot every 0.02s.Spark timerWhen working, the resistance of paper tape movement is smaller than that of electromagnetic dot timerExperimental errorSmall.electric sparkDotting principle:alternating currentThe power supply voltage of will change periodically. When the voltage reaches a large value (generally considered to be the maximum), an electric spark will be released, and then the electric spark will appear periodically. Because the paper is not conductive, the electric spark cannot be attracted to the paper surface, but the carbon film as a conductor plays the role of "lighting", and "marking" on the paper tape.
Electromagnetic dotting timer: electromagnetic dotting timer is a kind of timer that cantime interval, on paper tapeLinear orderInstrument for dotting.When the coil of the electromagnetic dot timer is powered on, the coil generates a magnetic field, and the vibration plate in the coil is magnetizedPermanent magnetUnder the action of the magnetic field, it moves up or down. Since the direction of the alternating current changes twice per cycle, the magnetic pole of the vibration plate after being magnetized will occurPeriodic change, permanent magnetForceThe direction ofalternating currentThe time interval between two points is equal to the period of AC power.Relative resistance of electromagnetic dot timer during operationElectric spark ignition timerIs relatively large.
electric sparkManagementtimerThe timing principle of is the same as that of the electromagnetic dotting timer, but the vibrating needle andCarbon paperInstead of electric spark and toner, connect 220V to the electric spark timerPower Supply, pressPulse outputSwitch, from timerpulse current, connected to the positive poleElectroacupunctureAnd tonerPaper trayTo the tray shaft connected to the negative pole, generateSpark dischargeTherefore, a series of dots are printed on the paper tape, and they are discharged once in each cycle of AC, so the time interval between the dots of the spark igniter is equal to the cycle of AC, and when the power frequency is 50Hz, it is printed once every 0.02s.
Main components: shell,Pulse generator, pulse output switch, paper tray shaft.
Electric spark ignition timer ratioElectromagnetic dotting timerThe error is small.The reason is that the resistance of electric spark timer is far less than that of electromagnetic timer.
Structure diagram of J0203 electromagnetic dot timer
These twotimerThe common point is that the "time-space" distribution information of moving objects is recorded on paper tape, and the timing benchmarks are all taken fromHousehold electricityThe time interval between every two points in China is 0.02s, so there are the samedata acquisitionMethods and data processing and analysis methods, andInclined planeTrolley, rail car, etc. are used together, but in terms of reliabilityaccuracyAnd class hoursUtilizationAnd so on.The electric spark timer successfully overcomes the shortcomings of the electromagnetic timer.
1、 Elimination of interference between movement and timing
1. Movement and timing interfere with each other
When the electromagnetic dotting timer is working, the magnetic field caused by the mains through the coilPeriodic change, instigationShrapnelDrive the dotting needle to dot the paper tape dragged by the moving object at a frequency of 50 Hz.The electromagnetic dotting timer produces impact and friction against the paper tape in motion, which interferes with the movement of the paper tapeFree fall experimentEspecially significant.
Spark timerUse high voltage pulseSpark dischargeThe discharge needle is not in contact with the paper tape, so it will not interfere with the movement of the paper tape.
2. Eliminate the interference of movement on timing
The timing cycle of the electromagnetic dot timer will be interfered by the movement of the paper tape, so we conduct the following analysis and experiments.withVibratorDownward dotting at the overbalance position is regarded as the zero phase, so that the phase when dotting on the paper tape will be at a point slightly smaller nearby, as long as the dotting can be kept at the same phaseIsochronityCan be guaranteed.When the paper tape is stationary, if the electromagnetic dotting timer is properly adjusted, the dotting isochronity can be guaranteed.However, when the paper tape moves, the paper tape under the impact of the dotting needle also reacts to the vibrator, thus affecting the phase drift during the dotting, which destroys the isochronity of the dotting.
2、 Improvement of operation performance
1. Reduce adjustment and improve working reliability
When using the electromagnetic dot timer, the adjustment of the dot phase is often a technical difficulty that is difficult for ordinary teachers and students to master;For the adjusted timerenvironmental factorHowever, even if it takes a lot of time and energy to adjust, it is often difficult to adjust, which reduces the reliability of the work and reduces the experimentalConfidenceAnd class hour utilization.
useSpark timerSince the above adjustment is not required, students can collect data in 2-5 minutesexperimental data So that students can focus onControl experimentThe conditions of the phenomenon, as well as research and analysis of the law of motion.
2. Breakthrough in safety performance
The electromagnetic timer is not in usesafety problemOf.Safety is one of the keys that spark timing can't be used in teaching for a long time.The recording sensitivity of early electric spark point timers is low, which depends on the electric spark on white paperHole burningIts voltage is more than 6kV, and its current is 80mA, which is dangerous to human life.LaterThermosensitive paperThe voltage is also greater than 6 kV, and the current is 5 to 20 mA, which is also extremely unsafe.The dot recording method we adopt is a kind of recording methodFax recordSimilar to the micro current transfer method, the voltage is as high as220VHowever, the current is controlled below 200 μ A, which solves the safety problem when using the spark timer.
3、 Experiment cost
The electromagnetic timer is a low costTeaching instrumentIts price and experimental consumption are relatively low.In the early 1980s, the price ratio between electric spark timers and electromagnetic dotting timers was more than 10:1, and the material consumption ratio was more than 50:1 (when the former used thermal paper). After long-term research and improvement, we finally found a suitable circuit. In charging and discharging, we used the phase information of mains electricity to greatly simplify the circuit, so that the price ratio of the two products was less than 2:1.Because the spark timer is not usedLow voltage power supply（Student power supply）The electromagnetic timer needs to use the supporting student power supply. If this factor is taken into consideration, the above price ratio will be reversed. Therefore, the spark timerCheaper。In terms of material consumption, due to the use of appropriate dielectric materials, spark discharge traces are amplified and printed on ordinary white paper, which is cheaper than the telegraph tape used for electromagnetic dot timers and can be self-made.
To sum up, it can be seen that the new electric spark timer is fully equipped with the conditions to replace the electromagnetic dotting timer, and is suitable for popularization in China.
Experimental process
Announce
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1. Place the long wooden board flat on the experimental table, and make the pulley extend out of the table.
2. Fix the dotting timer on the side of the board without pulley, and connect the circuit.
3. Tie a string to the trolley, and the string crossesFixed pulley, a suitable hook code is hung below.
4. Fix the paper tape passing through the dot timer at the back of the trolley.
5. Stop the trolley near the dot timer,turn on electricity, release the trolley and let it move.
6. Disconnect the power supply and take out the paper tape.
7. Replace it with a new paper tape and redo it twice.
Note: Avoid using the first two points of the paper tape.The power must be connected before releasing the trolley
Paper tape analysis
Experimental process
1. Analysis of the initial point of free fall movement: check whether the distance between the first two points of the paper tape is close to 2mm, and the paper tape close to 2mm is the free fall movement experiment paper tape that starts from standstill.
2. Whether the experimental paper tape has been studiedUniform speed movementAnalysis of: measure whether the distance difference between adjacent points on the paper tape is equal. If it is equal, it is a constant speed movement, otherwise it is not;That is, the distance difference between two adjacent points of the paper tape in uniform speed movement meets s (n+1) - s (n)=aT ^ 2
3. Calculate a point in constant speed movementInstantaneous velocity；The displacement of a moving object at a certain sectionaverage velocityIt is equal to the instantaneous velocity of the object at the midpoint of the displacement;That is, V (n)=[s (n)+s (n+1)]/2t
S (n) refers to the displacement from the N-1st timing point to the N-th timing point, s (n+1) refers to the displacement from the N-th timing point to the N+1 timing point, [s (n)+s (n+1)] refers to the displacement from the N-1st timing point to the N+1 timing point (that is, the required point is included in their middle, i.e. N), t refers to the time interval between two adjacent counting points (N-1 to N, N to N+1). 2T is the sum of time intervals
(1) Acceleration calculation of ideal paper tape: because the distance difference between two adjacent counting points described by ideal paper tape is completely equal, that is:S2-S1=S3-S2=…=S(n)-S(n-1)=△S=aT^2；So its acceleration a=△ S/T ^ 2
(2) Actual experimental paper tape acceleration calculation: due to certain errors in the experiment process, the distance difference between two adjacent counting points is not completely equal, which is to reduce theAccidental error, with spacinganalytical methodThe calculation amount can be reduced by using S1, S2, S3...... To represent the distance between adjacent counting points, the time interval between two counting points is T, and according to △ S=aT ^ 2
Before the experiment, the dotting timer should be fixed to avoid shaking when pulling the paper tape, and the paper tape should be pulled gently first, without obvious blocking.
two
Use the dotting timer to fix the dotting timer on the test bench. Turn on the power first, and release the paper tape after the dotting timer is stable to make the time interval more accurate.
three
The power supply used by the dotting timer is AC power supply, and the voltage of the electromagnetic dotting timer is 4~6V;The voltage of electric spark ignition timer is 220V.
four
The trolley cannot be too close to the pulley end, and the forward space is too small, which will leave too few timing points on the paper tape, which will bring inconvenience to the measurement and cause large errors.
five
The pulley position shall not be too low, which will make the connecting line between the weight and the trolley contact with the board, and the pull direction of the line is not parallel to the board surface, with large resistance and error.
six
When manually pulling the paper tape, the speed should be faster to avoid too dense dots.
seven
When using the spark timer, pay attention to correctly threading the paper tape, and the toner paper is located below the paper tape;When using the electromagnetic dotting timer, the paper tape shall pass through the limit hole and be pressed under the carbon paper.
eight
The dotting timer should print small dots of appropriate weight on the paper tape. If the small horizontal lines, repeated dots or dots are not clear, adjust the height of the vibrating needle from the carbon paper to make it larger.
nine
Do not install the carbon paper in reverse. Adjust the position of the carbon paper after each tape is printed. If the dots are not clear enough, consider replacing the carbon paper.
ten
The paper tape should be smoothed to reduce friction and thus reduce error.
eleven
The puncher cannot work continuously for a long time.Cut off the power supply in time after each paper tape is punched.After the paper tape is installed, turn on the power again and conduct the experiment.
twelve
When measuring the paper tape, do not measure the displacement of each section in sections. The correct way is to estimate the next millimeter when reading after one measurement (the distance between each measurement point and the starting point O can be measured uniformly first).
thirteen
When processing paper tape data, the displacement difference of dense points is measured with large error and should be rounded off;Generally, five points are taken as one counting point.
fourteen
When drawing points, connect as many points as possible to a straight line (or curve). The points that cannot be connected to the line should be separated on both sides of the line.Points with excessive error can be rounded off.
