Research methods

An Improved Jet Recorder of Bar Displacement

V. B. Kanevsky, State Central Institute of Physical Culture (GTsOLIFK)

English summary of Tyazhelaya Atletika. Ezhegodnik 1982 (Moscow: Fizkultura i Sport, 1982), pp. 68–69: the text is paraphrased, not translated; figures are the original images.

Principle and merits

A liquid jet traces the bar path (method of M. S. Khlystov, 1974).
Full-size (1:1) trace with no contact with the recording surface.
Dots at equal time intervals, so velocity and acceleration can be computed anywhere along the path.
Simple enough for almost any sports club to build.

Problem

The weak motor slows, sometimes by 20% or more, when coloured liquid is fed to the rotating shutter, which distorts velocity and acceleration.
Remedy: speed control. DPM-20, DPM-25 and DPM-30 DC motors are easiest, since their speed is linear in supply voltage (Fig. 1).

Straight-line graph of supply voltage against motor speed — Fig. 1. Supply voltage (V, volts) of the DPM-20 motor against speed (n, rpm).

Manual speed control (Fig. 2)

1. Switch on (VK1) before the lift and let the speed settle.
2. Set a slightly high voltage with R2, so the speed is a little too high.
3. Feed liquid under pressure to the shutter; speed and voltage drop.
4. Adjust pressure and voltage until the voltmeter shows the required speed, then record.

Example: dots every 0.01 s need 6000 rpm, which is 8 V on a DPM-20.
Check the speed with instruments periodically.
Electronic stabilisation also works. For DPM motors, see Radio, 1978, No. 4, p. 38.

Circuit diagram of the transformer-rectifier supply with variable resistor and voltmeter — Fig. 2. Power circuit with manual control: VK1 — switch; Pr — fuse; L1 — mains indicator lamp MN-3; R1 — resistor 80–100 kΩ; TR — transformer; R2 — variable resistor PPZ, 100 Ω; D1–D4 — rectifier diodes D7, D226 or similar; C1, C2 — capacitors 500 µF, 25 V; R3 — resistor 200 Ω; IP1 — voltmeter; M1 — DPM-20 motor.

Accuracy

A recorder on the bar end is affected by bar bending (A. S. Medvedev, A. A. Lukashev, V. B. Kanevsky, 1980).
Speed control plus an allowance for bending gives more reliable space–time data.