Electronic Fundamentals

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The interaction between the electrical components of the SRM – High Performance Ergometer allows precise power control for performance diagnostic examination and scientific data acquisition.

The PowerMeter

The SRM - PowerMeter Science
Reading slope and serial number

The PowerMeter is the central part of the ergometer. It is responsible for measuring the power output. The PM transmits a particular frequency (500 - 12.000 Hz) proportional to the actual torque and also a frequency proportional to the actual angular velocity via a transmitter to a receiver located on the frame. The signal is digital to eliminate transmission errors. Power supply of the PM is by a Lithium battery lasting 1800 hours.

Important: If there is no torque on the chain, the PM does not measure any frequency, which means the angular velocity is zero. Setting this zero offset correctly is important to receive the right measurement of power output.

In the factory we calibrate all SRM PowerMeters in order to determine the frequency change of PowerMeter when changing the torque on the cranks. This frequency is called slope. This slope, measured in Hz/Nm, has to be entered initially into the PowerControl. The slope only has to be reentered if a different PowerMeter is employed, which normally has also a different slope.

The PowerControl IV

The Power Control IV, abbreviated as PC on the handlebars in the photo
The Power Control IV
Rear Power Control IV with two connections
Rear Power Control IV with three connections

The PowerControl of the SRM Ergometer is responsible for storing the measured data, controlling the eddy current brake and transmitting signals to the computer via a serial cable.

Power output is calculated in the PC in the following manner:

Average torque of a full crank rotation x average angular velocity of a full crank rotation = average power of a full crank rotation.

Average values are calculated on the time basis of one second. Thus the power value is always displayed with a one second delay. An example: The athlete pedals with 90 rotations per minute or 1.5 rotations per sec. The calculated performance is in this case the average value during 1.5 crank rotations. To conduct this calculation the next full crank rotation has to be completed in order to know which value of the previous time basis has to be employed. Basis of performance calculations are therefore always full rotations which are calculated on the time basis (1 sec).

A common time basis is necessary in order to synchronize all measured values of heart rate, cadence, power, and speed. Without calculation based on a common time basis and the relationship to the position of the crank, the measured value for power would fluctuate wildly..

Heart rate is captured with a Polar heart rate monitor not-coded-model.

Power supply of the PowerControl unit is ensured via an integrated accumulator, which is constantly charged when the power supply is switched on. To employ the PowerControl on the Ergometer one has to ensure that the zero offset and slope are correctly entered. All other values such as sampling frequency, wheel size, etc. are in this case not applicable.

Since the PowerControl saves all data, the user has a second set of data which he can use e.g. if the computer has crashed or has lost its data.

Please read out the data from the PowerControl regularly and clear the memory..


In the following list you find the technical meanings of the plugs at the rear of the PowerControl. Please consider that the number of plugs can distinguish between the different model types. And please be careful in handling the plugs, because the pins inside the plugs are very sensitiv.

Plug 1 (female, 4 pins) (s. picture)
  • Power signal
  • Connection to the serial com interface
Plug 2 (male, 4 pins) (s. picture)
  • Eddy current brake instructions from PowerControl to the power supply
  • Charging
  • Speed
Plug 3 (optional) (female, 5 pins) (s. picture)
  • Torque Analysis

Keyboard shortcuts

In the following flowchart you find the different keybord shortcuts of the PC IV:

Alternativer Text

Ethernet hub

An Ethernet hub is a network hardware device for connecting multiple Ethernet devices together and making them act as a single network segment. If, for example, a notebook has only 1 Ethernet port this offers a solution. Because if you want to you use both the Ergometer and the Torque Analysis you will need 2 Ethernet ports. It is most likely is does not matter which Ethernut hub you will use but we recommend to take the Dexlan Ethernet hub because is has been tested with the SRM Ergometer.

The Power Supply

Power supply with a luminous green light

The power supply has been especially designed to suit the SRM Performance Ergometer. The power supply changes the magnetic field in the eddy current brake very rapidly to ensure a constant rotational speed in isokinetic mode. In the isokinetic mode it is necessary to reduce the braking power to near zero when the cranks are in vertical position. In horizontal position the braking power has to be up to 4000 watts. This is necessary to maintain the predetermined rotational pedaling speed. This means that at a cadence of 120 rotations per minute the magnetic field in the brake has to be established and abolished four times per second.

The power supply operates with 220 Volts, and incorporates fuses of 1.3 A. The power point which is used for the power supply has to be well earthed (grounded) in order to avoid that the athletes become electrostatically charged. WARNING: Operation of the ergometer through an unsatisfactorily earthed power point can damage the health of the athlete and the physiologist and can also lead to wrong ECG readings.

The operating mode of the power supply is indicated by a green LED display. The yellow LED display indicates the operation of the brake. The yellow LED has to flash during isokinetic mode.

Place the power supply in dry, well aired condition. Avoid switching the power supply on and off in quick succession. Never clean the power supply with water or other fluids, the electronics will be damaged.

The current transmitted by the power supply is +/- 24 Volt. The power supply charges the PowerControl unit. If the PowerControl is not charged over a period of about 3 months, it may lose the programmed operating parameters. In this case the user has to charge up the PC via the dedicated charger and enter all operating parameters again with the software SRMWIN.

The power supply does usually not need any maintenance. Wipe with a dry cloth and be sure that no moisture gets into it.

Light Barrier

The light barrier measures the speed of the fly masses on the so called interrupter disc. She is a necessity for the power measurement. If speed is not measured correctly, the power measurement is not correct either. The displayed speed values in the SRMWin-Software - e.g. during a performance test – do not correspond to real speed values.

The following images show you, where to find the light barrier in the gearbox. For the purpose of cleaning the light barrier and also for changing its position, you have to take off the fly masses..