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Thursday 14 July 2016

Treadmills with 2 speed sensors

Treadmills with 2 x Speed Sensors (eg. LiveStrong/Vision)

Some treadmills are fitted with 2 x speed sensors .
One is a standard magnetic reed switch fitted near the roller pulley and the other is an optical sensor mounted at the rear of the motor. The magnetic sensor is used for general speed measurement whilst the optical sensor is used for accurate control and safety.
Faults with the optical speed sensor are often confused with MCB problems, and due to the presence of the magnetic reed switch sensor, the optical speed sensor is frequently overlooked and not tested at all.
Problems with the optical sensor can usually be cured simply by blowing dust/dirt out of the sensor lens as part of a service.

Symptom of faulty optical speed sensor
Motor runs erratically.
Running belt speeds up/down and has trouble holding a steady speed (irrespective of whether there is a person on the machine).
Motor cuts out after a while.
Motor runs momentarily but then cuts out almost straight away.

System Operation
The system uses a magnetic speed sensor to monitor the running belt speed and allows the console display (kph/mph) to be re-calibrated accurately between different models. This allows the same electronics to be shared among multiple treadmills which saves costs.
However, the magnetic sensor is relatively slow to respond which can lead to speed fluctuations (a perceptible lag while the system makes automatic adjustments to maintain a set speed). This is also why an E1 speed sensor error usually takes up to 10 seconds to appear.
The optical sensor works much faster. It allows the system to respond very quickly to speed fluctuations so as to provide a very steady belt speed. Also, it allows the system to detect a problem far sooner than the magnetic sensor and hence shut down the system rapidly in the event of a problem.
Therefore, by using the two speed sensors, the manufacturer can share electronics between different models whilst also providing a very responsive control system.  

Operation of the Optical Speed Sensor
The optical speed sensor is an electronic device consisting of an in-built LED and sensor. Light passes from the LED lens to a receiver which is mounted directly opposite. When the light beam is interrupted a signal is passed to the MCB (the sensor generates an electrical on/off signal to indicate speed). The light beam is interrupted by use of a castellated round disk mounted on the rear of the motor which passes between the light beam of the sensor which blocks or allows light to pass as it rotates.
The optical speed sensor itself has no moving parts and is inherently reliable.

Common Problems and Fixes:
Dust on Lens
By far the most common problem on treadmills is dust and dirt on the lens or receiver of the LED. This blocks the light beam and causes the sensor to give an inconsistent output or no output at all. This can be resolved by cleaning the lens with an air blast or removing the dust with a small, clean paint brush.

Wires & Connections
Optical sensors can have 3, 4 or 5 signals wires depending on the configuration. These small wires can be trapped or snagged under screws and can easily become disconnected. Check connection and wire continuity using an multimeter. Repair and damaged wires or replace device. (Wiring and pinouts vary and cannot be easily tested using a multimeter without knowing the circuit)

Physical damage
Usually a thin metal castellated disk mounted to the motor shaft is used to interrupt the light beam of the sensor. This disk is prone to being bent or damage and can rub or scuff the side of the plastic enclosure of the LED or the circuit board. This can sometimes create debris which can block the light beam or in extreme cases cause complete failure of the sensor. Clean the lens and remove any plastic debris and re-try otherwise replace the device.

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