QVE00112 for Motor Control Circuits

These controls may be of the open loop type, where light just provides a no-contact, non-wearing circuit input to monitor the output of the motor, or a closed loop type, where the circuit monitors motor speed as a tachometer and maintains a fixed, selected, speed over a range of load and line conditions.

Closed loop, tachometer feedback control systems utilizing the QVE00112 and an encoder wheel, provide superior speed regulation when the dynamic characteristics of the motor system and the feedback system are matched to provide stability.

Mechanical Configuration
Mechanical Configuration

The mounting tab of the QVE00112 allows to position and fix the device on the motor chassis so that the encoder wheel is properly positioned between the two towers in the horizontal and the vertical directions : the horizontal position must align the optical center of the QVE00112 with the wheel slots (ideally the middle of the slot), and the vertical position must set the wheel at mid distance of the two towers to allow for the wobbling effect on the wheel when it rotates and prevent contact with the device.

The wires which are schematically shown on this view are essential to allow the supply lines and the output signal to be connected to a control board that most probably could not be inserted in the immediate vicinity of the motor.

In terms of electrical design, the QVE00112 offers simplicity of use and the benefits of an advanced internal circuitry :

First of all, the LED driver is integrated. The user does not have to design and add the external components to drive the LED. The internal driver has been designed to drive the LED taking into account the usual parameters, such as loss of efficiency due to temperature and aging.

The second integrated function is the conversion of the analog photodiode response into a logic output. The output is an open collector type that allows the device to interface with a controller board that is typically powered at a lower voltage than the motor drive board. Just use a pull up resistor to the controller board Vcc to use the output of the QVE00112 (Vout) as an input to the controller. That level of integration removes the need for amplifiers and ADC circuits.

As a result of the integration, the QVE00112 pin out is as simple as it can be : 3 connexions : Vcc (5V), GND and Vout.

Motor Control Block Diagram

The device has been specifically designed to meet the industrial temperature range requirements (-40°C to +85°C): the housing is able to withstand the highest temperatures that could be present around the motor without deforming or significant rigidity loss. The active semiconductor parts are also designed to function in that range without significant changes in their characteristics.

In terms of performance, the following features make the QVE00112 fit in applications where a standard transistor interrupter would not :

  1. The aperture size (0.356mm) results in a very narrow beam travelling from the LED to the photodetector. The effect of this narrow beam is a full transition from low to high when a blocking shield moves across the device throat by 0.05mm. Assuming an encoder wheel with a radius of 15mm (to the middle of the slot), as many as 1,000 slots could be stamped on the wheel and be readable with the QVE00112. This equates to a resolution of 0.36 deg.
  2. The rise time and fall time of 70ns (for a pull up of 360 Ohm) allow a rotation speed of about 400,000 RPM for a wheel as described above. That is about 1,000 times better than what is achievable with a standard phototransistor device.

For less demanding applications, those features, added to the inherent noise immunity of optical devices, contribute at generating a very sharp and clean waveform that reduces the occurrences of missed pulses, making the overall monitoring system more reliable and accurate.

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