I suggest that what you need is a servomotor combined with a solenoid brake. The solenoid actuates a flat plate in line with the axis of the motor shaft. When power is applied, the plate is pulled off a brake-pad and the servo motor is free to turn. When the power is cut, the spring-return on the solenoid causes the plate to clamp against the brake pad, holding the servo-motor shaft in position.

This combination is commonly used in industrial robotic arms, where the arm must not be allowed to crash to the floor when the power to the robot (and its servo motors) has been cut.

I second Nicholas. You can use a normally closed solenoid valve, so it does not comsume any power when it is deactivated. However, they will consume once the are actuated ( free the servos). So, keep an eye on the power rating of them as their are types that consume alot of power.

Dont give them pwm signal when you dont want to use them

@ Summit: in this case they code move if there is enough force.

A solenoid.. yes I had never thought of that. Something like this indeed:

https://www.sparkfun.com/products/11015

However, this will consume power when we actually use the servo to keep the solenoid in place. Alternatively, is there an wasy way to determine absolute (rather than relative) position of our servo in any other manner? If we are able to determine absolute position, than we have a starting point for control.

@Mohamed i didnt got u

@sumit: i meant that if u don't provide a pwm signal to the servos and there is a mechanical force that is big enough to change the servo position, then you won't be able to hold the servo position. Otherwise, you need to keep providing a pwm signal all the time.

Dear Napoleon,

The kind of Power-off-brake I was referring to is illustrated nicely on this Wikipedia page: http://en.wikipedia.org/wiki/Electromagnetic_brake

However, I think I now understand that the 'real' problem is not that you wish to keep the output shaft locked without power, but rather that you wish to know the angle of the output shaft at power-up.

To do this what you need is an "Absolute rotary encoder". This a (typically optical) measurement device which uses no power when it is off, but it will instantly tell you what the angle of the output shaft is at when power is applied.

See this explanation: http://en.wikipedia.org/wiki/Rotary_encoder#Absolute_rotary_encoder

One final trick which you may find useful:

Once you disconnect power from the servomotor, the output shaft can freely rotate. To prevent this, use a DPDT relay wired such that when the servo controller's power is cut it disconnects the motor from the controller and then shorts the DC motor's armature windings together (through a suitable power resistor). This causes the back-emf of the motor's own windings to act as a 'dynamic brake', which resists any attempts to rotate the output shaft.

Here is a demonstration of this in action: http://www.youtube.com/watch?v=oz3-sPTc34c

Regards,

Nicholas Lee

Dear Napoleon,

you can use worm gear to provide your system self-locking at specify position and this method not need to power .

@ Ihsan : but where to connect the worm gear to acheive that??

Dear Mohamed,

the worm gear can be connected between the servomotor shaft and the application which is want to powered

Dear Napoleon,

Depending on your application, you can use several alternatives.

- Use a irreversible reducer .

Pro. Do not need programming.

Con. Low energy efficiency. Backlash. Decreases the motor output speed (sometimes it is a Pro). Consumtion of volume between the motor output and the load. Decrease the rigidity between motor and load.

- Use an electric brake.

Pro. No backlash. Maximun precision. Low space ( Integrated brake in the motor.Integrated electronics in driver board)

Con. Brake coil consumtion.

- Use a motor holding forces, such as stepper motors. Pro Cheap and simple. Con limited braking force. Noise.

Though we need to use spur / helical gear for servo applications, it is possible to use in some applications a worm gear. With worm gear of proper rating, load cannot drive motor. If motor is powered off (transient / deceleration rate to be taken care), the motor will remain in the position where it is left.

One possible option is to buy brushless motors equipped with brakes. Many manufacturers of brushless motors for industrial applications already sell optional versions of their servos where, usually at the back of the shaft, near to the resolver, they mount an optional brake that can be switched on/off easily, using a pair of wires connected to the brake solenoid. Such option, in most cases, isn't too expensive. Note that the torque of such brakes is quite limited, but usually a bit higher the peak torque of the motor. On most applications they are considered 'emergency' brakes (i.e. they are engaged and lock the shaft when current is cut off from their wires and the position control is not working), anyway I think they could be used in your application, with some customization of the control algorithm.

As an alternative solution, you could try using servo motors having self locking gears (e.g., worm gear). It may be a cheaper solution compared to using magnetic brakes.

If I just wanted to know the position of the servo, in order to have a reference when I turn it on, can I use a potentiometer?

Sure, you can use a potentiometer. Absolute encoders are also a possibility. Among them, you choose according to your requirements such as positional accuracy.

Depending on your application and accuracy required, a potentiometer may work. Optical encoders are another option for better accuracy, but these range in resolution from model to model.

Some servo manufacturers do already provide feedback information on position, speed, temperature, etc. Have a look:

http://www.robotis.com/xe/dynamixel_en

http://herkulex.com/

http://www.openservo.com/

Gentlemen,

i bought recently CNC machine MIKRON VC 750 controlled by Hiedenhain TNC 415b.

unfortunately  i am new to CNC during assembly the machine, i turned the axises when the machine was turned off.

when i turn on the machine i get wrong position tool axis fault.

i guess the servo motor loss its position.

does anyone, can help me with this situation. 

B.Regards

Ali Khafeja