Optional Integrated Linear Encoder Improves Position Accuracy
Many servo press systems use a rotary encoder mounted to a servo motor to provide position feedback. The motor drives a synchronous belt system or a gearbox that drives a ball screw. The control system counts the encoder pulses to determine the system position.
This arrangement generally works well. It's the same system we use on most of our servo presses.
But there are some drawbacks and limitations that should be considered.
Each press component between the encoder and the press tool contributes position error. These errors can be exacerbated by the high forces within the machine. Some significant contributors are:
Most of these effects are small and can usually be neglected. But for applications that require very high position accuracy and these error contributors should be considered.
Servo Press Direct Provides the Press Tools and Fixtures You Need.
Every servo press application needs tooling. At Servo Press Direct, we strive to be your one-stop supplier for all of your press system needs. Along with servo presses, we can design and manufacture the tooling for your needs.
At Servo Press Direct we use the following definitions:
Tool - The device that applies the pressing force onto the part. It is usually placed in the servo press ram.
Fixture - The device that supports and locates the part being pressed upon. It is usually mounted to the servo press base.
In designing tools and fixtures the following should be considered
Press forces throughout the operation - It's important that the pressing forces load the servo press load cell properly throughout the press cycle. Often overlooked in tooling design are the forces applied when the parts being pressed fail to from as expected.
Tooling forces - Forces within the tooling components can be very high. This may require the use of hardened tool steel or hard materials such as tungsten carbide.
Lifetime cost - Some tooling is expected to last for millions of press cycles, others only need to perform a few hundred cycles. These factors can affect tooling design and material selection and have a significant affect on cost.
Commercially available tooling - Sometimes the least expensive way to create tooling is to adapt commercially available tooling to a servo press. This is especially true in wire crimping and electrical connector insertion applications.
At Servo Press Direct our engineers use the latest solid modelling software to design tooling. Our machine shop is capable of CNC milling and turning, grinding, welding, and heat treating.
Servo Press Load Cells and Their Effect on the System
Most servo presses, including ours at Servo Press Direct, use a load cell to provide force feedback to the machine control system. Load cells have a long history of reliable operation.
A typical load cell use a steel body that deflects slightly under load and one or more strain gages that change electrical resistance in proportion to the deflection.
Electronics are used to measure this change in electrical resistance and generate a signal to send to the controller.
There are a number of ways to incorporate a load cell into a servo press actuator. One common method is to incorporate the load cell into the ball screw thrust bearing mount. This method measures the thrust force acting on the bearings.
This method allows for a large robust load cell. The load cell is stationary so there are no moving wires. The load cell is reasonably insensitive to off-axis loads.
But friction in the actuator decreases the load cell accuracy.
All screw driven actuators need an anti-rotation mechanism to resist the screw torque. Some manufacturers will use a brass key that slides in a slot. The resultant friction detracts directly from the force registered by the load cell. This ambiguity should be considered when considering system accuracy.
The load cell can also be mounted at the end of the actuator ram. In this configuration the press tool holder can be in direct contact with the load cell.
Since the tool holder directly contacts the load cell the force inaccuracy from the anti-rotation mechanism friction is eliminated.
But the ram-end mounted load wires need to move. Our machines accommodate this by running a special cable track inside the actuator. With our actuators there are no external load cell wires.
Ram end mounted load cells are more sensitive to off-axis loads. They work best when the load is transferred through the ram centerline.
At Servo Press Direct we design our actuators for flexibility. They can be provided with either thrust bearing mounted or ram end mounted load cells.