admin
**Question:** We have a 300-ton servo press but no tonnage monitoring system. We want to detect broken punches and overloads without buying a $15,000 tonnage monitor. Can we use the servo drive data?
---
**Answer:**
Yes, you can use servo drive torque data as a proxy for tonnage monitoring. It is not as accurate as a strain gauge system, but it can detect broken punches and significant overloads.
## Using Drive Torque as Tonnage Proxy
The servo drive reports motor torque in real time. Torque is proportional to forming force (with some losses from the mechanical transmission).
**How to access torque data:**
- Siemens SINAMICS: r0080 (torque actual value, in Nm)
- Yaskawa: Un000 (torque reference monitor)
- Mitsubishi MR-J4: monitor parameter Md.Trq (torque)
- ABB ACS880: parameter 01.10 (motor torque %)
## Setting Up Monitoring
1. **Establish baseline**: run 50 good parts and log the peak torque value for each stroke. Calculate the average and standard deviation.
2. **Set limits**:
- High limit (overload): average + 3 standard deviations (typically 110-120% of average)
- Low limit (broken punch): average - 3 standard deviations (typically 70-80% of average)
3. **Configure drive fault output**: most drives can output a digital signal when torque exceeds a threshold. Wire this to the press safety circuit to stop on overload.
## Limitations vs. Dedicated Tonnage System
- Cannot distinguish between different punches in a progressive die
- Torque-to-force conversion changes with wear and lubrication
- Response time is 1-5ms vs. 0.1ms for strain gauges
- Cannot detect gradual punch wear (only sudden changes)
## Low-Cost Hardware Option
A load cell under one of the press columns connected to a simple PLC: 50-ton load cell costs $200-$500, basic PLC with analog input costs $300-$800. Total under $2,000 for a functional tonnage monitor.